RU2772883C1 - Iron-manganese alloy with improved weldability - Google Patents

Abstract

FIELD: metal alloys.

SUBSTANCE: invention relates to a ferromanganese alloy, which can be used for the manufacture of parts and welded assemblies operating under varying temperatures. The alloy contains, wt. %: 25.0 ≤ Mn ≤ 32.0, 7.0 ≤ Cr ≤ 14.0, 0 ≤ Ni ≤ 2.5, 0.05 ≤ N ≤ 0.30, 0.1 ≤ Si ≤ 0.5, optionally 0.010 ≤ rare earth elements ≤ 0.14, the rest is iron and residual elements obtained as a result of manufacture.

EFFECT: invention is aimed at developing an alloy having high dimensional stability under the influence of temperature changes.

9 cl, 2 tbl

Description

The present invention relates to an iron-manganese alloy intended for the manufacture of parts and weldments for applications that require high dimensional stability under the influence of temperature changes, in particular at cryogenic temperatures.

The alloy according to the invention is particularly intended for use in the electronics and cryogenic fields.

The alloys most commonly used for such applications are nickel-iron alloys, and in particular Invar® alloys, typically containing about 36% nickel. Such alloys have suitable dimensional stability, in particular at cryogenic temperatures, but have the disadvantage of a relatively high cost, due in particular to their relatively high nickel content. In addition, the weldability of these alloys with other metals does not always give satisfactory results, especially in terms of the mechanical strength of inhomogeneous welds.

Therefore, the present invention proposes an alloy suitable for the above applications, and therefore having appropriate properties, especially at cryogenic temperatures, while being less expensive than Invar®.

Iron-based alloys, also containing carbon and manganese, are sold by the Korean company Posco. These steels include by weight:

0.35% ≤ C ≤ 0.55%

22.0% ≤ Mn ≤ 26.0%

3.0% ≤ Cr ≤ 4.0%

0 ≤ Si ≤ 0.3%

the rest is iron and residual elements resulting from manufacture.

However, these alloys do not fully meet the requirements.

Although they are satisfactory in terms of thermal expansion coefficient and toughness at ambient and cryogenic temperatures (-196°C), the present inventors have noted that they exhibit high sensitivity to hot cracking and therefore have relatively poor weldability.

Also, the authors of the present invention additionally noticed that these steels have a high sensitivity to corrosion. However, good corrosion resistance is important for the aforementioned applications, in particular for thin strip, to limit the risks of fatigue or stress fracture of parts and structures made from these alloys. Therefore, these alloys are not entirely suitable for the above mentioned applications.

Thus, one of the objectives of the invention is to develop an alloy that can be used to make parts and weldments for applications that require high dimensional stability under the influence of temperature changes, for example, for cryogenic applications, while the alloy has a relatively low cost. .

To this end, the invention provides an iron-manganese alloy comprising, by weight:

25.0% ≤ Mn ≤ 32.0%

7.0% ≤ Cr ≤ 14.0%

0 ≤ Ni ≤ 2.5%

0.05% ≤ N ≤ 0.30%

0.1 ≤ Si ≤ 0.5%

optional 0.010% ≤ rare earths ≤ 0.14%

the rest is iron and residual elements resulting from manufacture.

In some specific implementations, the alloy of the present invention includes one or more of the following characteristics, taken alone or in any combination that is technically possible:

- the chromium content is 8.5 - 11.5 wt.%

- nickel content is 0.5 - 2.5 wt.%

- nitrogen content is 0.15 - 0.25 wt.%

- rare earth elements include one or more elements selected from: lanthanum, cerium, yttrium, praseodymium, neodymium, samarium and ytterbium,

- an iron-manganese alloy such as described above has an average coefficient of thermal expansion CTE from -180°C to 0°C, less than or equal to 8.5x10 ^-6 /°C,

- an iron-manganese alloy such as described above has a Néel temperature T _Néel greater than or equal to 40°C,

