EP1228253B1 - Steel composition, method for making same and parts produced from said compositions, particularly valves - Google Patents

Abstract

This invention relates to a steel composition comprising, expressed in percentages by weight: 0.25-0.35% C, 24-28% Cr, 10-15% Ni, 3-6% Mn, 1.75-2.50% Nb, 0.50-0.70% N, 0-0.30% Si, provided that C+N>=0.8%, the rest consisting mainly of iron and unavoidable impurities. The invention further relates to a method for making said compositions and valves produced from said compositions or using said method and exhibiting excellent mechanical strength and oxidation resistance at temperatures between 800 and 900° C.

Description

The present invention relates to more steel compositions particularly intended for the manufacture of intake valves and Exhaust systems for vehicles with internal combustion engines.

When used, this type of parts is subject to important mechanical stresses at temperatures that continue to grow with the increase in power and engine efficiency. On time current, when the engine intake includes a turbo, this temperature is generally between 200 and 400 ° C, but can reach 800 ° C at the exhaust when the fuel used is essence. The exhaust valves can thus be submitted at temperatures up to 900 ° C at each explosion followed by a exhaust. The materials used for these valves must also withstand sudden and significant changes in temperature.

This increase in operating temperatures of the valves makes it even more susceptible to oxidation and corrosion by some components contained in the fuels used, such as lead, sulfur or vanadium pentoxide, reducing their service life.

As for the direct oxidation of the metal, it represents the mechanism predominant in European countries where regulations tend to impose unleaded gasoline and to reduce the sulfur content of fuels at very low values, for reasons of atmospheric pollution.

In addition to these different constraints when using finished parts, the steel or alloy used for their manufacture must also fulfill certain additional criteria. Indeed, the manufacture of valves has usually takes place in two stages. The metallurgist will first develop a grade of steel or alloy that it will then deliver to the manufacturer of valves in the form of rectified bars, but also peeled or according to any other surface condition specified by the customer. This manufacturer will then shear these bars, an operation also called cutting in plots. In a first manufacturing process, the slice cut is performed at high temperature, and is followed by transformation by extrusion of slugs into valves at temperatures ranging from 1150 to 1200 ° C, which supposes that the granular structure of the delivered bar remains stable up to forging temperatures.

In a second manufacturing process, called upset, the slugs are obtained by shearing at room temperature, which requires a weakly brittle metal to avoid non-uniform shearing and cracking of these slugs. In addition, during this cold shearing, problems with the segregation of carbides in the plots, which in particular, excessive wear of the tools.

Steels of the prior art pose particular problems during the shearing because the appearance of cracks in the parts imposes adjustments frequent production lines.

The materials traditionally used for the manufacture of such valves are, in particular, stainless austenitic steels, which have iron-nickel-chromium base and are divided between high-grade steels manganese content (up to 10% by weight) and high grade steels nickel (up to 21% by weight). Their resistance to high oxidation temperature is not always satisfactory, especially when, for example, for example, the engine operates in a marine atmosphere and ingests chlorine, or even when an increase in engine efficiency implies warmer combustion gases. These shortcomings bring the developers to increase the chromium content ever more, which has disadvantage of favoring the formation of ferrite at high temperature and of embrittling intermetallic phases at the temperatures of use. Austenitic steel having improved resistance to high temperature containing Cr, Ni, Mn and one or more Carburigenic elements such as Nb have been proposed in EP-A1-0467756.

The present invention is therefore essentially intended to remedy the aforementioned drawbacks of known steel compositions, providing steel compositions having in particular a resistance to oxidation, mechanical characteristics as well as improved processing properties, which are particularly suitable for allow the manufacture of exhaust valves having a uniform mechanical and oxidation resistance excellent in the range of 800 to 900 ° C.

For this purpose, a first object of the invention is constituted by a steel composition comprising, expressed in percentages by weight: VS 0.25 - 0.35% Cr 24 - 28% Or 10 - 15% mn 3 - 6% Nb 1.75 - 2.50% NOT 0.50 - 0.70% Yes 0 - 0.30% it being understood that C + N ≥ 0.8%,
the balance being iron and unavoidable impurities.

In a preferred embodiment of the invention, the steel composition comprises, expressed in percentages by weight: VS 0.25 - 0.32% Cr 25 - 26% Or 11.50 - 12.50% mn 4.80 - 5.20% Nb 1.90 - 2.30% NOT 0.61 - 0.70% Yes 0 - 0.30% with the understanding that C + N ≥ 0.9%,
the balance being iron and unavoidable impurities.

