Iron-manganese alloy with improved weldability

Abstract

To provide an iron-manganese alloy intended to be used for manufacturing a component and a welded assembly for use that requires to have high dimensional stability under temperature variation, in particular, at cryogenic temperature.SOLUTION: The present invention relates to an iron-manganese alloy that comprises, in weight percentages: 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 earths≤0.14%, the balance being iron and residual elements resulting from manufacturing.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to iron-manganese alloys intended to be used for manufacturing parts and welded assemblies for applications requiring high dimensional stability under the influence of temperature changes, particularly at cryogenic temperatures.

[0002] The alloys of the present invention are more particularly intended to be used in the field of electronics and cryogenic applications.

Background Art

[0003] The alloys most frequently used for such applications are generally nickel-iron alloys containing about 36% nickel, and more particularly Invar® alloys. Such alloys have excellent dimensional stability characteristics, particularly at cryogenic temperatures, but have the disadvantage of being relatively expensive as a result of their particularly high nickel content. In addition, the weldability of these alloys with other metals is not always entirely satisfactory, particularly with regard to the mechanical strength of the non-uniform welds.

[0004] Therefore, in the present invention, there is a search for an alloy that is suitable for the above applications, thus having good characteristics particularly at cryogenic temperatures, while being less costly than Invar®.

[0005] It is known that iron-based alloys containing carbon and manganese are commercially available from Posco in Korea. These steels contain, by weight: 0.35% ≦ C ≦ 0.55% 22.0% ≦ Mn ≦ 26.0% 3.0% ≦ Cr ≦ 4.0% 0 ≦ Si ≦ 0.3% and the balance is iron and residual elements resulting from the manufacturing process.

[0006] However, these alloys do not provide complete satisfaction.

[0007] They are satisfactory with respect to the coefficient of thermal expansion and toughness at room temperature and cryogenic temperatures (-196 °C). However, the inventors of the present invention have noted that they exhibit high sensitivity to high-temperature cracking and, therefore, relatively poor weldability.

[0008] In addition, the inventors of the present invention have also observed that these steels have high sensitivity to corrosion. For the above applications, especially for thin strips, better corrosion resistance is important to limit the risk of fatigue failure or stress rupture of parts and structures made of these alloys. Therefore, these alloys are not entirely satisfactory for the above applications. SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] Therefore, it is an object of the present invention to propose an alloy that can be used in a satisfactory manner for manufacturing parts and welded assemblies for applications that require high dimensional stability while having a relatively low cost, for example, for cryogenic applications, under the influence of temperature changes. MEANS FOR SOLVING THE PROBLEMS

[0010] For this purpose, the present invention relates to an iron-manganese alloy containing, 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% Optionally 0.010% ≤ rare earths ≤ 0.14% and the balance being iron and residual elements resulting from the manufacturing process. MODE FOR CARRYING OUT THE INVENTION

[0011] In some specific embodiments, the alloy of the present invention comprises one or more of the following characteristics obtained alone or in any technically possible combination: - The chromium content is between 8.5 and 11.5 wt%. - The nickel content is between 0.5 and 2.5 wt%. - The nitrogen content is between 0.15 and 0.25 wt%. - The rare earth contains one or more elements selected from lanthanum, cerium, yttrium, praseodymium, neodymium, samarium and ytterbium. - The iron-manganese alloy as described above has an average coefficient of thermal expansion CTE of 8.5×10 -6 / °C or less between -180°C and 0°C. - The iron-manganese alloy as described above has a Néel temperature T ネール of 40°C or higher. - When the iron-manganese alloy as described above is prepared as a thin strip with a thickness of 3 mm or less, it has at least one of the following characteristics: - At a temperature of liquid nitrogen (-196°C) and with a cold-rolled test specimen of 3 mm thickness, a KCV toughness of 80 J / cm 2 or more, for example 100 J / cm 2 or more; - A yield strength Rp 0.2 of 700 MPa or more at -196°C; - A yield strength Rp 0.2 of 300 MPa or more at room temperature (20°C). - The iron-manganese alloy as described above is austenitic at cryogenic temperatures and room temperature.

