EP3252175B1 - Molded steel alloy, corresponding part and manufacturing method - Google Patents

Description

The present invention relates to a steel alloy made in a foundry, the composition of which comprises, inter alia, iron (Fe), carbon (C), nickel (Ni) and cobalt (Co).

The present invention will find its application mainly in the field of the manufacture of foundry tools from a steel alloy, said tools being used subsequently in the forming of various parts, in particular themselves obtained from a alloy comprising, for example, titanium.

We know of the European patent document EP 0 343 292 a steel alloy comprising the following elements, in the proportions indicated: C between 0.4 and 0.8%; If less than or equal to 1%; Mn less than or equal to 1%; Neither between 30 and 40%; Co between 2 and 8%; S less than or equal to 0.2%; P less than or equal to 0.2%; Mg and / or Ca less than or equal to 0.3%; with Ni + Co * 0.75 between 32 and 40%, the rest being Iron and inevitable impurities. The alloy is brought to a temperature between 600 and 1000 ° C and is then quenched.

This alloy is of interest in the manufacture of precision mechanical machines, for example, machine tools, measuring instruments, semiconductor manufacturing machines and optical machines, due to a low coefficient of expansion at temperatures including between 20 and 100 ° C.

Regarding the document JP 2003 138336 which discloses a steel alloy having 26 to 32% Ni, 5 to 12% Co, 0.5 to 1.7% C, less than 1% Si, less than 0.5% Mn, the remainder being iron, said alloy comprising a proportion of rare earths (lanthanum, cerium, praseodymium and neodymium) of between 0.01 and 0.009%, and having a coefficient of low thermal expansion up to a temperature of 200 ° vs.

However, in the field targeted by the alloy of the present invention, the manufacture of steel alloy foundry tools, the alloy in question must have a low coefficient of thermal expansion at temperatures much greater than 100 °. C, and even as 200 ° C.

Also known in the state of the art, tools made specifically in the foundry to form titanium alloy parts, at temperatures ranging from 400 to 1000 ° C. The expansion coefficients of these tools are relatively high (greater than 17.10 ^-6 K ^-1 ).

However, new parts development processes require a tool with a coefficient of thermal expansion close to that of the alloys to be formed, especially for titanium alloys, of the order of 10.3.10 ^-6 K ^-1 to 830 ° C, in the temperature range used by forming.

The tooling must also have good creep resistance. Indeed, such a characteristic makes it possible to prevent the tools from deforming at high temperature.

For forming temperatures above 400 ° C, low-expansion tooling is available only in welded construction. However, these are likely to deform at high temperatures.

For molding temperatures up to 400 ° C, known in the state of the art, tools made in the foundry from a number of alloys having a coefficient of thermal expansion and acceptable mechanical strength , which is not the case for higher temperatures.

The patent application is thus known FR 3 025 807 also filed by the applicant, and which discloses a spheroidal graphite or graphite cast iron alloy, known under the name Ferrynox N29K, and comprising the following elements: carbon (C) between 1.2% and 3.5% - silicon (Si) between 1.0% or 1.2% and 3% - nickel (Ni) between 26% and 31% - cobalt (Co) between 15% and 20% and optionally: magnesium (Mg) between 0.02% and 0.10% - manganese (Mn) less than or equal to 1.5% - chromium (Cr) less than or equal to 0.5% and / or phosphorus (P) less than or equal to 0.12 or 0.04% and / or sulfur (S) less than or equal to 0.11 or 0.03%, and / or molybdenum (Mo) less than or equal to 0.5%, and / or copper (Cu) less than or equal to 0.5 %, the rest being Iron and unavoidable impurities.

This alloy makes it possible to obtain tools having a coefficient of thermal expansion of interest, that is to say low and stable, for temperatures up to 400 ° C. The tools thus manufactured have an interesting application in the manufacture of composite parts or thermoplastic materials.

However, the coefficient of expansion at temperatures above 400 ° C is too great.

For temperatures above 400 ° C, there are still alloys with a low coefficient of thermal expansion for tools made in the foundry, but their resistance to creep is low.

However, some materials, such as titanium alloys for example, can be formed only at temperatures above 400 ° C.

