Cunisn alloy after hardening by spinodal decomposition, with controlled density and cleanliness of grain boundaries, and method for preparing such an alloy

Abstract

The invention relates to a material based on a CuNiSn alloy with spinodal decomposition, which consists of: - nickel between 14.0 and 16.0%, - tin between 7.0 and 9%, - optionally at least one minor alloying element chosen from manganese, niobium and titanium; - the remainder being copper and unavoidable impurities, and the metallurgical structure of which has: - an absence of pores of which the largest dimension is greater than 20 µm at any point in the material volume of said material; - the presence of residual discontinuous precipitation, the volume of which represents less than 1% of the total material volume; - a grain size MC of less than 60 µm.

Description

Description Title of the invention: CuNiSn alloy after spinodal decomposition hardening, with controlled density and grain boundary cleanliness, and method for preparing such an alloy

[0001] The present invention relates to the field of alloys and, more particularly, to ternary alloys comprising copper (Cu), nickel (Ni) and tin (Sn), these alloys being denoted CuNiSn.

[0002] Such alloys can be produced in foundries by continuous casting and semi-continuous casting, and these alloys are characterized by their mode of structural hardening, with spinodal decomposition and ordered precipitation.

[0003] The present invention relates more particularly to products made from an alloy denoted CuNi15Sn8, which require very high mechanical strength, maximum corrosion resistance, and excellent resistance to wear by friction.

[0004] CuNil 5Sn8 products are intended for use in very demanding industries, such as aeronautics, mining, or petrochemicals.

[0005] Known for being used in particularly extreme conditions, namely, in corrosive environments and / or in the absence of lubrication and / or in the presence of high mechanical impact effects, the CuNi15Sn8 alloy must not only meet standards defining levels of characteristics or maximum acceptable defect size, but must also minimize the risk of failure of products made from this alloy when extreme conditions are reached, or even prolonged over time.

[0006] Currently, conformity checks for the CuNi15Sn8 alloy are carried out:

[0007] - for mechanical characteristics, on a test specimen made from the alloy in question, by a destructive test, for example by a tensile test required by the standards; however, such a test only characterizes the mechanical characteristics of the test specimen, and not of the product in service;

[0008] - for material defects, by non-destructive testing, using ultrasound for example, and which will be characterized by a threshold; below this threshold, the material defect is not considered critical even though it could indicate a risk of material failure under the extreme conditions indicated, and to which said material is likely to be subjected, for a more or less prolonged period.

[0009] The following description will identify potential causes that could initiate damage to a material obtained from a CuNi15Sn8 alloy.

[0010] The inventors then, in an inventive process, succeeded in developing a material from a CuNi15Sn8 alloy which reduces the risk of damage to said material when it is exposed to extreme conditions such as those mentioned above, in the applications envisaged for the alloy according to the present invention, and developed a process for obtaining said material.

[0011] As for CuNiSn alloys, these are defined by a mass concentration of Ni between 5 and 20%, and of Sn between 5 and 10%, the balance being copper, and the addition of minor elements such as Mn, Nb, Fe ...; they have been studied since the 1970s.

[0012] CuNiSn alloys exhibit a structural hardening mode, with spinodal decomposition, which allows them to have high mechanical characteristics, comparable to those of copper beryllium (CuBe) alloys, without presenting the environmental and health disadvantages linked to the presence of beryllium in the alloy.

[0013] Indeed, significant regulatory restrictions are currently applied to alloys containing copper and beryllium.

[0014] In addition to these mechanical characteristics, it also has good corrosion resistance in aggressive environments, such as marine, hydrogenated or sulfide environments, as well as good tribological properties, namely a low coefficient of friction, and resistance to abrasion.

[0015] In this CuNiSn alloy class, the CuNi15Sn8 family with a proportion of Ni between 14 and 16% and a proportion of Sn between 7 and 9% has been particularly developed to produce materials with a very high level of mechanical characteristics.

[0016] In the rest of the description the CuNi15Sn8 family will simply be referred to as “CuNi15Sn8”, without repeating the term “family” preceding it.

[0017] The level of mechanical properties depends on the state of the alloy, namely:

[0018] - the TX state (solution-quenched-tempered) or

[0019] - the TS state (solution-quenched-work-tempered).

[0020] Thus, for example, according to the AMS 4596 standard, for a TX alloy state, the yield strength at 0.2% (Re 0.2%) can exceed 738 MPa, and the elongation (A%) 9.5%.

[0021] The process of obtaining the material includes a melting / casting sequence, followed by a homogenizing heat treatment, then a hot transformation, followed by a solution heat treatment and quenching and a hardening heat treatment.

[0022] Another example, according to the AMS 4597 standard, for a TS state, the elastic limit at 0.2% (Re 0.2%) can exceed 1069 MPa, and the elongation (A%) of 6%.

[0023] In this case, the production process follows the sequence consisting of melting / casting, followed by a homogenizing heat treatment, a hot transformation, then a solution heat treatment and quenching, a cold transformation, and finally a hardening heat treatment.

[0024] Thus, the difference between the TX state and the TS state, as can be seen from the above, consists of the introduction of a cold transformation step between the two heat treatment steps, which greatly increases the elastic limit to 0.2% of the final material.

[0025] Regarding the CuNi15Sn8 melting process, US patent 6716292 describes a method of stirring the liquid metal during solidification in a continuous casting process. This stirring is achieved through multiple feed slots located between a feed tank and the cavity of a die connected to a cooler. The presence of these slots and their relationship to the die cavity impart significant movement to the liquid metal alloy entering the cavity, resulting in generally uniform temperatures within the liquid alloy material as it descends through the die to the so-called "quasi-solidification" zone.

[0026] Stirring the liquid metal during solidification, as proposed in US patent 6716292, on an alloy with a large solidification interval is very favorable to obtaining a homogeneous and relatively fine microstructure.

[0027] Indeed, the stirring action inside the mold will break the dendrites. Each broken dendrite arm will become a solidification nucleus and promote a fine-grained, equiaxed solidification structure.

[0028] Here, the vortex is only accelerated during the introduction of the molten metal into the mold, and it is preferable to avoid an excessively deep molten well, which would dampen convection at the bottom of the well in the most critical area. If there is a lack of equiaxed structure at the center, the expert will seek to reduce the depth of the molten well by lowering the pouring rate and increasing the waiting time between two pull steps.

[0029] This process, described in this patent, therefore makes it possible to obtain an equiaxed solidification structure, without the need for a hot transformation step as in the processes previously established in the state of the art.

[0030] That being said, the elimination of the hot transformation step is not possible in order to claim compliance with aeronautical standards for the TX and TS alloy states, unlike the CX states, the latter being obtained without a hot transformation step, and following the following sequence: melting / casting, solution heat treatment, hardening heat treatment, and do not present the level of mechanical characteristics required by the TX and TS standards.

[0031] However, the adjustment parameters of the different heat treatment and hot and cold transformation stages have been the subject of several patents or patent applications, in particular WO2014176357, EP4095276, US11643713, US10858723 to specify in the case of already established processes the influence of certain stages and their parameterization on the improvement of the performance of CuNi15Sn8 beyond the requirements of the AMS 4596 and 4597 standards.

[0032] The process described in US patent 6716292 is, however, interesting for compliance with AMS standards, because it allows, through mixing, the reduction of the discontinuous phase precipitation at grain boundaries during the final hardening treatment; if this precipitation is not controlled, it weakens the material, which loses its ability to deform (A%) and its resilience (impact resistance).

[0033] It appears from the phase diagram of a CuNi15 alloy with a variable proportion of Sn, shown in Figure 1 of the attached drawings, that during solidification there is a significant difference between the liquidus (appearance of the first solid) and the solidus (disappearance of the last liquid) and that, for the alloy with 8% Sn, it is normal to find the presence of phase y with a mass concentration greater than 30% Sn.

[0034] The fragile y phase must absolutely be reabsorbed during the homogenization treatment.

[0035] The finer and more equiaxed the structure obtained after solidification, the greater the grain boundary density, and the more finely the phase is distributed at the grain boundaries.

[0036] Grain boundary density corresponds to the surface area of ​​grain boundaries per unit volume of the material. Grain boundary density is controlled by the mean grain size (MC), which is the average size of individual crystalline regions with the same atomic configuration in a polycrystalline material. Grain size is defined by an average diameter observed under an optical microscope under conditions defined by international standards (ASTM E112 revision 2024, in particular, defining the "mean grain diameter d"), meaning that the smaller the MC, the higher the density.

[0037] This fine distribution will reduce the time of the homogenization treatment but also reduce micro-shrinkages and lacunar structures linked to Sn diffusion from excessively large "volumes" of phase y.

[0038] The mixing process described in US patent 6716292 makes it possible to create a favorable structure to control discontinuous precipitation.

[0039] However, discontinuous precipitation at grain boundaries is still likely to be present, and it is still necessary to propose a solution to favour ordered phase precipitation during decomposition hardening treatment, and thus improve grain boundary cleanliness.