[0012] The present invention is a method for manufacturing a strip made of an alloy as previously defined, comprising the following successive steps: - A step of preparing an alloy as previously defined; - A step of forming a semi-finished product of the alloy; - A step of hot rolling this semi-finished product to obtain a hot-rolled strip; - Optionally, cold rolling the hot-rolled strip in one or more passes to obtain a cold-rolled strip also relates to a method comprising

[0013] The present invention also relates to a strip made from an iron-manganese alloy as previously defined

[0014] The present invention is a method for manufacturing a wire made from an iron-manganese alloy as previously defined, comprising the following steps: - Providing a semi-finished product made from an iron-manganese alloy; - Hot working this semi-finished product to form an intermediate wire; and - Processing the intermediate wire into a wire having a diameter smaller than that of the intermediate wire, comprising a step of drawing the wire also relates to a method comprising

[0015] The present invention also relates to a wire made from an iron-manganese alloy as previously defined

[0016] This wire is a wire particularly intended for the manufacture of welding wire or bolts or screws, and these bolts and screws are obtained in particular by cold forging this wire

[0017] The present invention will be more clearly understood by reading the following description given by way of example only

[0018] Throughout the description, the contents are given in weight percent

[0019] The alloy of the present invention, 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% Optionally 0.010% ≦ rare earths ≦ 0.14% and comprising The iron-manganese alloy has residues that are iron and residual elements resulting from production.

[0020] The alloy is a high-manganese austenitic steel.

[0021] The alloy of the present invention is austenitic at room temperature and at cryogenic temperatures (-196 °C).

[0022] Residual elements resulting from production mean elements contained in the raw materials used to prepare the alloy or elements derived from the equipment used for its preparation, such as furnace refractories. These residual elements have no metallurgical effect on the alloy.

[0023] The residual elements include, among others, one or more elements selected from 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).

[0024] For each of the residual elements listed above, the maximum content by weight 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 each ≤ 0.2 wt%; Sn, Nb, V, Ti each ≤ 0.02 wt%; Pb ≤ 0.001 wt%.

[0025] In particular, the selenium content is limited to the above range for the purpose of preventing the problem of high-temperature cracking that may result from an excessively high selenium content in the alloy.

[0026] In particular, the alloy of the present invention: - has an average coefficient of thermal expansion CTE of 8.5×10 -6 / °C or less between -180°C and 0°C; and - has a Néel temperature T ネール of 40°C or higher, when it is prepared as a thin strip with a thickness of 3 mm or less; - has a KCV toughness of 80 J / cm 2 or more, for example 100 J / cm 2 or more, at -196°C with a compressed test specimen 1 having a thickness of 3 mm; - has a yield strength Rp 0.2 of 700 MPa or more at -196°C; and - has a yield strength Rp 0.2 of 300 MPa or more at room temperature (20°C).

[0027] As a result, this alloy has satisfactory thermal expansion, toughness, and mechanical strength characteristics for the above applications, particularly for its use at cryogenic temperatures.

[0028] In addition, the alloy of the present invention has a corrosion resistance characterized by a critical corrosion current of less than exactly 230 mA / cm -1 in an H2SO4 medium (2 mol·l 2 -1), and a pitting potential V of more than exactly 40 mV determined with reference to the standard hydrogen electrode (SHE) in an NaCl medium (0.02 mol·l -1 -1). Therefore, the alloy of the present invention has a corrosion resistance greater than that of Invar®-M93. In this regard, it should be noted that Invar®-M93 is a material commonly used for the above applications, particularly at cryogenic temperatures.

[0029] The alloy of the present invention has an approximately 350 mA / cm -1 in an H2SO4 medium (2 mol·l 2It also has corrosion resistance far exceeding that observed in prior art Fe-Mn alloys having a critical corrosion current exceeding [value] and a pitting potential V of -200 mV or less with reference to the standard hydrogen electrode (SHE).

[0030] The alloy of the present invention further has satisfactory weldability and particularly good resistance to hot cracking. As will be explained below, it shows a crack length of 7 mm or less in the Varestraint test under 3% plastic strain. As a result, the alloy of the present invention has resistance to cracks much larger than those observed in prior art Fe-Mn alloys.