- an iron-manganese alloy such as described above, when manufactured in the form of a thin strip of 3 mm or less, has at least one of the following characteristics:

- impact strength KCV (sample with V-notch) of the sample for reduced tests with a thickness of 3 mm and at cryogenic temperature (-196°C), greater than or equal to 80 J/cm ² and, for example, greater than or equal to 100 J/ cm²,

- yield strength Rp _0.2 at -196°C greater than or equal to 700 MPa,

- yield strength Rp _0.2 at ambient temperature (20°C) greater than or equal to 300 MPa,

- iron-manganese alloy, such as described above, is austenitic at cryogenic temperatures and at ambient temperature.

The invention also relates to a method for manufacturing an alloy strip as defined above, the method comprising the following successive steps:

- preparation of an alloy such as previously defined;

- formation of a semi-finished product from the specified alloy;

- hot rolling of this semi-finished product to obtain a hot-rolled strip;

optionally, cold rolling the hot strip in one or more passes to obtain a cold strip.

The invention also relates to a strip made from an iron-manganese alloy such as previously defined.

The invention also relates to a method for manufacturing an iron-manganese alloy wire as defined above, the method comprising the following steps:

- preparation of a semi-finished product from an iron-manganese alloy;

- hot processing of a semi-finished product for the formation of an intermediate wire; and

- processing of the intermediate wire into a wire of a smaller diameter than the intermediate wire, and said processing includes the stage of wire drawing.

The invention also relates to a wire made from an iron-manganese alloy such as previously defined.

This wire, in particular, is a filler wire or a wire intended for the manufacture of bolts or screws, these bolts and screws being obtained, in particular, by cold heading of this wire.

The invention will be better understood by reading the following description, given by way of example only.

Throughout the description, the content is given in mass percent.

The alloy of the present invention is an iron-manganese alloy comprising, by weight:

25.0% ≤ Mn ≤ 32.0%

7.0% ≤ Cr ≤ 14.0%

0 ≤ Ni ≤ 2.5%

0.0% ≤ N ≤ 0.30%

0.1 ≤ Si ≤ 0.5%

optional 0.010% ≤ rare earths ≤ 0.14%

the rest is iron and residual elements resulting from manufacture.

Said alloy is an austenitic steel with a high manganese content.

The alloy according to the invention is austenitic at ambient temperature and at cryogenic temperature (-196°C).

Residual elements resulting from fabrication are elements that are contained in the raw material used to make the alloy or that come from the equipment used to make it, such as furnace refractories. These residual elements have no metallurgical effect on the alloy.

The residual elements specifically include one or more elements selected from the following: carbon (C), aluminum (Al), selenium (Se), sulfur (S), phosphorus (P), oxygen (O), cobalt (Co) , copper (Cu), molybdenum (Mo), tin (Sn), niobium (Nb), vanadium (V), titanium (Ti), and lead (Pb).

For each of the above residual elements, the maximum weight content is preferably selected as follows:

C ≤ 0.05 wt.% and preferably C ≤ 0.035 wt.%;

Al ≤ 0.02 wt.% and preferably Al ≤ 0.005 wt.%;

Se ≤ 0.02 wt.%, and preferably Se ≤ 0.01 wt.%, more preferably Se ≤ 0.005 wt.%;

S ≤ 0.005 wt.% and preferably S ≤ 0.001 wt.%;

P ≤ 0.04 wt.% and preferably P ≤ 0.02 wt.%;

O ≤ 0.005 wt.% and preferably O ≤ 0.002 wt.%;

Co, Cu, Mo ≤ 0.2 wt% each;

Sn, Nb, V, Ti ≤ 0.02 wt% each;

Pb ≤ 0.001 wt%

In particular, the selenium content is limited to the aforementioned ranges in order to prevent hot cracking problems that may arise from too high a selenium content in the alloy.