• en figure 1 : des compositions d'acier non conformes à la présente invention comportant 0,286% de C, 4,93% de Mn, 11,92% de Ni, 25,21% de Cr, 0,292% de Si, mais 1,5% de niobium et 0,5% d'azote,
• en figure 2 : des compositions d'acier selon la présente invention identiques aux précédentes mais comportant 1,75% de niobium et 0,525% d'azote,
• en figure 3 : des compositions d'acier selon la présente invention identiques aux précédentes mais comportant 2% de niobium et 0,55% d'azote.

Indeed, the present inventors have surprisingly discovered that the steel compositions thus defined all have a solidification mode very close to a eutectic between the γ phase of the austenite and a phase which has proved to be a carbonitride of niobium Nb (C, N). Three phase diagrams are shown in FIGS. 1 to 3, which respectively correspond to:

• in FIG. 1: steel compositions not in accordance with the present invention comprising 0.286% of C, 4.93% of Mn, 11.92% of Ni, 25.21% of Cr, 0.292% of Si, but 1, 5% of niobium and 0.5% of nitrogen,
• in FIG. 2: steel compositions according to the present invention identical to the preceding ones but comprising 1.75% of niobium and 0.525% of nitrogen,
• in FIG. 3: steel compositions according to the present invention identical to the preceding ones but comprising 2% of niobium and 0.55% of nitrogen.

Figures 4, 6 and 7 show steel structures according to the invention at different stages of implementation. Figure 5 shows a steel structure of the prior art.

These theoretical phase diagrams have been plotted against the carbon content of the compositions, because it must imperatively be between 0.25 and 0.35% by weight for hardness problems but also because, beyond this interval, carbide precipitates extremely harmful are formed.

If we consider Figure 1, in which the curve surmounted by a 1 represents the austenite phase and the curve surmounted by a 7 represents the phase of niobium carbonitride, it can be seen that the curve of niobium carbonitride does not rise above that of austenite for carbon contents greater than 0.5% by weight, which implies that the theoretical eutectic γ / Nb (C, N) (represented by the letter E) is located at right of the diagram.

On the other hand, if we consider Figure 2, on which the curves the same meanings as for Figure 1, we see that the eutectic is obtained for a carbon content of 0.30% by weight.

When cooling a steel casting having the composition theoretical of eutectics, we find that niobium carbonitrides are formed when one reaches the temperature of said eutectic precipitate very early, and then distribute evenly throughout the remainder of the liquid flow who surrounds them. The resulting structure at the end of operations conventional thermomechanical transformation by rolling and then cooling of the rolled bars is homogeneous and quite remarkable. It is presented in Figure 4. This structure retains a good homogeneity throughout the bar section as a result of heat treatments or heating at very high temperatures (> 1100 ° C) as shown in Figure 6.

For comparative purposes, FIG. Banded structure conventionally obtained with steel compositions austenitic stainless steels of the prior art. These segregated strips are not homogeneous, dark bands containing carbides while the light bands do not have any. These bands are actually obtained after stretching the piece of steel, which contains dendrites from the phase austenitic and interdendritic and intergranular network of carbides from a reaction of end of solidification.

These structural differences lead to differences in important behavior, especially during hot processing newly developed steel flows. Indeed, if the structure of the steel composition is heterogeneous, as is the case of the compositions of the prior art, the final structure of the pieces produced will be itself heterogeneous resulting in variations in the properties of steel.

Moreover, the heterogeneity of the structure may have other disadvantages when making parts. So, during manufacturing of valves for vehicles with crush-fueled engines, the car manufacturer shears round steel wires with a diameter of 6 to 13 mm in automated production lines. The structure of the steel being not homogeneous, the shearing will not be uniform which leads to the appearance of cracks and requires frequent adjustments of production lines.

In a preferred embodiment, the nitrogen contents, in niobium and carbon, which are the three elements forming the carbonitride of niobium Nb (C, N) are chosen so that the compositions resultants are hyper-eutectic in phase diagrams theoretical. The phase diagram of Figure 3 represents an example of such a composition for which eutectic E corresponds to a content carbon of about 0.15% by weight. Hyper-eutectic compositions according to the invention, for which the carbon content is between 0.25 and 0.35%, preferably between 0.25 and 0.32% by weight, exhibit the advantage of seeing the precipitation of niobium carbonitrides take place very early during the solidification process thus allowing a distribution optimal precipitates within the casting.