[0040] Furthermore, the process has several drawbacks.

[0041] Indeed, it requires;

[0042] - a continuous vertical downward casting with the production of only the same CuNiSn alloy family; a change of family would require a complete draining of the installation, necessarily very costly.

[0043] - a significant cost of graphite tooling, because of its complex shape, and which wears out by reaction with the liquid metal (presence of nickel and oxygen which are very reactive with graphite); this tooling can, therefore, only be used once.

[0044] - a slow flow rate so that the entire liquid "well" can be impacted by the "vortex", therefore low production rates.

[0045] Regarding international patent application WO2014 / 176357, it describes a process for producing the CuNi15Sn8 alloy with particularly interesting characteristics; the yield strength can exceed 655 MPa, the elongation can exceed 15% and the resilience can exceed 40 J.

[0046] In this production process, a cold forming operation is introduced between the solution treatment and the hardening treatment. This cold forming is characterized by a reduction in cross-section of between 15 and 80%.

[0047] The melting and pouring stage implemented in this process is not detailed, but the steps that follow this melting and pouring are.

[0048] Thus, the homogenization treatment that follows pouring is carried out at a temperature above 760 °C, and takes place over a period of between 4 and 48 hours.

[0049] The hot processing is then carried out at a temperature between 700 and 900 °C, with preheating for the hot working which takes place for a period of at least 6 hours.

[0050] Next, the solution treatment is carried out at a temperature between 802 and 899 °C (1475°F to 1650°F), applied for a time of 0.5 to 6h, and is followed by a quench of less than 2 min, after the end of the temperature time of the solution treatment.

[0051] The international patent application published under number WO 2014 / 150880 discloses a CuNi15Sn8 alloy product with a homogeneous grain size, and the process for obtaining it.

[0052] This application recalls that the established and standardized rule for obtaining a homogeneous grain size is to start with a homogenization treatment, although this may promote cracking, which forms before and after solidification, linked to the mechanical stresses generated during solidification.

[0053] Therefore, this application proposes a process without homogenization treatment, with an adapted first hot transformation, with a foundry structure followed by a series of heat treatments, before carrying out a second hot transformation.

[0054] The first hot transformation is carried out after reheating at a low temperature between 590°C and 760°C for a long time between 10 and 14 hours.

[0055] This step is followed by a heat treatment very close to that of homogenization of patent WO2014 / 176357, since it is indicated at a preferential temperature of 870°C to 980°C with a very long time between 12 and 48h.

[0056] This step is followed by treatment at a temperature of 900 to 950°C for 4 to 6 hours before a second hot transformation. This third treatment can also be carried out as a cooling step during the second heat treatment.

[0057] This application also discloses a complete process that enables the achievement of a homogeneous grain size on a spinodal decomposition alloy.

[0058] The demand also explains the importance of a homogeneous microstructure. This promotes consistent characteristics throughout the product, which is particularly difficult to achieve in the CuNi15Sn8 alloy.

[0059] It is recalled that several metallurgical phases can coexist in spinodal alloys, in different ratios, depending on the temperature, and the phase transformations will evolve during the homogenization and hot deformation treatment steps, which will cause structural heterogeneity and cracking.

[0060] We understand, through this request, that it is "difficult" to avoid an absence of cracking and a heterogeneity of microstructure and that a rough with a homogeneous grain size and an absence of cracking is to be sought, before carrying out the treatments of solution setting and hardening by spinodal decomposition.

[0061] The term "spinodal alloy" or "spinodal alloy" to define the product subject to application WO 2014 / 150880 corresponds here to a product in the "rough" state which has not undergone hardening treatment, namely solution treatment and precipitation treatment.

[0062] This classification as "difficult" is linked to the complexity of the process to be implemented and the very long cumulative duration of the various high-temperature heat treatments. Indeed, this is a minimum of 22.5 hours, with a solution treatment duration of 0.5 hours, and if the third treatment is carried out during the cooling phase of the homogenization treatment, compared to the minimum duration of 10.5 hours in the previously cited international patent application WO2014 / 176357.

[0063] It is interesting to note that grain size homogeneity is defined in the range of 40 and 60 pm in the description, without breakage by cracking, and without homogenization treatment.

[0064] This beach is also claimed in the European version EP3461923B1 belonging to the same family as the international application WO 2014 / 150880.

[0065] However, no indication is given of advantageous characteristics of the product after the steps of solution preparation and hardening treatment by spinodal decomposition have been implemented.

[0066] In particular, it is not mentioned whether the grain size and its homogeneity are maintained after these final steps have been carried out.

[0067] Document EP 3085798 describes an alloy which contains 5 to 25% Ni, 5 to 10% Sn, 0.005 to 0.5% by mass of an element denoted A, which may be at least one element chosen from the group consisting of Nb, Zr and Ti. The alloy still contains more than 0.005% carbon, with a molar ratio of 10 or less between carbon and element A. This document suggests that when the alloy contains both carbon and nitrogen in appropriate quantities, nitrogen carbides form and create a blocking effect, allowing the crystalline grains to become finer, thus increasing elongation and tensile strength. Therefore, this alloy requires the addition of carbon to control grain size. However, many parameters will influence the carbon percentage and are likely to vary and be difficult to control, including the type of furnace or crucible material, the type and quantity of the molten metal covering material (such as graphite, coke, or carbon black), as well as the contact time, temperature, and surface area with the carbon.Furthermore, the carbides sought in this alloy have dimensions on the order of several micrometers, up to 20 pm, which remain likely to weaken the material finally obtained.

[0068] It therefore follows from the above that the production of CuNi15Sn8 alloy products must avoid the presence of numerous imperfections, among which are the segregation of tin-rich phases (interdendritic segregations and γ-phase), cracking, gaps, large grains, grain size heterogeneities, discontinuous precipitation (YDOS) from the grain boundaries of a phase, which weaken the material.

[0069] As a reminder, such imperfections are likely to cause damage to a product made from a CuNi15Sn8 alloy, particularly when it is intended for more or less prolonged use in extreme conditions such as those mentioned in the preamble to this description.

[0070] In order to verify compliance with requirements, and to avoid products that may be damaged in use, checks must be carried out on products made of CuNi15Sn8 alloy.

[0071] The control criteria imposed by the standards result from the levels of characteristics given by tensile tests, hardness and chemical composition measurement.

[0072] Mandatory steps in the process are sometimes specified, in particular the conditions for heat treatment.

[0073] In some standards, implicit requirements sometimes remind us that the product must be free from imperfections that are harmful to the use of the product and that an ultrasonic inspection may be required, but with a millimeter detection threshold.

[0074] For controls carried out by sampling, if the test specimen does not contain a defect that changes the macroscopic characteristic from the requirement, the product will be considered good.

[0075] Furthermore, it has been explained previously that the test allows the test specimen to be characterized, and not the product under its usual conditions of use.

[0076] For ultrasonic inspections, if the defect does not exceed the imposed threshold, the product will be considered good.

[0077] However, such a defect, even below the threshold, can generate a risk of failure of the final product under its conditions of use, which are, it should be remembered, often extreme, in the fields of application covered by the present invention.

[0078] To address these issues, the inventors succeeded in developing a material that meets the best performance standards for the TX (solution-quenched-tempered) and TS (solution-quenched, hardened, work-hardened, tempered) states, and which surprisingly exhibits new characteristics, namely:

[0079] - an absence of porosity with a dimension 10 to 100 times smaller than the millimeter thresholds imposed by ultrasonic testing;

[0080] - an absence, or a presence below a threshold considered to be very low, of discontinuous precipitated phase at grain boundaries;

[0081] - a calibrated grain size, in increments of 20 pm.

[0082] The inventors have also developed a process that makes it possible to obtain such innovative characteristics.

[0083] Furthermore, the inventors have succeeded in demonstrating that the innovative characteristics obtained at the end of the process are maintained on the material after the final structural hardening stage.

[0084] Note that, in the description, "alloy" refers to a chemical formula that characterizes the chemical composition.

[0085] The term "material" refers to a state of matter for a given alloy, characterized, in particular, by mechanical and / or electrical and / or physical properties, and / or by its metallurgical structure.

[0086] The term "product" refers to a material having a specific geometric shape for a given use.

[0087] Finally, in the context of the present invention, a "non-round section" is understood to mean a section of the bar (and the ingot mold) of any shape, except for a round section, i.e., a circular / circle-shaped section, that is, one exhibiting rotational symmetry. A non-round section therefore corresponds to a non-circular section. In other words, strictly round or circular sections are excluded; included are sections considered to lack an axis of rotational symmetry (for example, rectangular / quadrilateral, square, polygonal, non-circular ellipse sections, etc.).