Consequently, the tools used for their forming must not only have a low coefficient of thermal expansion but also good creep resistance, in order to avoid deformation of said tools at temperatures above 400 ° C. during forming, for example of titanium alloy parts.

The object of the invention is to enable the manufacture by molding of a steel part whose thermal expansion is low for high temperatures, in particular up to 1000 ° C., and having good creep resistance.

In particular, the invention aims to design an alloy that allows the manufacture of a tool having a low coefficient of expansion and good creep resistance, including at very high temperatures.

• du carbone (C) entre 0,08% et 0,4%,
• du silicium (Si) entre 0,15% et 2%,
• du nickel (Ni) entre 24% et 31%,
• du cobalt (Co) entre 15% et 30%,
• du niobium (Nb) entre 0,01% et 2,5%,

le reste étant du Fer et des impuretés inévitables.For this purpose, the invention relates to a steel alloy characterized in that it consists, in% by mass relative to the total mass of the alloy, of the following elements:

• carbon (C) between 0.08% and 0.4%,
• silicon (Si) between 0.15% and 2%,
• nickel (Ni) between 24% and 31%,
• cobalt (Co) between 15% and 30%,
• niobium (Nb) between 0.01% and 2.5%,

the rest being iron and unavoidable impurities.

Optionally, the alloy of the invention comprises molybdenum (Mo) at a content of less than or equal to 3% by weight and / or manganese (Mn) at a content of less than or equal to 1.5% by weight and / or chromium (Cr) at a content of not more than 1.5% by mass and / or phosphorus (P) at a content of not more than 0.04% by mass and / or sulfur (S) at a less than or equal to 0.03% by weight and / or copper (Cu) at a content less than or equal to 0.5% by weight.

• la teneur en nickel (Ni) est comprise entre minimum 24%, ou minimum 25% ou minimum 26% et maximum 30%, ou maximum 30,5% ou maximum 31% en masse ;
• la teneur en cobalt (Co) est comprise entre minimum 15%, ou minimum 16% et maximum 19%, ou maximum 27%, ou maximum 30% en masse ;
• la teneur en niobium (Nb) est comprise entre minimum 0,01%, ou minimum 0,05% et maximum 1,5%, ou maximum 2%, ou maximum 2,5% en masse ;
• la teneur en carbone (C) est comprise entre minimum 0,08%, ou minimum 0,1%, ou minimum 0,12% et maximum 0,2%, ou maximum 0,3%, ou maximum 0,4% en masse ;
• la teneur en silicium (Si) est comprise entre au minimum 0,15%, ou au minimum 0,2% et au maximum 1%, ou au maximum 1,5%, ou au maximum 2% en masse ;
• la teneur en molybdène (Mo) est comprise entre au minimum des traces ou au minimum 0,05% ou au minimum 0,5% et au maximum 1,2%, ou au maximum 1,5%, ou au maximum 3% en masse ;
• la teneur en Manganèse (Mn) est comprise entre au minimum des traces ou au minimum 0,05%, ou au minimum 0,1% et au maximum 1%, ou au maximum 1,5% ;
• la teneur en cuivre (Cu) est inférieure ou égale à 0,5% en masse, de préférence inférieure ou égale à 0,4%, de préférence inférieure ou égale à 0,3% en masse.
• la teneur en chrome (Cr) est inférieure ou égale à 1,5% en masse, de préférence inférieure ou égale à 1% en masse, de préférence inférieure ou égale à 0,4% en masse, de préférence inférieure ou égale à 0,3% en masse.

According to particular embodiments, the alloy of the invention may comprise one or more of the following characteristics:

• the nickel (Ni) content is between minimum 24%, or minimum 25% or minimum 26% and maximum 30%, or maximum 30.5% or maximum 31% by mass;
• the cobalt (Co) content is between minimum 15%, or minimum 16% and maximum 19%, or maximum 27%, or maximum 30% by weight;
• the niobium content (Nb) is between minimum 0.01%, or minimum 0.05% and maximum 1.5%, or maximum 2%, or maximum 2.5% by weight;
• the carbon content (C) is between minimum 0.08%, or minimum 0.1%, or minimum 0.12% and maximum 0.2%, or maximum 0.3%, or maximum 0.4% in mass ;
• the silicon content (Si) is between at least 0.15%, or at least 0.2% and at most 1%, or at most 1.5%, or at most 2% by weight;
• the molybdenum (Mo) content is between minimum traces or at least 0,05% or at least 0,5% and at most 1,2%, or at most 1,5%, or at most 3% in mass ;
• the Manganese content (Mn) is between minimum traces or at least 0.05%, or at least 0.1% and at most 1%, or at most 1.5%;
• the copper (Cu) content is less than or equal to 0.5% by weight, preferably less than or equal to 0.4%, preferably less than or equal to 0.3% by weight.
• the chromium (Cr) content is less than or equal to 1.5% by weight, preferably less than or equal to 1% by weight, preferably less than or equal to 0.4% by weight, preferably less than or equal to 0; , 3% by weight.