Note, although the compositions can be described as hyper-eutectic in the theoretical phase diagrams, that in practice industrialists the inventors still observe the primary precipitation of the austenite phase: this disagreement between theory and experimental reality can be justified by supercooling, germination and phase growth.

As can be seen in these diagrams, one of the advantages of compositions according to the invention is that the eutectic γ / Nb (C, N) is conserved even with low carbon levels, because nitrogen is substituted for carbon in the compound Nb (C, N). We can therefore keep the favorable effect of eutectics on solidification structures while limiting the rate of carbon in steel, which has several interesting consequences, as will be seen now.

One of the favorable consequences of the limited carbon contents is that there is a very wide range of temperatures (about 1,175 ° C to 1,300 ° C) in which the structure consists exclusively of austenite and niobium carbonitrides. In particular, the harmful carbide M ₂₃ C ₆ is completely dissolved, which allows good behavior of the metal during thermomechanical processing operations such as rolling or extrusion / forging.

The presence of niobium carbonitride in this area of Moreover, temperatures have the advantage of limiting the magnification of grains during recrystallization heat treatments, dissolution, and / or softening of the finished products. The recrystallized structures are therefore homogeneous, very appreciable property and very difficult to obtain from reproducible way by implementing the steel compositions of the art prior.

The present inventors have also sought to limit the carbon content of the compositions according to the invention in order to reduce the potential rate of intergranular precipitation of the harmful carbide M ₂₃ C ₆ , during the final heat treatment of stabilization of the parts or during use. of these pieces. This potential precipitation rate, however, remains high in the compositions according to the invention, nitrogen being substituted for carbon to form nitrides and carbonitrides. But, it is quite surprising that the ductility at room temperature, measured by the elongation at the tensile test A _5d , remains very good. The characteristics of resistance to oxidation are also excellent.

The present inventors have also found that the structure obtained after the solidification of ingots undergoes a modification important after conventional transformation operations thermomechanical (rolling, etc ...).

Indeed, we see that the network of carbonitride rods eutectic Nb (C, N) disappears, giving way to a distribution relatively homogeneous Nb (C, N) globular carbonitrides in the processed products, such as rolled bars, for example, as can see it in figure 6.

When the nitrogen, niobium and carbon contents are such that the resulting compositions are hypo-eutectic in the theoretical phase diagrams, niobium carbonitrides do not precipitate that at the end of solidification, resulting in a distribution a priori less advantageous. We then obtain a structure called "Chinese characters" (Chinese scripts) as shown in Figure 7. However, we see there also surprisingly that the network of rods is globulise after forging allowing a subsequent transformation without problems individuals. On the other hand, the band structure is more apparent.

Excellent properties observed for steel compositions according to the invention are obtained thanks to the precise balancing of the elements alloy.

Chromium is mainly used to obtain good performance in oxidation thanks to the passivated layer of oxide that it forms on the surface of the metal. It also has a beneficial influence on the mechanical strength at high temperature. Its content in the compositions according to the invention is 24 to 28%, preferably 25 to 26% by weight.

Nickel has a desired gamma-gen effect. It is limited because of its price, at a content just sufficient for the solidification of the matrix in austenitic mode. Its content in the compositions according to the invention is 10 to 15%, preferably 11.5 to 12.5% by weight.

Carbon has a desired hardening effect, but too much high temperature leads to the precipitation of weakening and harmful carbides for the resistance to oxidation. Its content in the compositions according to the invention is 0.25 to 0.35% by weight, preferably 0.25 to 0.32%.

Nitrogen is a highly gamma-generating element that allows compositions according to the invention to remain in the austenitic field by delaying the precipitation of the intermetallic phases. Its content is however limited because of difficulties in introducing it in steel compositions because of its low solubility limit in liquid steel. Its content is 0.5 to 0.7%, preferably 0.61 to 0.7% in weight. These levels also correspond to the near-saturation at the equilibrium of the liquid metal at conventional temperatures of elaboration, which is an advantage, because this addition is then easy with the means usual known to those skilled in the art. In addition, as the solidification of the steel according to the invention gives rise to two phases (austenite and niobium carbonitride) which can accept a lot of nitrogen, there is no inadvertent reaction of degassing in ingots that could generate unwanted bubbles or blowholes.

Manganese makes it easier to introduce nitrogen into the composition by increasing the value of its phase solubility limit liquid and solid, but its quantity is limited because of its harmfulness for the resistance to oxidation. Its content in the compositions according to the invention is 3 to 6%, preferably 4.8 to 5.2% by weight.