[0088] To this end, and considering the definitions given above, the present invention proposes a material based on a CuNiSn alloy with spinodal decomposition, with controlled density and grain boundary cleanliness, based on copper (Cu), nickel (Ni) and tin (Sn), said alloy being composed of, in % by mass relative to the total mass of the alloy:

[0089] - nickel (Ni) in a proportion between 14.0 and 16.0%

[0090] - tin (Sn) in a proportion between 7.0 and 9%

[0091] - optionally at least one minor alloying element chosen from at least manganese (Mn), niobium (Nb) and titanium (Ti); when the alloy contains such alloying elements, Mn is preferably present in a proportion of between 0.1 and 0.3%, Nb is advantageously present in a proportion of between 0.04 and 0.09%, and Ti is added most preferably in a proportion of between 0.002 and 0.07%.

[0092] - the remainder of the alloy being copper and unavoidable impurities, the proportion of unavoidable impurities being less than 0.5% by mass,

[0093] said material being characterized by:

[0094] - an absence of porosity with a dimension greater than 20 pm at any point in the volume of material of said alloy material;

[0095] - the presence of residual discontinuous precipitation below a very low defined threshold, the proportion of the total volume of discontinuous precipitation representing less than 1% of the total volume of material of said alloy material;

[0096] - a MC grain size of less than 60 pm.

[0097] The maximum dimension of the pores is determined by making observations on a polished sample of the material, using an optical microscope.

[0098] The total volume of residual discontinuous precipitation is obtained by measuring the proportion of this precipitation over two-dimensional fields. An extrapolation is then performed from several of these 2D fields to obtain a 3D proportion, and this is then converted back to a 3D proportion, neglecting the anisotropy effect. This method is notably used and defined in the publication by Yi OUYANG, Xue-ping GAN, Shi-zhong ZHANG, Zhou LI, Ke-chao ZHOU, Ye-xin JIANG, Xian-wei ZHANG, "Age-hardening behavior and microstructure of Cu-15Ni-8Sn-0.3Nb alloy prepared by powder metallurgy and hot extrusion," *Transactions of Nonferrous Metals Society of China*, Volume 27, Issue 9, 2017, pages 1947-1955.

[0099] The MC grain size of copper alloys measures the average diameter of the grains, that is, the individual crystalline regions with the same atomic configuration in a polycrystalline material that constitutes the alloy material. Grain size is measured according to the standardized method ASTM E112 revision 2024 "Standard Test Methods for Determining Average Grain Size".

[0100] According to specific embodiments of the present alloy:

[0101] - the MC grain size is between 20 and 40 pm;

[0102] - the MC grain size is less than 20 pm;

[0103] - the material has a yield strength at 0.2% (Re) greater than 620 MPa and an elongation greater than 6%;

[0104] - the material has a yield strength at 0.2% (Re) greater than 755 MPa and an elongation greater than 10%;

[0105] - the material has a yield strength at 0.2% (Re) greater than 650 MPa and an elongation greater than 1%;

[0106] - the material has a yield strength at 0.2% (Re) greater than 1020 MPa and an elongation greater than 3%.

[0107] The mechanical characteristics of the material, namely the yield strength at 0.2% (Re) and the elongation (A%), are measured according to ASTM E8.

[0108] The present invention also relates to a method for manufacturing a CuNiSn alloy material as described above, from a composition comprising nickel (Ni) in a proportion of between 14.0 and 16.0%, tin (Sn) in a proportion of between 7.0 and 9%, optionally at least one minor alloying element selected from at least manganese (Mn), leniobium (Nb), titanium (Ti), the remainder of the alloy being copper and unavoidable impurities, the proportion of unavoidable impurities being less than 0.5% by mass, said method comprising at least the following steps, taken in order:

[0109] 1) melting of the different constituents of the composition of said alloy and continuous vertical pouring through an ingot mold having a non-round section, allowing to obtain a bar also having a non-round section;

[0110] 2) application, to said bar having a non-round cross-section, at the outlet of a continuous vertical casting, of a homogenization treatment;

[0111] 3) application, to the said non-round section bar after homogenization, of a first hot transformation and obtaining a hot-transformed bar;

[0112] 4) application, on said hot-processed bar, of a second hot-processing;

[0113] 5) carrying out a solution treatment on said doubly hot-processed bar and obtaining a rough product;

[0114] 6) application of a spinodal decomposition hardening heat treatment on said rough product and obtaining the CuNiSn alloy material whose metallurgical structure, on the one hand, is devoid of porosity whose largest dimension is greater than 20 pm at any point in the volume of said alloy, on the other hand, has residual discontinuous precipitation below a very low defined threshold, the proportion of the total volume of discontinuous precipitation representing less than 1% of the total volume of material and, finally, has a grain size MC less than 60 pm.

[0115] According to preferred features of the present method of the invention:

[0116] - said process includes a step 5') of cold deformation following the solution treatment step;

[0117] - during step 1) of continuous vertical melting and casting, a first indirect cooling is applied at the level of the mold, followed by direct cooling at the mold outlet, during which the temperature of the external surface of the non-round section bar decreases, preferably continuously from the mold outlet to an external surface temperature less than or equal to 300 °C;

[0118] - step 2) of heat treatment for homogenization is carried out at a temperature above 870 °C, for a duration of more than 5 hours;

[0119] - step 3) of first hot transformation is carried out by forging, blooming, hammering, rolling, extrusion, at a temperature above 800 °C and for a holding time of more than 8h, including the duration of the homogenization heat treatment of step 2), and with a reduction of cross-section to more than 1 / 3 compared to the initial cross-section;

[0120] - step 4) of second hot transformation, known as "near to shape" or "close to the final shape", is carried out at a temperature of 850 to 900 °C, with a preheating time before step 4) of second hot transformation for a duration greater than or equal to 2h, to obtain a grain size MC between 40 and 60 pm;

[0121] - step 4) of second hot transformation, called "near to shape" or "close to the final shape", is carried out at a temperature of 800 to 850 °C, with a preheating time before step 4) of second hot transformation for a duration greater than or equal to 2h, to obtain a grain size MC between 20 and 40 pm;

[0122] - step 4) of second hot transformation, called "near to shape" or "close to the final shape", is carried out at a temperature between 750 and 800 °C, with a preheating time before step 4) of second hot transformation for a duration greater than or equal to 2h, to obtain a grain size MC less than 20 pm;

[0123] - step 5) of the solution treatment is carried out in a static furnace at a temperature between 780 and 795 °C, for a holding time at this temperature of between 1 and 2 h, before a quench carried out by immersion in water of the double hot transformed and solution-treated bar, with a maximum time of 2 min between the moment when said bar is taken out of the furnace and the end of its immersion in water;

[0124] - when the process does not include step 5') of cold deformation, step 6) of heat treatment by spinodal decomposition is carried out at a temperature between 390 and 425 °C for a duration of between 1 and 6 h, preferably for a duration of between 1 and 2 h in a first example of embodiment, and preferably for a duration of between 3 and 6 h in a second example of embodiment;

[0125] - when the process includes a cold deformation step 5'), the spinodal decomposition heat treatment step 6) is carried out at a temperature between 320 and 400 °C for a duration between 1 and 6 h;

[0126] The present invention also relates to a vertical continuous casting installation for implementing step 1) of vertical continuous casting of the process for obtaining a material based on a spinodal decomposition CuNiSn alloy of the invention, said installation being designed to allow the formation of a casting bar from the molten CuNiSn alloy and comprising, at its upper part, through which the molten alloy is introduced from a feed hopper located upstream of said installation, a bottomless ingot mold of section S1 and height h, for the production of said casting bar of section S2 and length LB greater than h, said ingot mold being associated with first cooling means to form an indirect cooling zone of the molten alloy in the upper part of said installation.

[0127] This latter design is particular in that it also includes, downstream of the said direct cooling zone, a direct cooling zone in which the outer surface of the said casting bar is in direct contact with a coolant, for example, water, and in that section S1 of the said mold, and consequently section S2 of the said casting bar, consists of a quadrilateral with right angles, preferably rounded with a large radius, and with flat or, preferably, convex faces, the ratio of the lengths of two adjacent faces of the quadrilateral being advantageously, but not limited to, different from one. The radius of the angles of the quadrilateral is advantageously between 10 and 50 mm.

[0128] Other objects and advantages of the present invention will become apparent during the following description relating to embodiments which are given only as indicative and non-limiting examples.

[0129] Understanding this description will be facilitated by referring to the attached drawings, in which:

[0130] [Fig. 1] corresponds to a calculated 15% nickel isopleth in the copper-rich corner of the Cu-Ni-Sn system, illustrating the different metallurgical phases of the CuNi15 alloy as a function of the Sn content and temperature. The diagram is from Zhao, J.-C., and M.R. Notis. “Spinodal Decomposition, Ordering Transformation, and Discontinuous Precipitation in a Cu-15Ni-8Sn Alloy.” Acta Materialia 46, no. 12 (July 1998): 4203–18.