[0031] More particularly, in the alloy of the present invention, a manganese content of 32.0 wt% or less enables an average coefficient of thermal expansion of less than 8.5×10 -6 / °C to be obtained between -180°C and 0°C. This coefficient of thermal expansion is satisfactory for use of the alloy in the intended applications, particularly for cryogenic applications.

[0032] In addition, a manganese content of 25.0 wt% or more associated with a chromium content of 14.0 wt% or less enables good dimensional stability of the alloy to be obtained at room temperature and cryogenic temperatures (-196°C). In particular, in that case, the Néel temperature of the alloy strictly exceeds 40°C and there is no risk of reaching this point at the normal use temperature of the alloy. The use of the alloy at high temperatures above the Néel temperature risks causing a marked change in the expansion of parts and assemblies welded at room temperature. The coefficient of thermal expansion of the high-manganese steel described above is in the region of 8×10 -6 / °C at temperatures below the Néel temperature, but it is in the region of 16×10 -6 / °C at temperatures above the Néel temperature.

[0033] Chromium, at a content of 14.0 wt% or less, enables good KCV toughness to be obtained in a rolled test specimen 3 mm thick and at cryogenic temperatures (-196°C), and the KCV toughness at -196°C is particularly 50 J / cm 2The above is the case. On the contrary, the inventors have confirmed that a chromium content exceeding exactly 14.0% by weight has a risk of producing an alloy that is too brittle at extremely low temperatures.

[0034] In addition, chromium enables good weldability to be obtained at a content of 7.0% by weight or more. The inventors have found that the weldability tends to decrease at a chromium content of less than exactly 7.0% by weight. Chromium also contributes to the improvement of the alloy's resistance to corrosion.

[0035] Preferably, the chromium content is between 8.5 and 11.5% by weight. A chromium content within this range leads to a better balance between a high Curie temperature and high corrosion resistance.

[0036] Nickel enables an average coefficient of thermal expansion of 8.5×10 -6 / °C or less to be obtained between -180°C and 0°C at a content of 2.5% by weight or less. 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 a decrease in the coefficient of thermal expansion at a nickel content exceeding exactly 2.5% by weight.

[0037] Preferably, the nickel content is between 0.5 and 2.5% by weight. A nickel content of 0.5% by weight or more further improves the toughness of the alloy at extremely low temperatures (-196°C).

[0038] Nitrogen contributes to the improvement of corrosion resistance at a content of 0.05% by weight or more. However, its content is limited to 0.30% by weight in order to maintain satisfactory weldability and toughness at extremely low temperatures (-196°C).

[0039] Preferably, the nitrogen content is between 0.15 and 0.25% by weight. A nitrogen content within this range makes it possible to further improve the balance obtained between the mechanical properties and the corrosion resistance.

[0040] Silicon present in the alloy at a content between 0.1% and 0.5% by weight acts as a deoxidizer in the alloy.

[0041] Optionally, the alloy contains rare earths at a content between 0.010% and 0.14% by weight. The rare earths are preferably selected from yttrium (Y), cerium (Ce), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm) and ytterbium (Yb) or a mixture of one or more of these elements. In a particular example, the rare earths include a mixture of cerium and lanthanum, or yttrium used alone or in a mixture with cerium or lanthanum.

[0042] In particular, the rare earths consist of lanthanum and / or yttrium, and the total content of lanthanum and yttrium is between 0.010% and 0.14% by weight.

[0043] As a variant, the rare earths consist of cerium, and the cerium content is between 0.010% and 0.14% by weight.

[0044] As a variant, the rare earths consist of a mixture of lanthanum, yttrium, neodymium and praseodymium, and the total content of lanthanum, yttrium, neodymium and praseodymium is between 0.010% and 0.14% by weight. In this case, the rare earths are added, for example, in the form of mischmetal at a content between 0.010% and 0.14% by weight. Mischmetal contains lanthanum, yttrium, neodymium and praseodymium in the following ratios: Ce: 50%, La: 25%, Nd: 20% and Pr: 5%.

[0045] The presence of rare earths, more particularly the above-mentioned content of a mixture of cerium and lanthanum or yttrium, enables an alloy to be obtained that has very good resistance to hot cracking and, as a result, further improved weldability.

[0046] For example, the content of rare earths is between 150 ppm and 800 ppm.

[0047] The alloy of the present invention can be prepared using any suitable method known to those skilled in the art.