Niobium, besides its favorable carburizing properties for the mechanical resistance to heat, allows to obtain the eutectic previously described. Its content in the compositions according to the invention is from 1.75 to 2.50%, preferably 1.90 to 2.30% by weight.

Silicon is limited to a content of not more than 0.30% by weight, although it improves the resistance to oxidation because it is strongly sigmagene and further lowers the solubility of nitrogen.

The steel compositions according to the invention can be manufactured according to the methods applicable to the standard materials referred to, in taking into account these particularities.

Thus, one can not develop under vacuum because it is necessary to saturate the liquid in nitrogen. It may be used for this purpose an electric furnace or a reactor AOD or any other means suitable for the preparation of steels containing high levels of the nitrogen alloy element, including processes secondary refining by electroslag remelting slag. The remelting can be done, for example, under slag with electrode consumable if you are looking for a great inclusiveness.

These operations may be followed by a process of conventional hot thermomechanical transformation such as forging or rolling then a softening treatment, which will preferably maintained at 1,050-1,100 ° C for 1 to 16 hours in air or in another fluid, which guarantees a complete recrystallization fine grain, and satisfactory ductility characteristics.

Thermal treatments for dissolution and recrystallization as well as the preheating of products for valve manufacture may be carried out between 1100 and 1200 ° C; the most brought a grain magnification which remains limited.

The stabilizing heat treatment is intended to guarantee a certain structural and dimensional stability at operating temperatures. It may be achieved, for example, in the form of 700-1000 ° C for 1 to 16 hours in air or other fluid. On the one Generally speaking, it is preferable to carry out this treatment at a temperature greater than or equal to the operating temperature of the part in service.

TESTS

R_m = résistance maximale

R_p0,2= limite élastique conventionnelle à 0,2% de déformation

A_5d = allongement en % sur la base 5 d (d = diamètre de l'éprouvette).

Tous les pourcentages mentionnés sont des pourcentages en poids.The symbols used in the following have the following meanings:

R _m = maximum resistance

R _p0,2 = conventional yield _stress at 0.2% deformation

A _5d = elongation in% on the base 5 d (d = diameter of the test piece).

All percentages mentioned are percentages by weight.

The different tests were carried out on one hand out of two compositions according to the invention called A and B, and secondly on a composition C outside the claims of this patent and created specifically for comparison purposes, as well as on three compositions of known reference steel D, E and F.

D : X50CrMnNiNbN 21.9 (norme DIN = 1.4882)

E : X33CrNiMnN 23.8 (norme DIN 1.4866)

F : X35CrNiMnMoW 25.9.

The three known reference steels are as follows:

D: X50CrMnNiNbN 21.9 (DIN standard = 1.4882)

E: X33CrNiMnN 23.8 (DIN 1.4866 standard)

F: X35CrNiMnMoW 25.9.

R _m , R _p0,2 and A _5d are measured using a tensile test. AT B * VS D E F VS 0.30% 0.30% 0.286% 0.52% 0.35% 0.35% Cr 25.46% 25.35% 25.21% 20.70% 22.75% 25.50% Or 12.00% 12.10% 11.92% 3.60% 7.50% 9.00% mn 4.90% 4.84% 4.93% 8.60% 3.25% 5.00% Nb 2.00% 1.98% 1.55% 2.10% - 0.45% NOT 0.644% 0.55% 0.50% 0.47% 0.275% 0.515% Yes 0.22% 0.25% 0.292% 0.35% 0.70% 0.18% W - - - 0.99% - 0.725% MB - - - - - 0.725% V - - - - - 0.45% Fe complement complement complement complement complement complement C + N 0.944 0,850 0.786 0,990 0.625 0.865

Mechanical properties at room temperature and at elevated temperatures

The values of mechanical strength of the valve steels being very strongly dependent on their thermal state, the values that are going to be compared in the following are average values obtained for different thermal states of use all including a dissolution in solution to high temperature followed by aging at lower temperature.

Indeed, apart from the statistical fluctuations in resistance from one batch to another (of the order of a few tens of MPa), notes that the rise in the aging temperature and / or increasing the maintenance time at the aging temperature induce a collapse of the resistance levels, in particular the elastic limit, which is related to the coalescence of carbides and other precipitates.

This implies that the values of mechanical resistance measured on a sample treated by short-term aging at a temperature less than that in service have no meaning because these values are decrease rapidly when putting the parts into service.