In particular, the alloy according to the invention has:

- average coefficient of thermal expansion CTE in the range from -180°C to 0°C, less than or equal to 8.5x10 ^-6 /°C; and

- a Néel temperature T _Néel greater than or equal to 40°C,

and when it is made in the form of a thin strip with a thickness of 3 mm or less;

- impact strength in KCV on a reduced test piece 3 mm thick and at cryogenic temperature (-196°C), greater than or equal to 80 J/cm ² and, for example, greater than or equal to 100 J/cm²;

- yield strength Rp _0.2 at -196°C greater than or equal to 700 MPa; and

- yield strength Rp _0.2 at ambient temperature (20°C) greater than or equal to 300 MPa.

Therefore, this alloy has thermal expansion, toughness and mechanical strength properties that are satisfactory for its use in the above applications, in particular at cryogenic temperatures.

The alloy of the present invention additionally has a corrosion resistance characterized by a critical corrosion current in the H ₂ SO ₄ (2 mol l ^-1 ) medium, strictly less than 230 mA/cm ² , and a pitting potential V in the NaCl medium (0.02 mol l ^-1 ) strictly above 40 mV, with the pitting potential relative to the standard potential of a standard hydrogen electrode (SHE). Therefore, the alloy of the present invention has a corrosion resistance greater than or equal to that of Invar®-M93. In this context, it should be noted that Invar®-M93 is a material commonly used in the above applications, in particular at cryogenic temperatures.

The alloy of the present invention also has corrosion resistance that is much higher than prior art Fe-Mn alloys, which have a critical H ₂ SO ₄ (2 mol l ^-1 ) corrosion current greater than about 350 mA/cm ² , and the pitting corrosion potential V is less than or equal to -200 mV relative to a standard hydrogen electrode (SHE).

In addition, the alloy of the present invention has satisfactory weldability and, in particular, suitable hot cracking resistance. As explained below, it gives a crack length of 7 mm or less in the Warestrain test at 3% plastic strain. As a result, the alloy of the present invention has much greater crack resistance than observed prior art Fe-Mn alloys.

More specifically, in the alloy according to the invention, manganese content of 32.0 wt.% or less allows you to obtain an average coefficient of thermal expansion below 8.5x10 ^-6 /°C at a temperature of -180°C to 0°C. This coefficient of thermal expansion is satisfactory for the use of the alloy in the intended applications and in particular for cryogenic applications.

In addition, a manganese content greater than or equal to 25.0 wt.%, with a chromium content less than or equal to 14.0 wt.%, allows you to obtain a suitable dimensional stability of the alloy at ambient temperature and cryogenic temperature (-196°C). In particular, the Néel temperature of the alloy in this case strictly exceeds 40° C., and there is no risk of reaching this point at normal use temperatures of the alloy. The use of an alloy at temperatures higher than the Néel temperature can cause severe deviations in the expansion of parts and assemblies welded at ambient temperature. The expansion coefficient of high manganese steel described above is in the range of 8x10 ^-6 /°C at temperatures below or equal to the Néel temperature, while it is in the range of 16x10 ^-6 /°C for temperatures above the Néel temperature.

Chromium in an amount equal to or less than 14.0 wt.%, allows to obtain a suitable impact strength KCV for reduced test on a sample with a thickness of 3 mm and at cryogenic temperature (-196 ° C), in particular, impact strength KCV at -196 °C is equal to or greater than 50 J/cm². On the contrary, the inventors have found that a chromium content strictly above 14.0 wt.%, can cause the alloy to be too brittle at cryogenic temperatures.

In addition, a chromium content greater than or equal to 7.0 wt.%, allows you to get a good weldability. The inventors have found that weldability tends to deteriorate when the chromium content is strictly less than 7.0 wt.% Chromium also contributes to the improvement of the corrosion resistance of the alloy.