[0048] For example, it is prepared in an electric arc furnace and then refined using conventional methods (decarbonization, deoxidation, and desulfurization) which can include, in particular, the step of applying a reduced pressure. As a variant, the alloy of the present invention is prepared from raw materials with low residues in a vacuum furnace.

[0049] Next, hot-rolled or cold-rolled strips are produced from the prepared alloy.

[0050] For example, the following methods are used to produce the hot-rolled or cold-rolled strips.

[0051] The alloy is cast in the form of a semi-finished product such as an ingot, a remelted electrode, a slab, in particular a thin slab having a thickness of less than 200 mm obtained by continuous casting, or a billet.

[0052] When the alloy is cast in the form of remelted electrodes, these are preferably remelted under vacuum or with a conductive slag to obtain semi-finished products of higher purity and greater uniformity.

[0053] The semi-finished product thus obtained is hot-rolled at a temperature between 950 °C and 1220 °C to obtain a hot-rolled strip.

[0054] The thickness of the hot-rolled strip is, in particular, between 2 mm and 6.5 mm.

[0055] In one embodiment, the hot rolling is preceded by a chemical homogenization heat treatment at a temperature between 950 °C and 1220 °C for a time between 30 minutes and 24 hours. The chemical homogenization is carried out, in particular, on slabs, in particular thin slabs.

[0056] The hot-rolled strip is cooled to room temperature to form a cold-rolled strip and is wound into a coil.

[0057] Optionally, the cold-rolled strip is then cold-rolled to obtain a cold-rolled strip having a final thickness advantageously between 0.5 mm and 2 mm. The cold rolling is carried out in a single pass or in several successive passes.

[0058] At its final thickness, the cold-rolled strip is optionally subjected to a recrystallization heat treatment at a temperature above 700 °C for a time ranging from 10 minutes to several hours in a static furnace. As a variant, it is subjected to a recrystallization heat treatment in a continuous annealing furnace for a time ranging from a few seconds to about 1 minute, at a temperature above 900 °C in the soaking zone of the furnace, and at a frost point between -50 °C and -15 °C under a protective atmosphere of the N2 / H2 type (30% / 70%). The frost point defines the partial pressure of water vapor contained in the heat treatment atmosphere.

[0059] The recrystallization heat treatment can be carried out under the same conditions as for the cold rolling 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 selected, for example, to be 1.5 mm when the final thickness of the cold-rolled strip is 0.7 mm.

[0060] As a mere example, a method for preparing an alloy and manufacturing hot-rolled and cold-rolled strips from this alloy is shown.

[0061] All other methods known to those skilled in the art for this purpose can be used for preparing the alloy of the present invention and for manufacturing the final product from this alloy.

[0062] The present invention also relates to strips, in particular hot-rolled or cold-rolled strips made from alloys such as the alloy described above.

[0063] In particular, the strip has a thickness of 6.5 mm or less, and preferably 3 mm or less.

[0064] For example, the strip is a cold-rolled strip manufactured according to the above method, or a hot-rolled strip obtained after the hot-rolling step of the above method.

[0065] The present invention also relates to a wire made from the above alloy.

[0066] More particularly, the wire is a filler wire used to weld parts together.

[0067] As a variant, the wire is intended for the manufacture of bolts or screws, and these bolts and screws are obtained in particular by cold forging this wire.

[0068] For example, the wire is manufactured by implementing a method comprising the following steps: - preparing a semi-finished product with an alloy as described above; - hot working this semi-finished product to form an intermediate wire; and - processing the intermediate wire into a wire with a diameter smaller than that of the intermediate wire, including a step of drawing the wire.

[0069] In particular, the semi-finished product is an ingot or a billet.

[0070] These semi-finished products are preferably formed into intermediate wires by hot working between 1050 °C and 1220 °C.

[0071] In particular, in this hot working step, the semi-finished product, i.e., the ingot or the billet, is hot worked in particular to reduce the cross-section and give it, for example, a square cross-section with sides of about 100 mm to 200 mm. In this way, a semi-finished product with a reduced cross-section is obtained. The length of this semi-finished product with a reduced cross-section is in particular between 10 meters and 20 meters. Advantageously, the reduction of the cross-section of the semi-finished product is obtained by one or a plurality of successive hot rolling passes.