[0131] [Fig. 2] schematically and in cross-section represents a vertical continuous casting installation for implementing the first step of the process of the present invention

[0132] [Fig.3] schematically illustrates the different zones found within a cylindrical cross-section material (in this case a foundry bar) obtained by continuous casting of a CuNi15Sn8 alloy, with a diameter of 300 mm or more, when cooling is carried out only indirectly (i.e. differently compared with the present invention), such indirect cooling resulting in the formation of three zones, each with a particular structure.

[0133] The present invention relates to a material based on a spinodal decomposition CuNiSn alloy, with controlled density and grain boundary cleanliness, based on copper (Cu), nickel (Ni), and tin (Sn), said alloy being composed of, in % by mass:

[0134] - nickel (Ni) in a proportion between 14.0 and 16.0%

[0135] - tin (Sn) in a proportion between 7.0 and 9%

[0136] - optionally at least one minor addition element chosen from at least manganese (Mn), niobium (Nb) and titanium (Ti);

[0137] - the remainder of the alloy being copper and unavoidable impurities, the proportion of unavoidable impurities being less than 0.5% by mass, in other words impurities to be avoided whose sum of proportions cannot exceed 0.5%;

[0138] the metallurgical structure of said material being characterized by:

[0139] - an absence of porosity with a dimension greater than 20 pm at any point in the volume of matter of said material;

[0140] - the presence of residual discontinuous precipitation below a very low defined threshold, the share of the total volume of discontinuous precipitation representing less than 1% of the total volume of matter.

[0141] - a MC grain size of less than 60 pm.

[0142] Advantageously, the metallurgical structure of said material is also free of inclusions (oxides, sulfides, carbides, etc.) with a size greater than one micrometer.

[0143] Advantageously, the metallurgical structure of the material has a MC grain size of less than 60 pm, with a homogeneous grain size, the difference between the maximum and minimum values ​​of which is less than 20 pm.

[0144] Preferably, with regard to the minor addition elements present in the composition of the alloy constituting the material of the invention, Mn is present in a proportion between 0.1 and 0.3%, and / or Nb in a proportion between 0.04 and 0.09%, and / or titanium Ti in a proportion between 0.002 and 0.07%.

[0145] The alloy composition is preferentially made up of these three addition elements, Mn, Nb and Ti, in addition to the main elements Cu, Ni and Sn.

[0146] Returning now to the dimensions of the porosities, corresponding to empty spaces of material indicated above, namely that these have a size of less than 20 pm at any point in the material of the alloy of the invention, are understood to be the largest dimension of said porosities.

[0147] Generally, these porosities have an overall spherical or elliptical shape, so that, in the alloy according to the present invention, their diameter or their greatest axis has a dimension of less than 20 pm.

[0148] Thus, the alloy of the present invention has the characteristic that a very low residual porosity can indeed be present, however, the maximum dimension of the pores is, at any point in the volume of the material, less than 20 pm.

[0149] The measurement of the size of residual pores in the material of the invention is carried out by optical microscopy on a polished section of said material, advantageously with a magnification of 1000x. More specifically, the average diameter is measured in the case of spherical pores, or the largest dimension in the case of elliptical pores or any other shape. The measurements are preferably carried out on at least three fields.

[0150] It should also be noted that, preferably, the total volume of all these residual porosities, in other words the sum of the volume of each of these porosities, represents less than 0.2% of the total volume of the alloy material.

[0151] The volume of these porosities can be determined in the same way as the volume of discontinuous precipitation is determined (see above and below), according to the method described in Ouyang et al. (2017).

[0152] Indeed, the alloy of the present invention also exhibits residual discontinuous precipitation at grain boundaries but below a threshold considered to be very low, namely that the total volume of these discontinuous precipitations, in other words the sum of the volume of each of these discontinuous precipitations, must represent less than 1% of the total volume of the alloy material.

[0153] The volume of discontinuous precipitation can be determined, in particular, by optical microscopy and image analysis of polished sections after etching the polished sections with a metallographic etching reagent, by measuring the proportion of this precipitation in two-dimensional fields. An extrapolation is then performed from several of these 2D fields to obtain a 3D proportion, and this is then scaled back to a 3D proportion, neglecting the effect of anisotropy. This method is notably used and defined in the publication by Ouyang et al., "Age-hardening behavior and microstructure of Cu-15Ni-8Sn-0.3Nb alloy prepared by powder metallurgy and hot extrusion," Transactions of the Nonferrous Metals Society of China, Volume 27, Issue 9, 2017, pages 1947-1955.

[0154] As previously stated, the volume of the pores can possibly be determined in the same way as that used for the determination of the volume of discontinuous precipitation, except that the polished sections analyzed do not undergo attack by a metallographic etching reagent when determining the volume of the pores.

[0155] The MC grain size measures the average diameter of the grains, that is, individual crystalline zones having the same atomic configuration in a polycrystalline material that constitutes the alloy material, these grains having a dimension or size less than 60 pm, is understood to be the average diameter of said grains.

[0156] The MC grain size, which measures the average diameter of the grains, is measured according to the standardized method ASTM E112 revision 2024 "Standard Test Methods for Determining Average Grain Size >>.

[0157] Preferably, the size of MC grains is between 20 and 40 pm.

[0158] In another embodiment of the alloy, which is equally advantageous, the MC grain size is less than 20 pm.

[0159] In view of all the characteristics of the alloy material of the present invention which have been mentioned above, it is possible to describe said material as having a density and grain boundary cleanliness which are scrupulously controlled.

[0160] Grain boundary cleanliness can be considered as an absence of inclusions (i.e. oxides, sulfides, carbides, etc.) larger than a micrometer, an absence of tin-rich phase residues (segregations), an absence of discontinuous precipitation at grain boundaries (i.e. below a threshold considered to be very low), which correspond to embrittling precipitated phases.

[0161] Thus, in summary, an absence or a very low defined threshold of discontinuous precipitated phase is a marker of clean grain boundary, which characterizes the alloy material of the present invention.

[0162] Similarly, the absence of inclusions (i.e. oxides, sulfides, carbides, etc.) with a size greater than 1 pm is characteristic of a clean joint found in the alloy material of the invention.

[0163] On the contrary, an unfavorable discontinuous precipitation begins at grain boundaries, usually during the spinodal decomposition hardening treatment, and propagates through the matrix of the alloy material over time.

[0164] With the attainment of a level of mechanical characteristics considered high which is sought with the material of the invention, the absence of discontinuous precipitation at grain boundaries is also sought, and shows an absence of germination points (the tin-rich phases) and a slow growth kinetics of these, with a maintenance of the constraints therefore an absence of large precipitates.

[0165] As will be explained and demonstrated in the following description, the particular characteristics of the CuNi15Sn8 alloy of the invention, or a material obtained from this CuNi15Sn8 alloy, namely the MC grain size, the presence of residual porosities but of considerably reduced size compared to existing ones, ten to one hundred times smaller than the thresholds imposed by ultrasonic testing, and a very low proportion of discontinuous precipitation at grain boundaries, are linked to the implementation conditions of certain particular steps of the production process of said material according to the invention, which will be described below.

[0166] Indeed, the present invention also relates to a method for manufacturing a material based on a CuNi15Sn8 alloy, as described above, namely having the aforementioned composition and the particular metallurgical characteristics indicated, said manufacturing method comprising, at least, the following steps, taken in order:

[0167] 1) a first step of melting the different constituents composing the CuNi15Sn8 alloy in the proportions indicated above, then of continuous vertical casting, which is carried out by means of a rapid process, which will be detailed later, and by means of simplified tooling, which has the advantage of being able to be reused many times, and allowing in particular to obtain a casting bar in CuNi15Sn8 alloy (the product) and whose material is characterized by a porosity considered to be particularly low; for the record, the presence of a very low residual porosity means, in the context of this application, that the maximum dimension of the residual porosities is, at any point in the volume of the material, less than 20 pm;

[0168] 2) a homogenization treatment on the product (bar) obtained following the vertical continuous casting step, in order to absorb the y phase and the interdendritic segregations, both rich in tin Sn, said homogenization treatment being in particular favored by the shape of the product obtained during the previous casting step; note that this particular shape will be described and detailed later in the description;

[0169] 3) a first hot transformation step on the homogenized product, introduced in the particular process according to the present invention, and which will have the effect, in a particularly advantageous way, of allowing a structural recrystallization which leads to a suppression of the foundry dendritic structure in favor of a homogeneous microstructure of smaller size; such a step also makes it possible to constitute a preliminary operation to the final shaping of the product;

[0170] 4) a second hot transformation step, which is carried out at a temperature adjusted according to the solution treatment carried out in the following step; such a step has the effect of fixing the grain size of the final product, after the final treatment by spinodal decomposition, in increments of 20 pm;

[0171] 5) a specific solution treatment, with defined parameters, leading to a controlled microstructure that slows down the discontinuous precipitation kinetics, and

[0172] 6) a hardening treatment by spinodal decomposition allowing the obtaining of a final material in CuNi15Sn8 alloy whose metallurgical state presents a residual discontinuous precipitation, below a very low defined threshold, as already defined previously in the description.