The invention also relates to a part manufactured at least in part of a steel alloy, said alloy having a composition as defined above.

This piece may in particular consist of a tool, whose purpose is to allow the forming of other parts from alloys, for example titanium-based. In other words, said tool manufactured from the alloy of the invention is capable of receiving an alloy at a high temperature, between 400 and 1000 ° C, and it must have a low coefficient of expansion and good strength. creep in this temperature range, and especially up to a forming temperature of 1000 ° C.

• on élabore un alliage en fonderie qui présente une composition telle que définie ci-dessus ;
• on coule ledit alliage dans un moule pour obtenir ladite pièce.

The invention also relates to a method for manufacturing a part as defined above, characterized in that it comprises at least the following steps:

• a casting alloy is produced which has a composition as defined above;
• casting said alloy in a mold to obtain said piece.

The invention will be better understood on reading the detailed description which follows, given solely by way of example and with reference to the single figure on which is represented the thermal expansion behavior of four alloys (GX40CrNiSi25-20, GXX40NiCrSi38-19, Ferrynox N29K and Example1), one of which alloy example of the invention (Example 1), by means of the evolution of their average coefficient of thermal expansion (in 10 ^-6 m / m ° C or 10 ^-6 K ^-1 ) as a function of the temperature (in ° C) and also referring to the table below giving the creep behavior of these different alloys.

The invention relates to a particular alloy composition. This alloy makes it possible to obtain parts whose coefficient of thermal expansion is low and the resistance to high creep up to a temperature of 1000 ° C.

The part is for example a tool, in particular a tool for forming metal parts.

All the composition indications are subsequently given in% by weight of the total mass of the alloy.

A first aspect of the invention is the chemical composition of the alloy.

The alloy is a cast steel alloy made in a foundry. Its basic component is Iron (Fe). It also includes unavoidable impurities resulting from the development of said alloy.

The alloy of the invention has a composition consisting, in addition to iron (Fe) and said impurities, of carbon (C) between 0.08% and 0.4%, silicon (Si) between 0.15% and 2%, Nickel (Ni) between 24% and 31%, Cobalt (Co) between 15% and 30% and Niobium (Nb) between 0.01% and 2.5%.

The nickel (Ni) content of the alloy may be preferably from at least 25% or 26% to at most 30% or 30.5% by weight.

The cobalt (Co) content of the alloy may be preferably from at least 16% to at most 19% or 27% by weight.

The Niobium (Nb) content of the alloy may be preferably from at least 0.05% to at most 1.5% or 2% by weight.

The carbon content (C) of the alloy may be preferably from at least 0.1% or 0.12% to at most 0.2% or 0.3% by weight.

The silicon (Si) content of the alloy may be preferably from at least 0.2% to at most 1% or 1.5% by weight.

In addition, the alloy may comprise manganese (Mn) at a content between traces and 1.5%.

The Manganese content (Mn) of the alloy may be preferably from at least 0.05% or at least 0.1% to at most 1.0% by weight.

In addition, the alloy may comprise copper (Cu) at a content between traces and 0.5% by weight.

The copper (Cu) content of the alloy may preferably be less than 0.4%, more preferably less than 0.3% by weight.

In addition, the alloy may comprise molybdenum (Mo) at a content between traces and 3%.

The molybdenum (Mo) content of the alloy may preferably be from 0.05% or 0.5% to at most 1.2% or 1.5% by weight.

In addition, the alloy may comprise chromium (Cr) at a content between traces and 1.5%.