[0072] The semi-finished product with the cross-section reduced is then hot-worked again to obtain wire. The wire can in particular be wire rod. For example, it has a diameter between 5 mm and 21 mm, in particular a diameter of 5.5 mm. Advantageously, in this process, the wire is produced by hot rolling in a wire rod mill.

[0073] Test The inventors carried out laboratory castings of alloys having a composition as defined above and comparative alloys having a composition different from the above composition.

[0074] These alloys were prepared under vacuum and hot-worked by rolling to obtain strips with a width of 35 mm and a thickness of 4 mm.

[0075] This hot-rolled strip was then machined to obtain a scale-free surface.

[0076] The alloy composition of each of the tested strips is shown in Table 1 below.

[0077] The inventors carried out a ballast strain test on the obtained strips under 3.2% plastic strain in accordance with European standard FD CEN ISO / TR 17641-3 to evaluate the high-temperature cracking resistance. The inventors measured the total length of the cracks generated during the test and classified the strips into three categories: - Strips with a total crack length of 2 mm or less after the test were considered to exhibit excellent high-temperature cracking resistance; - Strips with a total crack length between 2 mm and 7 mm after the test were considered to exhibit good high-temperature cracking resistance; while - Strips with a total crack length strictly exceeding 7 mm after the test were considered to exhibit insufficient high-temperature cracking resistance.

[0078] The results of these tests are shown in the column headed "Ballast Strain Test" in Table 1 below. In this column, it is shown as follows: - «1»: Strips having excellent high-temperature cracking resistance; - <<2>>: Strip with good high-temperature cracking resistance; - <<3>>: Strip with insufficient high-temperature cracking resistance.

[0079] High-temperature cracking resistance is an important aspect of the weldability of alloys. The better the weldability, the greater the resistance to high-temperature cracking.

[0080] The inventors also tested the corrosion resistance by conducting a potential difference measurement test. For this purpose, the following tests were carried out: - Measurement of the critical corrosion current J in an H2SO4 medium (2 mol·l -1 ), and evaluation of general corrosion by comparison of this current with the current (J Mn鋼 ≈ 230 mA / cm Invar M93 ) measured for strips in Invar®-M93; 2 - Measuring the pitting potential V in an NaCl medium (0.02 mol·l ), and evaluation of localized corrosion by comparison of this potential V with the potential (V -1 / E Invar M93 ≈ 40 mV, where E SHE is the standard potential of the hydrogen electrode) measured for Invar®-M93. SHE

[0081] It is reconfirmed that Invar®-M93 has the following composition by weight percentage: 35% ≤ Ni ≤ 36.5% 0.2% ≤ Mn ≤ 0.4% 0.02 ≤ C ≤ 0.04% 0.15 ≤ Si ≤ 0.25% Optionally 0 ≤ Co ≤ 20% 0 ≤ Ti ≤ 0.5% 0.01% ≤ Cr ≤ 0.5% The balance is iron and residual elements resulting from manufacturing.

[0082] J Mn鋼 < J Invar M93 and V Mn鋼 / E SHE > V​Invar M93 / E SHE In the case of, the test steel is considered to be more corrosion-resistant than Invar M93.

[0083] J Mn鋼 >J Invar M93 or V Mn鋼 / E SHE <V Invar M93 / E SHE In the case of, the test steel is considered to have a corrosion resistance less than that of Invar (registered trademark)-M93.

[0084] The results of these tests are summarized in the column with the heading "≪Corrosion Resistance≫" in Table 1 below. In this column: - The notation "≫>Invar≪" corresponds to a strip where J Mn鋼 <J Invar M93 and V Mn鋼 / E SHE >V Invar M93 / E SHE ; - The notation "≪<Invar≪" corresponds to a strip where J Mn鋼 >J Invar M93 or V Mn鋼 / E SHE <V Invar M93 / E SHE ; and - The notation "≪~Invar≪" corresponds to a strip where J Mn鋼 ≒J Invar M93 or V Mn鋼 / E SHE ≒V Invar M93 / E SHE .