Preferably the chromium content is 8.5 to 11.5% by weight. The chromium content in this range results in an appropriate compromise between high Néel temperature and high corrosion resistance.

The nickel content equal to or less than 2.5 wt.%, allows you to get an average coefficient of thermal expansion in the range from -180°C to 0°C, which is less than or equal to 8.5x10 ^-6 °/C. This coefficient of thermal expansion is satisfactory for the use of the alloy in the intended applications. On the contrary, the inventors have found that there is a risk of deterioration in the coefficient of thermal expansion at a nickel content strictly above 2.5 wt.%.

Preferably the nickel content is 0.5 to 2.5% by weight. A nickel content greater than or equal to 0.5 wt.% further improves the impact strength of the alloy at cryogenic temperature (-196°C).

Nitrogen content greater than or equal to 0.05 wt.% contributes to the improvement of corrosion resistance. However, its content is limited to 0.30 wt.% to maintain satisfactory weldability and toughness at cryogenic temperature (-196°C).

Preferably the nitrogen content is 0.15 to 0.25% by weight. The nitrogen content in this range allows an appropriate compromise between mechanical properties and corrosion resistance to be obtained.

Silicon, present in the alloy in an amount of 0.1 - 0.5 wt.%, acts in the alloy as a deoxidizer.

Optionally, the alloy contains rare earth elements in an amount of 0.010 - 0.14 wt.%. The rare earth elements are preferably selected from yttrium (Y), cerium (Ce), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm) and ytterbium (Yb), or mixtures with one or more of these elements. In one specific example, the rare earth elements include a mixture of cerium and lanthanum, or yttrium is used alone or in a mixture with cerium or lanthanum.

In particular, rare earth elements are represented by lanthanum and/or yttrium, while the sum of the content of lanthanum and yttrium is 0.010 - 0.14 wt.%.

Alternatively, rare earth elements are represented by cerium, and the cerium content is 0.010 - 0.14 wt.%.

Alternatively, the rare earth elements consist of a mixture of lanthanum, yttrium, neodymium and praseodymium, with the sum of the content of lanthanum, yttrium, neodymium and praseodymium being 0.010-0.14 wt%. In this case, rare earth elements are added, for example, in the form of a misch metal in an amount of 0.010 to 0.14 wt.%. Mischmetal contains lanthanum, yttrium, neodymium and praseodymium in the following proportions: Ce: 50%, La: 25%, Nd: 20% and Pr: 5%.

The presence of rare earth elements, and in particular mixtures of cerium and lanthanum or yttrium in the amounts indicated above, makes it possible to obtain an alloy having suitable hot cracking resistance and hence further improved weldability.

For example, the content of rare earth elements is 150 - 800 parts per million.

The alloy of the invention may be prepared using any suitable method known to those skilled in the art.

For example, it is produced in an electric arc furnace followed by ladle refining using conventional methods (decarburization, deoxidation and desulphurization), which in particular may include a reduced pressure step. Alternatively, the alloy of the present invention is produced in a vacuum furnace from raw materials with a low content of residual impurities.

Then hot-rolled or cold-rolled strip is made from the prepared alloy.

For example, the following method is used to produce hot rolled or cold rolled strip.

The alloy is cast in the form of semi-finished products such as ingots, remelted electrodes, slabs, in particular thin slabs with a thickness of less than 200 mm, obtained, in particular, by continuous casting, or blanks.

When the alloy is cast as remelted electrodes, they are preferably remelted in a vacuum or in an electrically conductive slag to produce cleaner and more uniform semi-finished products.

The semi-finished product thus obtained is subjected to hot rolling at a temperature of 950 to 1220°C to obtain a hot-rolled strip.

The thickness of the hot-rolled strip is in particular 2 to 6.5 mm.

In one embodiment, hot rolling is preceded by heat treatment with chemical homogenization at 950-1220° C. for 30 minutes to 24 hours. In particular, chemical homogenization of slabs, in particular thin slabs, is carried out.