[0173] The sequence of these steps allows us to obtain a CuNi15Sn8 alloy material in the TX state (solution-quenched-tempered, for the record).

[0174] It is also possible to obtain an alloy material in the TS state (solution-quenched-work-tempered) by implementing the steps defined in general above, and by adding an additional step, noted as step 5') after the solution treatment step, and before the structural hardening (tempering) treatment of step 6).

[0175] All steps before the additional step 5') are unchanged, only the last step 6), after the additional step will need to be adapted and will be different from that proposed for the TX state.

[0176] Such a step 5') consists of a cold deformation treatment, with preferably the application of a section reduction rate greater than 30%.

[0177] It should be noted here that, whatever the TX or TS state ultimately sought, the innovative characteristics and performance displayed by the CuNi15Sn8 alloy material result from the combination of the first five steps of the process, noted 1) to 5) above, and the specific parameters applied during the implementation of each of these steps.

[0178] That being said, although the characteristics of the CuNi15Sn8 material of the invention result from the implementation of the first five steps of the manufacturing process according to the invention, it is important to remember that the performance improvement must also be observed and characterized in the product after structural hardening (step 6). Indeed, this improved performance must also be present in the final product in use, bearing in mind that this product is generally intended for various applications in industries known to be particularly demanding in terms of mechanical properties and strength, due to the product's exposure to often extreme conditions.

[0179] All the steps in the production process of the CuNi15Sn8 material, as well as the parameters for implementing them, which lead to the characteristics of the alloy in question (residual porosities of considerably reduced size and limited total volume, controlled grain size, discontinuous precipitation below a defined threshold considered very low), will now be detailed in the order in which they are carried out.

[0180] It should be noted that, where relevant, and to avoid repetition, it is appropriate to specify that the embodiments and features according to the invention can be freely combined in any way between the material, the process, and the installation. Therefore, all embodiments and features according to the invention are disclosed and may be claimed for the material, the process, or the installation.

[0181] Step 1 of the manufacturing process of the CuNi15Sn8 material of the invention consists of a continuous vertical melting / casting step, described in detail below, in connection with the illustration of a casting installation shown in Figure 2 of the accompanying drawings, developed specifically for the process of the present invention.

[0182] After the development of the CuNi15Sn8 alloy, considering its composition, namely, as a reminder:

[0183] - nickel (Ni) in a proportion between 14.0 and 16.0%

[0184] - tin (Sn) in a proportion between 7.0 and 9%

[0185] - optionally at least one minor alloying element chosen from at least manganese (Mn) in a proportion between 0.1 and 0.3%, niobium (Nb) in a proportion between 0.04 and 0.09% and titanium (Ti) in a proportion between 0.002 and 0.07%,

[0186] - the remainder of the alloy being copper and unavoidable impurities, the proportion of unavoidable impurities being less than 0.5% by mass,

[0187] The liquid metal, after melting of the various elements constituting the alloy in question, is transferred into a vertical continuous casting installation 1 illustrated in Figure 2, which is also the subject of the present invention.

[0188] The said vertical continuous casting installation 1 is composed, in the first instance, at the level of its upper part 11, at the level of which the molten metal 2 is introduced from a reservoir, or a feed hopper, located upstream, and not illustrated in the figure, by an ingot mold 3.

[0189] This ingot mold 3 corresponds, more particularly, to a bottomless ingot mold 3, and of limited height h, fed at the level of its upper part, corresponding to the upper part 11 of the vertical continuous casting installation 1, with molten liquid metal.

[0190] Such a bottomless ingot mold 3 makes it possible, in a particularly advantageous way, to allow the production of a casting bar 4, having a length much greater than the limited height h of said ingot mold 3.

[0191] In the continuous casting installation 1 according to the invention, the bottomless ingot mold 3 is associated with cooling means 5 so as to create, in the upper part of said installation 1, a first cooling zone 6 of the molten CuNi15Sn8 alloy 2, this consisting, more particularly, of an indirect cooling zone 6 of the molten metal 2.

[0192] In this indirect cooling zone 6, more particularly, the liquid metal 2 is therefore in contact with the internal surface 31 of the ingot mold 3, the opposite external surface of which is, in turn, cooled by means of a fluid, preferably water, so that, at the exit of the ingot mold 3, the external surface 41 of the product, in the form of a casting bar 4, is already solidified.

[0193] Downstream of the ingot mold 3, at which the liquid metal is cooled indirectly, there is a direct cooling zone 7 where the outer surface 41 of the product is in direct contact with a refrigerant fluid, again preferably consisting of water.

[0194] Preferably, in the direct cooling zone 7 there are means for spraying the casting bar 4 with water and / or means suitable for allowing immersion of said bar 4 in water, such as a tank.

[0195] The cooling of the liquid metal 2, and then of the bar 4 once said metal has solidified, is therefore carried out continuously over the entire height of the vertical continuous casting installation 1, first through an indirect cooling zone 6 at the level of the ingot mold 3, then through a direct cooling zone 7 of the bar 4.

[0196] In the direct cooling zone 7, it was determined by the inventors that it must be applied for a sufficient time on the product 4 so that any fixed point on the surface of the bar 4 exiting the ingot mold 3 has its temperature decrease continuously, and cannot rise to a temperature above a threshold, noted Tmax, after said bar 4 has exited said direct cooling zone 7.

[0197] For example, considering that the temperature of an external surface point 41 at the outlet of the ingot mold 3, in other words at the outlet of the indirect cooling zone 6, is 850 °C, and that the threshold temperature Tmax is defined as being equal to 300 °C at the outlet 71 of the direct cooling zone 7, the cooling condition of the bar is unsuitable:

[0198] - if, in the direct cooling zone 7, the surface point temperature initially falls from 850 to 700 °C, before rising again to a temperature of 750 °C; indeed, in this case, the temperature does not decrease continuously within said direct cooling zone 7;

[0199] - if, in the direct cooling zone 7, the temperature continuously falls from an initial temperature of 850 °C to a final temperature, at the outlet 71 of said direct cooling zone 7, equal to 300 °C, and that this temperature rises again, for example to 400 °C, after the bar 4 has exited said direct cooling zone 7; indeed, in this case, the temperature of the bar 4 downstream of the outlet 71 of said zone 7 would be greater than the predefined threshold temperature Tmax of 300 °C.

[0200] Thus, it follows from the above that the direct cooling condition in zone 7 is suitable insofar as, for example, the surface temperature of the bar 4, along this zone 7, decreases continuously from an initial temperature of 850 °C to a final temperature of 200 °C, and then this surface temperature rises again to a value of 290 °C downstream of the outlet 71 of said direct cooling zone, since this value is lower than the predefined threshold temperature Tmax.

[0201] It has been determined that, in the case of continuous casting of a CuNi15Sn8 alloy in the form of a round section bar, with a diameter greater than 300 mm, with a continuous casting installation comprising only an indirect cooling zone at the level of an ingot mold (case of cylindrical casting bars in a dry tank), in other words no direct cooling zone as in installation 1 according to the present invention, observation of the structure of a section of the cylindrical casting bar shows three zones.

[0202] Each of these zones has a particular structure, described below with reference to the attached figure 3:

[0203] - a first zone defined as "zone 1" on the said figure, consisting of a thin outer ring, and exhibiting a fine equiaxed microstructure;

[0204] - a second zone defined as "zone 2", consisting of an inner ring with a large columnar grain structure several centimeters in size;

[0205] - a third zone defined as "zone 3" in Figure 3 and consisting of a central zone made up of large grains, without a preferred direction, with the presence of porosities having a volume increasing as they approach the center of the cylindrical bar.

[0206] Zone 3 corresponds, in the case of indirect cooling only, to the change in cooling regime when the product leaves the mold, on which very significant indirect cooling is applied, and enters a zone without cooling, where the casting bar is cooled only by radiation with a much lower cooling power than during the indirect cooling implemented in the casting installation 1 according to the present invention.

[0207] It is in this zone 3 that the defects specific to the solidification of CuNi15Sn8 in known alloys of the state of the art will be found, with locally very high mass concentrations of Sn that can reach between 30 and 39%, and with porosities, corresponding, for the record, to empty spaces of material, which can be a few pm in the interdendritic spaces but can reach a significant size, on the order of a millimeter, when approaching the center of this zone 3.

[0208] In the vertical continuous casting installation 1 of the invention, the center of zone 3 corresponds to the bottom of the solidification well 8, located at a depth H, shown in Figure 2, below the free surface 9 of the liquid metal in the ingot mold 3.

[0209] The closure of the bottom of well 8 closes the access routes to feed the solidification shrinkage with new liquid with the correct composition (CuNi15Sn8) and a hot temperature, above 1200 °C.

[0210] It was recalled that, for CuNi15Sn8, the last solidification takes place with the presence of a liquid very rich in Sn which corresponds to the y phase with a melting temperature below 950°C.