[0085] The inventors also carried out a toughness test on a test specimen (about 3.5 mm thick) compressed at -196°C and measured the impact fracture energy of the strip (denoted as KCV) in accordance with the standard NF EN ISO 148-1. The fracture energy is expressed in J / cm 2 . This explains the toughness of the strip. The results of these tests are summarized in the column with the heading "≪KCV at -196°C≫" in Table 1 below.

[0086] The inventors also conducted dilatometry tests: - Determine the average coefficient of thermal expansion of the alloy between -180 °C and 0 °C; and - Determine the Néel temperature T of the alloy between 20 °C and 500 °C ネール The Néel temperature corresponds to the temperature above which an antiferromagnetic material becomes paramagnetic.

[0087] More particularly, the average coefficient of thermal expansion is determined by measuring the change in length in micrometers of a test specimen with a length of 50 mm at 0 °C between -180 °C and 0 °C. Next, the average coefficient of thermal expansion is obtained by applying the following formula:

[0088]

Equation

[0089] (In the formula, L0 - L1 represents the change in length in micrometers between 0 °C and -180 °C, L0 represents the length of the test specimen at 0 °C, T0 is 0 °C, and T1 is -180 °C).

[0090] The Néel temperature is determined by measuring L(T) (where L is the length of the specimen at temperature T) and then calculating the gradient dL / dT. The Néel temperature corresponds to the temperature at which the change in the gradient of this curve occurs.

[0091] The results of these tests are shown in the columns headed ≪CTE[-180 °C to 0 °C]≫ and ≪T ネール ≫ respectively in Table 1 below.

[0092] Finally, the inventors conducted a mechanical flat tension test at -196 °C to measure the yield strength at 0.2% elongation Rp 0.2 at -196 °C. The results of these tests are summarized in the column headed ≪Rp at -196 °C 0.2 ≫ in Table 1 below.

[0093]

Table 1

[0094] In the above Table 1, ≪n.d.≫ means that the value under consideration has not been determined.

[0095] The tests marked with an underline are those that comply with the present invention.

[0096] In this table: - For the 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% means. - The elements marked as ≪Others≫ 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% means.

[0097] For nitrogen, ≪mini≫ means N < 0.03 wt%. With these contents, nitrogen is considered a residual element.

[0098] For rare earths, i.e., Ce, La, and Y, ≪mini≫ means that the alloy contains these elements in trace amounts or less, preferably, the content of each of these elements is 1 ppm or less.

[0099] The tests numbered 6, 8, 10, 12, 15 - 17, 19, and 20 comply with the present invention.

[0100] The strips prepared in these tests show good and even better high-temperature cracking resistance (see the column of the ballast train test), and therefore, it is confirmed that they have good weldability.

[0101] In addition, this strip has corrosion resistance higher than that of Invar M93, an average coefficient of thermal expansion CTE of 8.5×10 -6 / °C or less between -180°C and 0°C, a Néel temperature of 40°C or higher, a KCV toughness of 80 J / cm 2 or more at -196°C, and a yield strength Rp of 700 MPa or more at -196°C. 0.2 It shows.

[0102] Therefore, the strips made of the alloy of the present invention show satisfactory properties of thermal expansion, toughness and mechanical strength for their use in applications where high dimensional stability is required under the influence of temperature changes, especially at cryogenic temperatures.

[0103] The alloys in the tests numbered 1 to 5 have a chromium content of less than exactly 7.0% by weight. It can be seen that the corresponding strips have poor high-temperature cracking resistance and therefore almost unsatisfactory weldability. Tests 1 and 3 also show that this poor high-temperature cracking resistance is not even compensated by the addition of carbon at relatively high levels.

[0104] The alloy in Test 11 has a chromium content of more than exactly 14.0% by weight. The corresponding strip shows significant brittleness at cryogenic temperatures and can be seen to exhibit a KCV toughness of less than exactly 50 J / cm 2 It is also observed that this alloy has a Néel temperature of less than exactly 40°C.

[0105] The alloy in Test No. 13 has a nickel content of more than exactly 2.5% by weight. It is observed that the corresponding strip has an average coefficient of thermal expansion CTE of more than exactly 8.5×10 -6 / °C between -180°C and 0°C.