The hot-rolled strip is cooled to ambient temperature to form a cold-rolled strip, and wound into coils.

Optionally, the cold rolled strip is then cold rolled to obtain a cold rolled strip having a final thickness of preferably 0.5 to 2 mm. Cold rolling is performed in one pass or in several successive passes.

The cold rolled strip is optionally subjected to recrystallization heat treatment in a constant temperature furnace for 10 minutes to several hours at a temperature above 700°C. Alternatively, it is subjected to recrystallization heat treatment in a continuous annealing furnace for a few seconds to about 1 minute, at a temperature above 900°C in the holding zone of the furnace and in a protective atmosphere of the type N ₂ /H ₂ (30%/70%) with a temperature freezing from -50°C to -15°C. The freezing point determines the partial pressure of the water vapor contained in the heat treatment atmosphere.

The recrystallization heat treatment can be carried out under the same cold rolling conditions to an intermediate thickness between the initial thickness (corresponding to the thickness of the hot rolled strip) and the final thickness. The intermediate thickness is chosen to be 1.5 mm, for example when the final thickness of the cold rolled strip is 0.7 mm.

The alloy preparation method and the production of hot-rolled and cold-rolled strip from this alloy are given as examples only.

All other methods for this purpose known to those skilled in the art can be used to obtain the alloy of the invention and to make end products from this alloy.

The invention also relates to strip, in particular to hot-rolled or cold-rolled strip made from an alloy such as described above.

In particular, the strip has a thickness of 6.5 mm or less, preferably 3 mm or less.

For example, said strip is a cold-rolled strip produced in accordance with the method described above, or a hot-rolled strip obtained after the hot rolling step of the method described above.

The invention also relates to a wire of the alloy described above.

In particular, the wire is a filler wire used for welding parts.

Alternatively, the wire is intended for the manufacture of bolts or screws, these bolts and screws being obtained, in particular, by cold heading of this wire.

For example, said wire is produced by a process comprising the following steps:

- obtaining a semi-finished product from an alloy, such as described above;

- hot processing of a semi-finished product for the formation of an intermediate wire; and

- processing the intermediate wire into a wire of a smaller diameter than the intermediate wire, and the processing includes the stage of drawing the wire.

In particular, the semi-finished product is an ingot or billet.

These semi-finished products are preferably formed by hot working at a temperature of 1050 - 1220°C to form the intermediate wire.

In particular, in this hot working step, semi-finished products, in particular ingots or billets, are hot worked to reduce the cross section, giving them a square cross section, for example with sides of about 100 to 200 mm. Thus, a semi-finished product with a reduced cross section is obtained. The length of this semi-finished product with a reduced cross section is 10 - 20 meters. Preferably, the reduction in the cross section of the semi-finished products is achieved by one or more successive hot rolling passes.

The reduced cross-section semi-finished products are then hot-worked again to produce wire. In particular, the wire may be a wire stock. For example, it has a diameter of 5 - 21 mm, in particular, the diameter is 5.5 mm. Preferably at this stage, the wire is produced by hot rolling on a bar mill.

Tests

The inventors conducted laboratory casting of alloys whose composition is defined above, and comparative alloys, the composition of which differs from the compositions described above.

These alloys are vacuum-prepared and hot-worked by rolling to a strip 35 mm wide and 4 mm thick.

This hot-rolled strip was then machined to obtain a dross-free surface.

The alloy composition of each of the tested strips is shown in Table 1 below.

The inventors conducted Warestrain tests with a strip obtained in accordance with the European standard FD CEN ISO/TR 17641-3 at a plastic deformation of 3.2% to evaluate hot crack resistance. The entire length of the crack formed during testing was measured and the strips were classified into three categories:

— a strip having a total crack length after the test of 2 mm or less is considered to have adequate hot cracking resistance;

— a strip having a total crack length after the test of 2 to 7 mm is considered to have suitable hot cracking resistance; and at the same time

— a strip having a total crack length strictly greater than 7 mm after the test is considered insufficiently resistant to hot cracking.