[0211] This characteristic of having a final liquid very rich in Sn will become increasingly effective as one approaches the center of the foundry bar, and this is what will exacerbate the solidification defects of CuNi15Sn8, namely:

[0212] - The segregation of volumes very rich in Sn;

[0213] - The porosity of micrometric interdendritic shrinkage cavities;

[0214] - The porosity of the central shrinkage pore is millimeter-sized.

[0215] The example given above, in connection with Figure 3 of the attached drawings, shows that, in order to obtain a material with improved performance and a reduced risk of failure when used in extreme conditions, it is necessary to avoid the following during the first continuous melting / casting stage of the manufacturing process:

[0216] - A central area with a diameter that is too large;

[0217] - A dendritic solidification structure that is too coarse in the central zone;

[0218] - Porosity that is too high in density and size;

[0219] - Segregations with Sn content greater than 30% with excessively large dimensions.

[0220] In an original way, in the case of the use of a conventional continuous casting, it was sought to reduce the diameter D and the height H of the pouring well 8, and thus achieve the characteristics targeted for the alloy of the invention, by acting on the one hand on the cooling of the product 4 and, on the other hand, on the shape of the latter, without however varying the pouring speed by reducing it.

[0221] Indeed, such a decrease in the pouring speed could have been able to reduce the diameter D and the height H, but this would have impacted the industrial performance of this process.

[0222] Thus, the tests carried out, the results of which are reproduced below, were carried out in casting speed ranges between 2000 and 4000 kg / h, that is to say without a decrease in casting speed compared to a traditional process.

[0223] Within this framework, and based on these elements, several tests were carried out.

[0224] In particular, the manufacturing process was tested:

[0225] - of a casting product having a round section, that is to say a circular section, therefore having a symmetry of revolution (whose shape is noted "symmetric" in the table below), on which only indirect cooling is applied;

[0226] - of a first casting product having a non-round section, that is to say a non-circular section, without an axis of symmetry of revolution (whose shape is noted as "asymmetric" in the table below), with indirect cooling only;

[0227] - of a second casting product also having a non-round section, (non-circular, "asymmetrical" shape again), with indirect cooling followed by direct cooling.

[0228] The results obtained are presented in Table 1 below:

[0230] It should also be noted that tests, not included in the table above, were conducted on a round-section casting bar (referred to as "symmetrical shape"), with indirect cooling followed by direct cooling. The results showed a low porosity rate in the bar material, although it exhibited undesirable (internal) cracking.

[0231] On the one hand, these tests demonstrate that the transition from a casting product with a round cross-section, or having a symmetry of revolution (noted "symmetrical shape" >>) to a non-round cross-section (non-circular, without symmetry of revolution and noted "asymmetrical shape" >>) makes it possible to reduce the maximum porosity rate, which goes from 3.8% to 3%, and to reduce the maximum porosity dimension, which decreases from 1200 pm with a round cross-section of product to 1000 pm with a non-round cross-section.

[0232] On the other hand, this introduces external cracking over the entire product, when it has a non-round section, compared to a round section, in the case of the application of indirect cooling only.

[0233] On the other hand, the results of these tests demonstrate that the introduction of direct cooling, in addition to indirect cooling and downstream of it, allows continuous cooling at the ingot mold outlet up to 300 °C on a non-round cross-section material, which has the following effect:

[0234] - to avoid the phenomenon of external cracking of the product;

[0235] - to reduce the central diameter D1 with porosity;

[0236] - to reduce the maximum porosity rate;

[0237] - to drastically reduce the maximum dimension of the pores.

[0238] Thus, the results of these tests demonstrate that product 4, obtained at the exit of step 1 of vertical continuous melting and casting, has the desired characteristics.

[0239] Preferably, the non-round (i.e., non-circular) cross-section of the product 4 obtained using the vertical continuous casting installation 1 is a quadrilateral with right angles, preferably rounded with a large radius, and with flat or advantageously convex faces. The ratio of the lengths of two adjacent faces may be equal to one or preferably different from one, the cross-section of the product, namely the casting bar, thus being preferably rectangular. The presence of rounded corners, i.e., rounded or arc-shaped corners, preferably with a large radius, at the corners of the quadrilateral obtained using the vertical continuous casting installation 1, makes it easier to extract and demold the product 4 and thus prevents it from jamming in the mold 3, while also reducing stress and cracking, particularly at the corners, where the angles and edges are sharp.The radius of the angles or rounded corners is preferably between 10 and 50 mm.

[0240] The ingot mold 3, in the vertical continuous casting installation 1 of the invention, must also have such a non-round section, conforming to the section that one wishes to obtain for the casting bar 4, and preferably as defined above.

[0241] In the manufacturing process of the material based on a CuNiSn alloy according to the invention, following the first casting step (melting-casting) described above in detail, it is necessary to carry out, in step 2), a homogenization treatment.

[0242] In the process of the invention, this homogenization treatment is carried out at a temperature above 870 °C, preferably above 900 °C, for a period of more than 5 h, in order to allow the absorption of tin-rich segregations, as well as the y phase.

[0243] Preferably, the maximum temperature of the homogenization treatment is 930 °C and the maximum holding time at this temperature is 7 h. Note that the duration indicated here corresponds to the holding time at the temperature, and not to the total cycle time which includes a temperature rise phase, the holding phase at the temperature, followed by the cooling phase.

[0244] The CuNiSn alloy thus homogenized then undergoes, in step 3) of the process according to the invention, a first hot transformation, particular to the present process, because this has the effect of allowing a structural recrystallization which eliminates the dendritic casting structure.

[0245] This first hot transformation can be carried out, among other things, by forging, blooming, hammering, rolling, extrusion, or any other hot transformation process known to a person skilled in the art, and must allow a reduction in cross-section of more than 1 / 3 compared to the initial cross-section.

[0246] The total time the product is held at a temperature above 800 °C, including the homogenization treatment time in step 2) and the duration of step 3) of the first hot transformation, is 8 hours or more.

[0247] Thus, the conditions applied during this step 3) of the process of the invention are:

[0248] - the application of a section reduction of more than 1 / 3 compared to the initial section.

[0249] - a temperature above 800 °C;

[0250] - a holding time at this temperature such that the total holding time at temperature including the homogenization treatment time of step 2) and the holding time of step 3) is greater than 8h; it is thus understood, given that the holding time at a temperature of 870 °C or more of step 2) is defined as greater than 5 h and a maximum of 7 h, that the holding time at temperature during step 3) is greater than 1 h to more than 3 h, depending on the treatment time of step 2), to result in a total holding time of the product at a temperature greater than 800 °C for more than 8 h.

[0251] The above can be summarized by the following formula:

[0252] Holding time at T °C > 800 °C step 3) > (8 - time step 2) homogenization), expressed in h.

[0253] Considering these implementation conditions of step 3) of first hot transformation, this allows the loss of the large columnar grain structure resulting from step 1) of continuous melt-casting and, by proceeding with a reduction of the cross-section of more than 1 / 3 of the initial cross-section, it is possible to obtain a recrystallized structure with a grain size of less than 200 pm, with suppression of the foundry dendritic structure.

[0254] Note here that, if the product obtained by following step 1) of melting has a non-round cross-section, in order, due to this particular shape and the specific cooling conditions implemented, to obtain in particular a drastic reduction in the proportion of porosities within the volume of material and their size, as explained above, the product obtained from step 3) of first hot transformation may, on the other hand, have a round cross-section.

[0255] Note that the formula used to calculate the reduction in cross-section when the initial cross-section of the product is not round, and has an area denoted "initial surface," while the final cross-section is round after hot processing, and has an area denoted "final surface," is (Initial Surface - Final Surface) / (Initial Surface). This formula applies regardless of the product's cross-section (round, rectangular, square).

[0256] Thus, the product after the first hot treatment of step 3) can consist of a bar with a round cross-section, and this is moreover preferably the case in the implementation of the process of the invention.

[0257] Following this step 3) of first hot transformation, on the product obtained after this treatment, a step 4) of second hot transformation is implemented.

[0258] This second hot transformation can be described as "near to shape" and can be carried out, among other things, by forging, blooming, hammering, rolling, extrusion, or any other hot transformation process known to a person skilled in the art.

[0259] In the product or material of the invention based on CuNiSn alloy, it is sought, for the record, to have the smallest and most homogeneous grain size possible at every point of the material, which will reduce the initiation of damage on areas of heterogeneity, increase the elastic limit of the product, and improve its resistance to wear from friction, and this in order to considerably reduce the risks of failure of the material during its prolonged use in extreme conditions.

[0260] Thanks to the implementation conditions of step 4) of second hot transformation of a product made according to the invention and resulting from step 3) of first hot transformation which, as a reminder, is specifically introduced in the process of the invention, the inventors were able to determine that it is possible to control the final grain size in the CuNiSn alloy material of the invention.

[0261] Thus, in the case of the present process, the MC grain size of the CuNiSn alloy was measured, after carrying out the solution treatment of step 5) and the hardening treatment of step 6) which will be detailed below, but according to the parameters which were implemented during the conduct of the present step 4) of second hot transformation.