The chromium (Cr) content of the alloy may preferably be less than or equal to 1% by weight, more preferably less than or equal to 0.4% and, most preferably, this chromium content is less than or equal to 0, 3%.

In addition, the alloy may comprise phosphorus (P) at a content between traces and 0.04% by weight.

In addition, the alloy may comprise sulfur (S) at a content between traces and 0.03% by weight.

Since the end of the 19 ^th century, there has been known an iron-based alloy with a low coefficient of expansion, Fe-Ni36. This alloy is known under the trade name INVAR®.

In its composition, this alloy comprises 36% by weight of nickel, the balance being iron.

The curve representing the expansion coefficient of iron / nickel alloys has an anomaly around 36% nickel: the coefficient of expansion is then much lower than for the other compositions. However, this is only valid for low temperatures, up to 130 ° C, above which the coefficient of expansion of the alloy is no longer stable. Thus, the use of this alloy at higher temperatures is not advantageous.

Various alloys were then developed from this base, in particular with cobalt as additive element. For example, iron / nickel / cobalt steel with 32% nickel and 5.5% cobalt has a lower coefficient of expansion than INVAR®, and most importantly, it retains this property at higher temperatures. However, its creep resistance at high temperature is low.

Molded refractory steels (GX40CrNiSi25-20, GX40NiCrSi38-19) are known, this time making it possible to obtain good creep resistance at high temperature (tensile strength in 1000h respectively equal to 28 and 30MPa at 900 ° C.), but with a high coefficient of expansion (respectively 18.6 and 17.3.10 ^-6 K ^-1 at 900 ° C).

According to examples, the alloy comprises, in addition to iron (Fe) and unavoidable impurities, only the following elements, within the limits indicated: VS Yes mn P S Cr Or MB Cu Co Nb mini 0.08 0.15 footsteps footsteps footsteps footsteps 24 footsteps footsteps 15 0.01 Maxi 0.4 2 1.5 0.04 0.03 1.5 31 3 0.5 30 2.5 Example 1 0.14 1.49 0.14 0.014 0,008 0.12 28.22 0.0051 0,038 16.7 0.59 Example 2 0.13 1.1 0.4 0,015 0,007 0.11 27.79 0.01 0,045 25.1 0.64

This alloy is a cast steel.

The coefficient of thermal expansion of Example 1, compared with thermal expansion coefficients of the three alloys of the state of the art (GX40CrNiSi25-20, GX40NiCrSi38-19, Ferrynox N29K) is shown in the single figure.

The coefficient of thermal expansion of the alloy having the composition of Example 1 is less than 12.6 * 10 ^-6 K ^-1 for a temperature below 980 ° C.

More generally, it is noted that the coefficient of thermal expansion of the alloy having the composition of Example 1 is significantly lower than the coefficient of expansion of the standardized alloys GX40CrNiSi25-20 and GX40NiCrSi38-19, and in almost the entire temperature range. which has been tested, especially for high temperatures.

Therefore, the alloy according to the invention has a low coefficient of expansion at temperatures between 400 and 1000 ° C, applied during the forming of parts made of titanium alloys, which is particularly interesting.

The creep resistance of the alloy according to the invention and having a composition according to Example 1 was also tested, and compared with those of the alloys GX40CrNiSi25-20 and GX40NiCrCrSi38-19 and the alloy known under the trade name Ferrynox N29K.

More specifically, and as a reminder, the Ferrynox N29K consists of a cast iron alloy with spheroidal graphite or lamellar graphite, with low expansion and subject of the patent application. FR 3 025 807 .
The creep resistance is as follows: Example 1 Ferrynox N29K GX40CrNiSi25-20 GX40NiCrSi38-19 Stress at break (MPa) Stress at break (MPa) Stress at break (MPa) Stress at break (MPa) 100h 1000h 100h 1000h 100h 1000h 100h 1000h at 700 ° C 85 68 70 52 100 80 80 at 800 ° C 62 46 49 33 75 50 90 50 at 900 ° C 39 25 21 14 47 28 48 30 at 980 ° C 21 14 12 8 28 16 28 17

The creep resistance of the steel alloy according to the invention having the contents according to Example 1 is much greater than that of the alloy called Ferrynox N29K.

This improved creep resistance is observed for the alloy of Example 1 on all the temperatures which have been tested and which are likely to be used during the manufacture of titanium parts, and either after 100h or after 1000h of exposure.