[0106] The comparison between Test 7 and Test 8 shows that, all other things being equal, an increase in nitrogen content enables improved corrosion resistance. The alloy in Test No. 9 has a nitrogen content strictly exceeding 0.30 wt%, and it is found to exhibit decreased weldability and KCV toughness at -196°C.

[0107] As also shown by the comparison between Test 14 and Test 15, a decrease in manganese content results in a decrease in the nailing temperature, all other things being equal.

[0108] It is also observed that the strips corresponding to Tests 14, 17, 19, and 20, which contain rare earths in a ratio between 0.010 and 0.14 wt%, have excellent high-temperature cracking resistance, and the crack length is less than 2 mm. In contrast, the strips corresponding to Tests 18 and 21 have a rare earth content strictly exceeding 0.14 wt%, and it can be seen that such strips have decreased weldability.

[0109] The mechanical strength of a homogeneous weld between two parts in the iron-manganese alloy of the present invention, or between a part in the iron-manganese alloy of the present invention and a part in a different alloy, particularly a heterogeneous weld between 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 the iron-manganese alloy.

[0110] More particularly, the homogeneous weld was obtained by taking two test bars from strips of the iron-manganese alloy of Example 16 in Table 1 (Table 1) and welding them end to end. The heterogeneous weld was also obtained by welding a test bar taken from a strip of the alloy of Example 16 in Table 1 (Table 1) to a test bar taken from a strip of Invar® M93 or a test bar taken from a strip of 304L stainless steel end to end.

[0111] For comparison, a uniform weld was obtained by welding together two test bars taken from strips of Invar® M93, and a non-uniform weld was obtained by welding end-to-end a test bar taken from a strip of Invar® M93 and a test bar taken from a strip of 304L stainless steel.

[0112] The results are shown in Table 2 below.

[0113]

Table 2

[0114] The tensile tests were carried out at room temperature as in a normal weld quality qualification test.

[0115] These tests show that the alloys of the present invention have satisfactory weldability with stainless steel and Invar®.

[0116] The alloys of the present invention can be advantageously used, particularly in the very low temperature range or in the field of electronics, in any application where good dimensional stability is required, with good corrosion resistance and good weldability.

[0117] Taking these properties into account, the alloys of the present invention are intended for applications where high dimensional stability is required, particularly at very low temperatures, under the influence of temperature changes, and can be advantageously used for manufacturing welded assemblies.

Claims

1. By weight: 25.0% ≤ Mn ≤ 32.0% 8.5% ≤ Cr ≤ 11.5% 0 ≤ Ni ≤ 2.5% 0.15% ≤ N ≤ 0.30% 0.1% ≤ Si ≤ 0.5% C ≤ 0.035% comprising an iron - manganese alloy, wherein the balance is iron and residual elements resulting from manufacture.

2. The alloy according to claim 1, wherein the nickel content is between 0.5 and 2.5% by weight.

3. The alloy according to claim 1 or 2, wherein the nitrogen content is between 0.15 and 0.25% by weight.

4. The alloy according to any one of claims 1 to 3, wherein the alloy further comprises 0.010% ≤ rare earths ≤ 0.14%.

5. The alloy according to claim 4, wherein the rare earths comprise one or more elements selected from lanthanum (La), cerium (Ce), yttrium (Y), praseodymium (Pr), neodymium (Nd), samarium (Sm), and ytterbium (Yb).

6. A method for manufacturing a strip made from the iron - manganese alloy according to any one of claims 1 to 5, comprising the following successive steps: preparing an alloy according to any one of claims 1 to 5; forming a semi - finished product of the alloy; hot - rolling the semi - finished product to obtain a hot - rolled strip; optionally, cold - rolling the hot - rolled strip in one or more passes to obtain a cold - rolled strip comprising.

7. A strip made from the iron - manganese alloy according to any one of claims 1 to 5.

8. A method for manufacturing a wire made from the iron - manganese alloy according to any one of claims 1 to 5, comprising the following steps: A step of preparing a semi-finished product made of an iron-manganese alloy according to any one of claims 1 to 5; A step of hot working this semi-finished product to form an intermediate wire; and A step of processing the intermediate wire into a wire having a diameter smaller than that of the intermediate wire, including a step of drawing the wire A method comprising: **Claim 9** A wire made of an iron-manganese alloy according to any one of claims 1 to 5.

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