[0262] Indeed, for the record, the performance improvement due to the implementation of certain specific steps of the process of the invention must be characterized on the final material, at the end of the execution of all the steps of said process, that is to say after the structural hardening of step 6) mentioned later.

[0263] It has thus been shown, as will be explained in detail in Example 1 below, that the MC grain size in the CuNiSn alloy material of the invention (at the end of step 6) is advantageously less than 20 pm when the temperature at which the second hot transformation step 4 is carried out is between 750 °C and 800 °C and when the applied section reduction is greater than 70% and in particular between 70% and 95%.

[0264] When the second hot transformation is carried out at a temperature between 800 °C and 850 °C with a reduction in cross-section greater than 70% and in particular between 70% and 97%, the MC grain size of the copper alloy is between 20 and 40 pm.

[0265] Finally, when the implementation temperature of this step 4) is between 850 °C and 900 °C and with a reduction in cross-section greater than 70% and in particular between 70% and 98%, the MC grain size in the CuNiSn alloy material of the invention at the end of all the steps of the process is between 40 and 60 pm.

[0266] The "step 4 implementation temperature" also known as the "hot transformation temperature" refers to the temperature at which the product (the bar) is preheated for a period of more than 2 hours before the section reduction is applied.

[0267] Note that the section reduction ratio applied during an operation to reduce the diameter of a round bar, from an initial diameter denoted Di to a final diameter Df smaller than the initial diameter Di, is calculated using the following mathematical formula: (Di 2 - Df 2 ) / Di 2 X100, the section reduction rate being expressed as a percentage.

[0268] It follows from the above that, by lowering the temperature of the second hot transformation of step 4) of the process, a higher concentration of dislocations at the grain boundaries is obtained, which will contribute to a smaller grain size after the solution treatment of the subsequent step 5) described below in detail.

[0269] Conversely, a low concentration of dislocations at gain joints can lead to grain growth and size heterogeneity during the subsequent solution operation.

[0270] It is therefore essential, for the control of the MC grain size in the final CuNiSn alloy material, at the end of the implementation of all the steps of the process of the invention, to find conditions, for step 5) of dissolution, which combine with the product from step 4) of second hot transformation to achieve this controlled grain size.

[0271] Thus, in the manufacturing process of a CuNiSn alloy material of the present invention, the subsequent step 5) of solutionization is carried out, in a particularly advantageous manner, at a temperature lower than that usually used for solutionization, namely more than 800 °C.

[0272] Indeed, it has been determined that products made with a solution temperature above 800° do not, after step 6) of hardening, exhibit a good compromise between mechanical strength and elongation, unlike the process of the invention, which allows an improvement of this compromise between mechanical properties and elongation.

[0273] In particular, in the process of the invention, step 5) of dissolving can be carried out statically on a batch of bars at a temperature between 780 and 795 °C with a holding time at this temperature of each bar in the batch of between 1 h and 2 h in order to homogenize the temperature between the different bars in the batch.

[0274] Note that the temperature indicated here refers to the temperature of the bar which is subjected to the solution treatment.

[0275] Such a step 5) of solution in the a+y domain allows a part of Sn to be consumed to form a limited fraction of Y- phase. Thus, during the subsequent hardening treatment, during step 6) of the process, the growth of discontinuous precipitation is delayed or even inhibited because Sn is no longer available.

[0276] A compromise was sought here between the beneficial delay of discontinuous precipitation during the hardening treatment, which allows good elongation to be maintained, and the sufficient level of high mechanical strength in the final material.

[0277] The results of example 1 below demonstrate that applying such a solution temperature with a bar held between 780 and 795 °C allows obtaining a final material with an excellent compromise between elastic limit and elongation, while the mechanical characteristics outside this specific range are less satisfactory.

[0278] Preferably, the solution stage is carried out at a temperature between 780 and 790 °C, this temperature corresponding to the setpoint temperature of the oven in which the solution is carried out.

[0279] The solution treatment is followed by quenching carried out by immersing each bar in water, preferably respecting a maximum time of 2 min between the moment when each bar, having a temperature between 780 and 795 °C, is removed from the furnace where the solution treatment is carried out, and the moment of the end of the immersion of said bar in the water.

[0280] Following step 5) of dissolving, a rough product is obtained, and on which the hardening treatment is applied, which corresponds to step 6) of the process of the invention.

[0281] For TX alloy states, such a hardening treatment is carried out at a temperature between 390°C and 425°C, advantageously between 400 and 425°C, and a time at temperature between 1 h 00 and 6 h 00, between 1 and 2 h according to a first example of embodiment, or between 3 and 6 h according to a second example of embodiment, even more preferable.

[0282] It should also be noted here that, for TX alloy states, the second hot transformation step 4) is often specified as a step involving a hot extrusion, hot forging or hot rolling process, enabling the dimension of the final product or material to be provided.

[0283] For TS alloy states, the temperature and hardening time can be reduced compared to a TX state, due to the work hardening of the product carried out during the cold forming step 5'), which can be implemented in the process of the invention to obtain such an alloy state. This operation can be carried out, among other methods, by rolling, drawing, hammering, or mandrel forming.

[0284] The temperature applied in step 6) of hardening will then preferably be between 320 °C and 400 °C, with a holding time at temperature again between 1 h 00 and 6 h 00, preferably between 1 h 00 and 4 h 00.

[0285] In the case of the CuNiSn alloy materials of the invention, after the solution treatment described in step 4) and after the hardening treatment of step 6), at any observation point on the bar material, the proportion of discontinuous precipitated phase must be less than 1% by volume, relative to the total volume of the material, and measured according to the method mentioned previously in the description. The porosities have a maximum dimension that is less than 20 pm (largest dimension or diameter, as the case may be, of globally elliptical or globally spherical porosities, respectively, measured according to the method also mentioned previously).The grain size can be adjusted in increments of 20 pm, i.e., either to a value less than 20 pm or to a value between 20 and 40 pm, or to a value between 40 and 60 pm, depending on the implementation conditions of step 4) of the second hot processing, i.e., the temperature conditions.

[0286] The final CuNiSn alloy material obtained by conducting the process has a yield strength at 0.2% greater than 620 MPa and an elongation greater than 6%.

[0287] It is also conceivable, in a particular configuration, that the final CuNiSn alloy material obtained by conducting the process has a yield strength at 0.2% greater than 755 MPa and an elongation greater than 10%.

[0288] It is also possible to introduce a cold transformation 5' with section reduction between step 5 and step 6, which will greatly increase the elastic limit level.

[0289] The final CuNiSn alloy material obtained by conducting the process then exhibits a yield strength at 0.2% greater than 650 MPa and an elongation greater than 1%.

[0290] It is also conceivable, in a particular configuration, that the final CuNiSn alloy material obtained by conducting the process with the introduced cold forming step 5' exhibits a 0.2% yield strength greater than 650 MPa and an elongation greater than 20%, or a 0.2% yield strength greater than 750 MPa and an elongation greater than 15%, or a 0.2% yield strength greater than 1020 MPa and an elongation greater than 1%, or a 0.2% yield strength greater than 1020 MPa and an elongation greater than 3%, or even an elongation greater than 6%. The material of the invention may also exhibit a 0.2% yield strength greater than 1100 MPa and an elongation greater than 6%.

[0291] Example 1: Determination of the MC grain size in a CuNiSn alloy material as a function of the temperature conditions implemented in step 4) of the second hot transformation of the process of the invention and determination of the mechanical characteristics (Re, Rm and A%) as a function of the solution temperature of step 5):

[0292] The table below summarizes different temperature conditions as well as different section reduction conditions which were implemented in step 4) of the second hot transformation of the process of the invention, and indicates the grain size of the CuNiSn alloy which was obtained in the material of final products, in the form of round section bars.

[0293] The MC grain size in CuNiSn alloy material measures the average diameter of the grains, that is, the individual crystalline regions with the same atomic configuration in a polycrystalline material that constitutes the alloy material. It was measured by applying the recommendations of ASTM E112 revision 2024, entitled "Standard Test Method for Determining Average Grain Size". <

[0295] It has thus been determined that it is possible, for example, to obtain a grain size of 20 to 40 pm, and that, for this, several conditions and parameters must be respected, namely:

[0296] - the implementation temperature of step 4) of the second hot transformation must be between 800 and 850°C;

[0297] - the diameter of the product at the end of step 3), before the second hot processing must be, for example, 180 mm, for bars which, after the carrying out of step 4), have a diameter between 32 and 100 mm, or, for example, 250 mm for bars which, after step 4), have a diameter between 65 mm and 140 mm after the second hot processing.

[0298] It has also been shown that the grain size can be further reduced to a dimension of less than 20 pm, with a hot second transformation temperature between 750 and 800 °C.

[0299] The bars from step 4), and obtained with a hot second transformation temperature of 900°C, were then subjected to solution tests with different temperature conditions, and with a holding time of 1 hour.