The skilled person will also consider the data from the literature with caution, especially when the thermal state of the parts tested is not specified.

This is why the compositions tested were put in solution to 1 160 ° C for 1 hour then cooled in water, then aged for 4 hours at 850 ° C, with the exception of the grade F which was dissolved in 1 120 ° C for 1 hour then cooled in water and then aged at 820 ° C for 4 hours.

Aging at 850 ° C corresponds to an estimated temperature greater than or equal to the temperature of use of the valves in modern engines where very high temperatures prevail. Materials Test temperature (° C) R _m (MPa) R _p0.2 (MPa) A _5d (%) ambient 1001 605 26 AT 800 ° C 419 263 27 850 ° C 348 226 29 ambient 964 563 26.5 B 800 ° C 394 249 35.5 850 ° C 342 226 40 ambient 957 558 28.5 VS 800 ° C 375 234 36 850 ° C 298 203 30 ambient 968 555 23.8 D 800 ° C 350 209 41 850 ° C 281 187 41 ambient 916 491 32 E 800 ° C 352 209 51.5 850 ° C 286 175 68.5 ambient 1033 606 24 F 800 ° C 373 244 34 850 ° C 307 191 49

We therefore note that for aging adapted to a use at very high temperature, the alloys according to the invention levels of mechanical strength higher than the steels of reference, especially since the nitrogen content is between 0.64% and 0.70% by weight, at least.

Creep-lengthening resistance

This behavior is determined from the value of the constraint causing 1% elongation by creep in 100 hours.

The three shades A, B and C were previously processed by placing in solution and aging at 850 ° C for 4 hours, while the reference steel grades have been treated conventionally for each steel, which is favorable to them in the comparison.

The results are collated in Table 3. Materials Test temperature (° C) Stress for 1% elongation (MPa) AT 815 76 B 815 62 VS 815 59 D 815 27 E 815 82 F 815 60

Corrosion and oxidation resistance tests 1. Resistance to corrosion by sodium sulphate + NaCl

This test reproduces the environment of the valves when in contact with engine combustion fumes diesel in the marine environment, where corrosion is aggravated by presence of chlorides.

The steel specimen is a cylinder 12 mm in diameter by 12 mm long cut in the axis of the products. We weigh the test tube and then we has a cold alumina crucible filled with a mixture of 90% by weight of sodium sulphate and 10% by weight of sodium previously melted for 20 minutes in the electric oven brought to about 927 ° C. The whole is left for 1 hour at room temperature in the oven.

The specimen is then removed from the crucible and allowed to cool the air. It is then scoured by immersion for about 15 minutes in an aqueous solution, previously heated to 100 ° C, and containing 12% of ferric sulphate and 2.6% of a 40% HF solution, then the lost mass.

Δm : perte de masse de l'éprouvette, en g,

S : surface initiale de l'éprouvette en dm²,

t : durée de l'essai de corrosion en heure.

The stripping / weighing cycle is repeated several times, then the mass of the test piece is plotted as a function of the cumulative duration of stripping. This graph must show a first line which represents the attack of the oxide formed in contact with the corrosive mixture, then a second line which represents the attack of the healthy steel by the pickling solution. The intersection of these two straight lines makes it possible to obtain the mass loss of the specimen Δm due to corrosion by the molten lead oxide. The corrosion rate C is then calculated according to the following formula: C = Dm St

Δm: mass loss of the specimen, in g,

S: initial surface of the specimen in dm ² ,

t: duration of the corrosion test in hours.

2. Resistance to oxidation in the air

The steel test piece is a 6 mm diameter cylinder on 20 mm long cut in the axis of the products, and having a hole of diameter 3 to 4 mm.

Pf : poids après oxydation,

Pi : poids initial,

S : surface initiale de l'éprouvette en dm².

This test consists of carrying the test piece, previously placed in an alumina crucible at a temperature of 871 ° C. for 100 hours in an electric oven, and then allowing the specimen to cool. This test piece is weighed before and after the oxidation and the weight gain is determined according to the formula: Weight gain = Pf - Pi S

Pf: weight after oxidation,

Pi: initial weight,

S: initial surface of the specimen in dm ² .

We then proceed to several successive stripping as for 1. The first stripping is carried out for 10 minutes, then their duration is gradually brought to 20, 40 and 60 minutes. We stop stripping when the sound metal is attacked.