The results of these tests are shown in the "Warestripe Tests" column in Table 1 below. This column indicates the following:

- "1": strip with appropriate resistance to hot cracking;

- "2": strip with suitable resistance to hot cracking;

- "3": strip with insufficient resistance to hot cracking.

Hot crack resistance is an important aspect of alloy weldability, and weldability is better the higher the hot crack resistance.

The inventors also tested the corrosion resistance by conducting potentiometric tests. For this, the following tests were carried out:

- assessment of general corrosion by measuring the critical corrosion current of steel J _Mn in H ₂ SO ₄ (2 mol l ^-1 ) and comparing this current with the current measured for a strip of Invar®-M93 (J _{Invar M93} ~ 230 mA/cm ² );

- assessment of localized corrosion by measuring the pitting corrosion potential V in NaCl (0.02 mol l ^-1 ) and comparing this potential V with that of Invar®-M93 (V _{Invar M93} /E _SHE ~ 40 mV), where E _SHE represents the standard potential of the hydrogen electrode.

Recall that Invar®-M93 has the following composition in mass percent:

35% ≤ Ni ≤ 36.5%

0.2% ≤ Mn ≤ 0.4%

0.02 ≤ C ≤ 0.04%

0.15 ≤ Si ≤ 0.25%

not necessary

0 ≤ Co ≤ 20%

0 ≤ Ti ≤ 0.5%

0.01% ≤ Cr ≤ 0.5%

the rest is iron and residual elements resulting from manufacture.

If J _{Mn steel} <J _{Invar M93} and V _{Mn steel} /E _SHE > V _{Invar M93} /E _SHE , the tested steel is considered to be more corrosion resistant than Invar M93.

If J _{Mn steel} > J _{Invar M93} or V _{Mn steel} /E _SHE < V _{Invar M93} /E _SHE , the tested steel is considered less corrosion resistant than Invar®-M93.

The results of these tests are summarized in the Corrosion Resistance column in Table 1 below. In this column:

- the designation “> Invar” corresponds to the strip for which J _{Mn steel} < J _{Invar M93} and V _{Mn steel} /E _SHE > V _{Invar M93} /E _SHE ;

- the designation “<Invar” corresponds to the strip for which steel J _{Mn steel} > J _{Invar M93} or V _{Mn steel} /E _SHE < V _{Invar M93} /E _SHE ; and

- the designation “~ Invar” corresponds to the strip for which J _{Mn steel} ≈ J _{Invar M93} or V _{Mn steel} /E _SHE ≈ V _{Invar M93} /E _SHE .

The inventors also performed impact tests at -196°C on reduced test specimens (~3.5 mm thick) and measured the impact energy of the strip (denoted KCV) in accordance with NF EN ISO 148-1. The energy of destruction is expressed in J/cm². It characterizes the impact strength of the strip. The results of these tests are summarized in the "KCV at -196°C" column in Table 1 below.

The inventors also carried out dilatometric tests:

- from -180°C to 0°C to determine the average coefficient of thermal expansion of the alloy; and

- from 20°C to 500°C to determine the Néel temperature T _Néel of the alloy. The Néel temperature corresponds to the temperature above which an antiferromagnetic material becomes paramagnetic.

где

представляет изменение длины в микрометрах между 0°C и -180°C, L₀ представляет длину испытуемого образца при 0°C, T₀ составляет 0°C и T₁ составляет -180°C.More specifically, the average coefficient of thermal expansion is determined by measuring the change in length in micrometers at a temperature from -180°C to 0°C of a test piece having a length of 50 mm at 0°C. The average coefficient of thermal expansion is then calculated using the following formula:

where

represents the change in length in micrometers between 0°C and -180°C, L ₀ represents the length of the test piece at 0°C, T ₀ is 0°C and T ₁ is -180°C.