The improvement of the creep resistance is due to the presence, in the composition of the alloy according to the invention, of niobium, which causes formation at the grain boundaries of niobium carbides. Niobium carbides allow creep blockage by preventing grains from slipping relative to each other.

In addition, the 1000h rupture stress value of a part obtained from the alloy of Example 1, namely 25 MPa at 900 ° C., is comparable to the breaking stresses of the alloys GX40CrNiSi25-20 and GX40NiCrSi38 -19, respectively 28 and 30 MPa at 900 ° C, as indicated above. However, these alloys GX40CrNiSi25-20 and GX40NiCrSi38-19 are considered to have good resistance to creep at high temperature.

Another aspect of the alloy according to the invention is that it is weldable.

A second aspect of the invention is a part made of an alloy as defined above. The piece is notably a tool. The tool may comprise only portions of the alloy according to the invention or be entirely made of this alloy.

A third aspect of the invention is the method of manufacturing a part made of an alloy according to the invention.

First, the casting alloy is made with the compositions which have been described above. In other words, the various constituents are mixed in the contents which have been indicated.

Said alloy is then cast in a mold of shape and dimensions configured to allow the manufacture of the desired part, the piece consisting for example of a tool.

After the cooling step, the workpiece can be heat treated.

The alloy is in particular used for the manufacture of tools subsequently used for forming metal parts (for example titanium alloy).

The technical field can be aeronautics.

Claims (12)

1. A steel alloy, wherein it consists, in percentage by weight, of the following elements:

   - carbon (C) between 0.08% and 0.4%,

   - silicon (Si) between 0.15% and 2%,

   - nickel (Ni) between 24% and 31%,

   - cobalt (Co) between 15% and 30%,

   - niobium (Nb) between 0.01% and 2.5%,

   optionally molybdenum (Mo) at a content of less than or equal to 3% by weight and/or manganese (Mn) at a content of less than or equal to 1.5% by weight and/or chromium (Cr) at a content of less than or equal to 1,5% by weight and/or phosphorus (P) at a content of less than or equal to 0,04% by weight and/or sulfur (S) at a content of less than or equal to 0.03% by weight and/or copper (Cu) at a content of less than or equal to 0.5% by weight, the remainder being iron and unavoidable impurities.
2. The steel alloy according to claim 1, wherein the nickel (Ni) content is between minimum 25% or minimum 26% and maximum 30% or maximum 30.5% by weight.
3. The steel alloy according to claim 1 or 2, wherein the cobalt (Co) content is between minimum 16% and maximum 19% or maximum 27% by weight.
4. The steel alloy according to any one of claims 1 to 3, wherein the niobium content (Nb) is between minimum 0.05% and maximum 1.5% or maximum 2% by weight.
5. The steel alloy according to any one of claims 1 to 4, wherein the carbon content (C) is between minimum 0.1% or minimum 0.12% by weight and maximum 0.2 % or maximum 0.3% by weight.
6. The steel alloy according to any one of claims 1 to 5, wherein the silicon content (Si) is between minimum 0.2% and maximum 1% or maximum 1.5% by weight.
7. The steel alloy according to any one of claims 1 to 6, wherein the molybdenum content (Mo) is between minimum 0.05% or minimum 0.5% and maximum 1.2% or maximum 1.5% by weight.
8. The steel alloy according to any one of claims 1 to 7, wherein the manganese content (Mn) is between minimum 0.05% or minimum 0.1% and maximum 1% by weight.
9. The steel alloy according to any one of claims 1 to 8, wherein the copper (Cu) content is less than or equal to 0.4% by weight, preferably less than or equal to 0.3% by weight.
10. The steel alloy according to any one of claims 1 to 9, wherein the chromium (Cr) content is less than or equal to 1%, preferably less than or equal to 0.4%, preferably less than or equal to 0.3% by weight.
11. A part made at least partially of a steel alloy, wherein said alloy has a composition according to any one of claims 1 to 10.
12. Method for manufacturing a part according to claim 11, wherein it comprises at least the following steps:

    - an alloy is prepared in foundry, which has a composition according to any one of claims 1 to 10;

    - said alloy is poured into a mold in order to obtain said part.

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