The graph of the mass of the specimen as a function of the cumulative duration of stripping again provides two lines of different slopes, the intersection of which gives the value of the mass Pr of sound metal. The following loss of mass is then calculated: Loss of mass = Pi-Pr S

The results of the corrosion and oxidation tests are summarized in the following table: Materials Corrosion in molten salts C (g / dm ² / h) Oxidation in the air Changes in the mass at the end of the test (g / dm ^2/100 h) Oxidized mass - Loss after pickling (g / dm ^2/100 h) AT 0.3 0.120 to 0.180 0.80 to 1.10 B 12.5 - 0.020 0.36 to 0.52 VS 19.2 0,020 from 0.79 to 0.78 D - - 0.091 1.45 to 2.50 E 0.2 0,047 0.45 to 0.75 F 63-70 - 0.45-0.60

Although the thermal states of these steels are not rigorously identical, it is found that the oxidation rates of the steels according to the invention (A and B) are lower than those of the reference steel D, and are of the same order as those of the best steels of the prior art, even best in the case of grade B.

An increase in the niobium content, all else being equal (C / B), improves the oxidation resistance, since it seems that nitrogen forms preferentially NbN nitride rather than Cr ₂ N nitride, thus leaving more room for oxidation. free chrome not fixed.

It can therefore be seen that the steel according to the invention can provide a very high good resistance to oxidation despite concentrations in C + N also higher than 1%.

Surprisingly, the present inventors have found a very marked improvement in the corrosion resistance in the Na ₂ SO ₄ + NaCl medium with the increase in the nitrogen content of the steel according to the invention. When the nitrogen content is in the highest range, this resistance to corrosion in the molten salts is equivalent to that of the best reference steel, despite a rate of intergranular precipitation nitrides and carbides much stronger.

It can therefore be seen that the steels according to the invention present both excellent mechanical properties at room temperature and at very high temperatures as well as excellent resistance to oxidation and corrosion by molten salts.

Thus, if the main application of the compositions according to the invention described here is the manufacture of valves for motor vehicles to internal combustion, it is clear that the invention is not limited to such application and that it can be used to make all the pieces withstand similar or similar constraints, as may be be the case for tools for hot deformation, elements of fixing (screws, nuts) or control devices, for example.

Claims (12)

1. A steel composition comprising, expressed in percentages by weight : C 0.25-0.35% Cr 24-28% Ni 10-15% Mn 3-6% Nb 1.75-2.50% N 0.50-0.70% Si 0-0.30%    whereby C+N≥0.8%, and the rest consist of iron and the unavoidable impurities.
2. The steel composition as claimed in claim 1, wherein the steel composition contains 25 to 26% by weight of chromium.
3. The steel composition as claimed in claim 1 or 2, wherein the steel composition contains 1.90 to 2.30% by weight of niobium.
4. The steel composition as claimed in any one of claims 1 to 3, wherein the steel composition contains 0.61 to 0.70% by weight of nitrogen.
5. The steel composition as claimed in any one of claims 1 to 4, whereby C+N≥0.9%.
6. The steel composition as claimed in claim 1, wherein the steel composition contains, expressed in percentages by weight: C 0.25-0.32% Cr 25-26% Ni 11.50-12.50% Mn 4.80-5.20% Nb 1.90-2.30% N 0.61-0.70% Si 0-0.30%    whereby C+N≥0.9%, and the rest consist of iron and the unavoidable impurities.
7. The steel composition as claimed in any one of claims 1 to 6, wherein the levels of carbon, nitrogen and niobium are also selected so that said steel composition is hypereutectic in the theoretical phase diagrams.
8. A method for the preparation of a steel having a composition according to any one of claims 1 to 7 comprising the steps of :

   preparing an electrode of the said steel composition ;

   electroslag remelting the consumable electrode ;

   and possibly forming said steel by a hot thermomechanical process such as forging or rolling.
9. The method as claimed in claim 8, wherein it further comprises a thermal tempering of the steel between 1050 and 1100°C after the possible thermomechanical forming step.
10. The method as claimed in claim 8 or 9, wherein it also comprises the further steps :

    solution annealing the steel at 1100-1200°C. ; and

    heat treating at a temperature greater than or equal to the temperature at which said steel will be used.
11. Parts, in particular valves, formed in a steel having a composition according to any one of claims 1 to 7 or obtained by a process according to any one of claims 8 to 10.
12. Use of a composition according to any one of claims 1 to 7 for working valves for engines working in a marine atmosphere.

Images