The Néel temperature is determined by measuring L(T), where L represents the length of the sample at temperature T, then the slope dL/dT is calculated. The Néel temperature corresponds to the temperature of the change in the slope of this curve.

The results of these tests are respectively shown in the columns "°CTE [-180°C to 0°C]°" and "T _Néel " in Table 1 below.

Finally, the inventors conducted a mechanical planar tensile test at -196°C to measure the yield strength at a relative elongation of 0.2% Rp _0.2 at -196°C. The results of these tests are summarized in the column headed "Rp _0.2 at -196°C" in Table 1 below.

In table 1 above "n.d." means that the value in question has not been determined.

The underlined tests correspond to the present invention.

In this table:

- for elements C, Al, Se, S, P, O "mini" means:

C<0.05 wt.%,

Al <0.02 wt.%,

Se<0.001 wt.%,

S<0.005 wt.%,

P<0.04 wt.%,

O <0.002 wt.%,

- elements marked "Other" include Co, Cu, Mo, Sn, Nb, V, Ti and Pb, and in this column "mini" means:

- Co, Cu, Mo <0.2 wt%,

- Sn, Nb, V, Ti <0.02 wt.% and

- Pb <0.001 wt%.

For nitrogen, "mini" means N<0.03 wt%. At this content, nitrogen is considered a residual element.

For rare earth elements, namely Ce, La and Y, "mini" means that the alloy contains no more than traces of these elements, preferably with a content of each of these elements of 1 ppm or less.

Test numbers 6, 8, 10, 12, 15-17, 19 and 20 are in accordance with the present invention.

It has been found that the strip prepared in these tests exhibits suitable and even superior hot cracking resistance (see Waretrain test column) and therefore has suitable weldability.

In addition, this band shows a corrosion resistance greater than or equal to that of Invar M93, an average coefficient of thermal expansion CTE from -180°C to 0°C is less than or equal to 8.5x10 ^-6 /°C, and a Néel temperature greater than or equal to 40 °C, KCV impact strength at -196°C is greater than or equal to 80 J/cm² and yield strength Rp _0.2 at -196°C is greater than or equal to 700 MPa.

Thus, the strip made from the alloy of the present invention exhibits satisfactory thermal expansion, toughness and mechanical strength properties for applications in applications that require high dimensional stability under the influence of temperature changes, in particular at cryogenic temperatures.

Alloys in tests under numbers 1 - 5 have a chromium content strictly less than 7.0 wt.%. It has been found that the corresponding strip has insufficient hot cracking resistance and therefore hardly satisfactory weldability. Tests 1 and 3 also show that this low hot crack resistance is not compensated by the addition of carbon even at a relatively high content.

The alloy in test 11 has a chromium content strictly above 14.0 wt%. It can be seen that the corresponding band exhibits high brittleness at cryogenic temperatures, resulting in a KCV resistance strictly less than 50 J/cm². It is also observed that this alloy has a Néel temperature strictly below 40°C.

The alloy in test No. 13 has a nickel content strictly above 2.5 wt.%. It is noted that the corresponding strip has an average coefficient of thermal expansion CTE between -180°C and 0°C, which is strictly above 8.5x10 ^-6 /°C.

Comparison of test results 7 and 8 shows that, other things being equal, increasing the nitrogen content improves corrosion resistance. The alloy in test number 9 has a nitrogen content strictly above 0.30 wt.%, and it can be seen that it shows degraded weldability and toughness KCV at -196°C.

In addition, as a comparison of tests 14 and 15 shows, a decrease in the manganese content, other things being equal, leads to a decrease in the Néel temperature.

It is also observed that the strip corresponding to tests 14, 17, 19 and 20, which contains rare earth elements in an amount of 0.010 - 0.14 wt.%, has a suitable resistance to hot cracking with a crack length of less than 2 mm. In contrast, the strip corresponding to tests 18 and 21 has a content of rare earth elements strictly above 0.14 wt.%, and such a strip was found to have poor weldability.

The mechanical strength of a homogeneous weld between two parts of an iron-manganese alloy of the present invention, or a non-uniform weld between a part of an iron-manganese alloy of the present invention and a part of another alloy, in particular 304L stainless steel and Invar® M93, was examined by tensile tests. These tests were carried out using the alloy of example 16 in Table 1 as an iron-manganese alloy.

More specifically, uniform welds are obtained by butt-welding two test rods taken from the strip of the iron-manganese alloy of Example 16 in Table 1. Non-uniform welds are also obtained by butt-welding a test rod taken from the strip of the alloy of Example 16 in Table 1 and the test rod , taken from an Invar® M93 strip, or a test rod taken from a 304L stainless steel strip.

For comparison, uniform welds are obtained by butt-welding two test rods taken from an Invar® M93 strip, and non-uniform welds by butt-welding a test rod taken from an Invar® M93 strip and a test rod taken from a 304L stainless steel strip.

The results are presented in table 2 below.

Table 2 Tensile Test Results

Butt weld type Example 16 - Example 16 Example 16 - 304L SS Example16-Invar M93 Invar M93 – Invar M93 304L SS-Invar M93 Mechanical strength Rm of assembly welded at 25°C (MPa) 615 475 425 410 330

Tensile tests were carried out at ambient temperature, as is usually the case for qualification tests of welds.

These tests show that the alloy of the invention has satisfactory weldability to stainless steel and Invar®

The alloy of the present invention can be successfully used in any application that requires suitable dimensional stability associated with suitable corrosion resistance and suitable weldability, in particular in the cryogenic range or in the field of electronics.

In view of their properties, the alloys of the present invention can advantageously be used to make welded assemblies for applications where high dimensional stability is required under conditions of temperature change, in particular at cryogenic temperatures.

Claims (23)

1. An alloy of iron-manganese, including by weight: 25.0 ≤ Mn ≤ 32.0%; 7.0 ≤ Cr ≤ 14.0%; 0 ≤ Ni ≤ 2.5%; 0.05 ≤ N ≤ 0.30%; 0.1 ≤ Si ≤ 0.5%; optional 0.010% ≤ rare earths ≤ 0.14%; the rest is iron and residual elements resulting from manufacture. 2. An alloy according to claim. 1, in which the chromium content is 8.5-11.5 wt.%. 3. Alloy according to claim. 1 or 2, in which the nickel content is 0.5-2.5 wt.%. 4. Alloy according to any one of paragraphs. 1-3, in which the nitrogen content is 0.15-0.25 wt.%. 5. Alloy according to any one of paragraphs. 1-4, in which the rare earth elements include one or more elements selected from: lanthanum (La), cerium (Ce), yttrium (Y), praseodymium (Pr), neodymium (Nd), samarium (Sm) and ytterbium (Yb ). 6. A method of manufacturing a strip of iron-manganese alloy according to any one of paragraphs. 1-5, including the following successive stages: - obtaining an alloy according to any one of paragraphs. 1-5; - formation of a semi-finished product from the specified alloy; - hot rolling this semi-finished product to obtain a hot-rolled strip; optionally cold rolling the hot strip in one or more passes to obtain a cold strip. 7. Strip of iron-manganese alloy according to any one of paragraphs. 1-5. 8. A method of manufacturing iron-manganese alloy wire according to any one of paragraphs. 1-5, including the following steps: - obtaining a semi-finished product from an iron-manganese alloy according to any one of paragraphs. 1-5; - hot processing of the semi-finished product to form the intermediate wire and - processing the intermediate wire into a wire of a smaller diameter than the intermediate wire, and said processing step includes a wire drawing step. 9. Wire alloy iron-manganese according to any one of paragraphs. 1-5.