[0300] The temperature conditions that were tested are as follows:

[0301] - in the single-phase domain y at a temperature of 800°C;

[0302] - in the two-phase a+y domain at different temperatures, between 700 and 790 °C.

[0303] These solutions were followed by a hardening treatment (step 6) for a period of 1 to 2 hours at a temperature between 400 and 425 °C.

[0304] The mechanical characteristics that were calculated from several tensile tests at room temperature on standardized CuNiSn alloy material specimens after implementation of the process of the invention as a whole, i.e. after the hardening treatment of step 6), and according to ASTM E8 standard are established in the table below:

[0306] The results obtained and shown in the table above demonstrate that, at a solution temperature of 750 °C, the elastic limit Re is too low.

[0307] For a dissolution temperature of 800 °C, the elongation is too small.

[0308] The results demonstrate that the best compromise in terms of mechanical characteristics, with regard to both the yield strength Re and the elongation A (%) is obtained at a solution temperature which is between 780 and 790°C.

[0309] In this compromise zone, keeping the solution temperature between 780 and 790 °C, and for certain targeted characteristics, an increase in the elastic limit to 0.2% and in the elongation value can be sought.

[0310] The solutions to further improve these already satisfactory mechanical characteristics are implemented in the process of the present invention by setting the temperature of the second hot transformation less than or equal to 890 °C and adjusting the parameters of the hardening heat treatment of step 6) accordingly.

[0311] Indeed, as a reminder, in this example, the solution treatment step (5) and the hardening step (6) were carried out on a bar obtained following a hot processing step (4) at 900 °C. A reduction in this second hot processing temperature, in combination with an adjustment of the heat treatment parameters, results in an improvement of the mechanical properties (Re and A%) at the end of the process.

[0312] Thus, for example, by lowering the second hot transformation temperature to 760°C and carrying out a solution treatment at 790°C for 1 h00 and a tempering treatment at 400°C for 3 h00, the elastic limit value is at least 755 MPa and the elongation value is at least 13%.

[0313] Further tests were conducted, implementing the procedure detailed below.

[0314] An alloy according to the invention is cast, then a homogenization step is carried out at a temperature above 870 °C for more than 5 hours, followed by a hot processing step at 890 °C and solution heating at a temperature of 790 °C for 1 hour. A cold forming process with a reduction in cross-section greater than 30% is then performed, followed by tempering (spinodal decomposition) at 360 °C for 2 to 4 hours.

[0315] The results, in terms of mechanical characteristics, measured according to ASTM E8, are illustrated in the table below:

[0316] With the implementation of such a process, the resulting material exhibits a yield strength at 0.2% (Re) greater than 1020 MPa and an elongation A% greater than 3%.

Claims

Demands

1. Material based on a spinodal decomposition CuNiSn alloy, based on copper (Cu), nickel (Ni) and tin (Sn), said alloy being composed of, in % by mass relative to the total mass of the alloy: - nickel (Ni) in a proportion between 14.0 and 16.0%, - tin (Sn) in a proportion between 7.0 and 9% - optionally at least one minor addition element chosen from at least manganese (Mn) in a proportion between 0.1 and 0.3%, niobium (Nb) in a proportion between 0.04 and 0.09%, and titanium (Ti) in a proportion between 0.002 and 0.07%; - the remainder of the alloy being copper and unavoidable impurities, the proportion of unavoidable impurities being less than 0.5% by mass, the metallurgical structure of said CuNiSn alloy material being characterized by: - an absence of porosity where the largest dimension is greater than 20 pm at any point in the volume of said material, the maximum dimension of the precipitates being determined by making observations on a polished sample of the material, using an optical microscope; - the presence of residual discontinuous precipitation below a very low defined threshold, the proportion of the total volume of discontinuous precipitation representing less than 1% of the total volume of material, measured according to the method defined in the publication by Ouyang et al, (“Age-hardening behavior and microstructure of Cu-15Ni-8Sn-0.3Nb alloy prepared by powder metallurgy and hot extrusion”, Transactions of Nonferrous Metals Society of China, Volume 27, Issue 9, 2017, pages 1947-1955); - a MC grain size of less than 60 pm, measured according to the standardized method ASTM E112 revision 2024.

2. Spinodal decomposition CuNiSn alloy-based material according to claim 1 characterized in that the MC grain size is between 20 and 40 pm, measured according to the standard method ASTM E112 revision 2024.

3. Spinodal decomposition CuNiSn alloy-based material according to claim 1 characterized in that the grain size MC is less than 20 pm, measured according to the standardized method ASTM E112 revision 2024.

4. Material based on a spinodal decomposition CuNiSn alloy according to any one of claims 1 to 3 characterized in that it has a 0.2% yield strength (Re) greater than 620 MPa and an elongation greater than 6%, measured according to ASTM E8.

5. Material based on a spinodal decomposition CuNiSn alloy according to any one of claims 1 to 4 characterized in that it has a 0.2% yield strength (Re) greater than 755 MPa and an elongation greater than 10%, measured according to ASTM E8.

6. Material based on a spinodal decomposition CuNiSn alloy according to any one of claims 1 to 3 characterized in that it has a 0.2% yield strength (Re) greater than 650 MPa and an elongation greater than 1%, measured according to ASTM E8.

7. Material based on a spinodal decomposition CuNiSn alloy according to any one of claims 1 to 3 or 6 characterized in that it has a 0.2% yield strength (Re) greater than 1020 MPa and an elongation greater than 3%, measured according to ASTM E8.

8. A process for obtaining a material based on a spinodal decomposition CuNiSn alloy according to any one of the preceding claims, from an alloy composition consisting of nickel (Ni) in a proportion of between 14.0 and 16.0%, tin (Sn) in a proportion of between 7.0 and 9%, optionally at least one minor alloying element selected from at least manganese (Mn), niobium (Nb), titanium (Ti), the remainder of the alloy being copper and unavoidable impurities, the proportion of unavoidable impurities being less than 0.5% by mass, said process comprising at least the following steps, taken in order: 1) melting of the different constituents of the composition of said alloy and continuous vertical casting through an ingot mold having a non-round cross-section, allowing to obtain a bar also having a non-round cross-section, with application of a first indirect cooling at the level of the ingot mold, followed by a direct cooling at the outlet of the ingot mold; 2) application, to said bar having a non-round cross-section, at the outlet of continuous vertical casting, of a homogenization treatment at a temperature above 870 °C, for a duration of more than 5h; 3) application, to said non-round section bar after homogenization, of a first hot transformation and obtaining a hot-transformed bar, at a temperature above 800 °C and for a holding time at temperature above 8h, including the duration of the homogenization heat treatment of step 2), and with a reduction of section to more than 1 / 3 compared to the initial section; 4) application, on said hot-processed bar, of a second hot-processing; 5) carrying out a solution treatment on said doubly hot-processed bar and obtaining a rough product; 6) application of a spinodal decomposition hardening heat treatment on said rough product, at a temperature between 390 and 425 °C for a duration between 1 and 6 h, and obtaining the CuNiSn alloy material whose metallurgical structure, on the one hand, is devoid of porosity whose largest dimension is greater than 20 pm at any point in the volume of said alloy, on the other hand, has residual discontinuous precipitation below a very low defined threshold, the proportion of the total volume of discontinuous precipitation representing less than 1% of the total volume of material and, finally, has a grain size MC less than 60 pm.

9. A method for obtaining a material based on a spinodal decomposition CuNiSn alloy according to any one of claims 1 to 7, from an alloy composition consisting of nickel (Ni) in a proportion of between 14.0 and 16.0%, tin (Sn) in a proportion of between 7.0 and 9%, optionally at least one minor alloying element selected from at least manganese (Mn), niobium (Nb), titanium (Ti), the remainder of the alloy being copper and unavoidable impurities, the proportion of unavoidable impurities being less than 0.5% by mass, said method comprising at least the following steps, taken in order: 1) melting the various constituents of the composition of said alloy and continuous vertical casting through an ingot mold having a non-round cross-section, making it possible to obtain a bar also having a non-round cross-section, with the application of a first indirect cooling at the ingot mold,followed by direct cooling at the ingot mold outlet; 2) application, to said bar having a non-round cross-section, at the outlet of a continuous vertical casting, of a homogenization treatment at a temperature above 870 °C, for a duration of more than 5 hours; 3) application, to said non-round section bar after homogenization, of a first hot transformation and obtaining a hot-transformed bar, at a temperature above 800 °C and for a holding time at temperature above 8h, including the duration of the homogenization heat treatment of step 2), and with a reduction of section to more than 1 / 3 compared to the initial section; 4) application, on said hot-processed bar, of a second hot-processing; 5) carrying out a solution treatment on said doubly hot-processed bar and obtaining a rough product; 5') application of a cold deformation treatment; 6) application of a spinodal decomposition hardening heat treatment on said rough product, at a temperature between 320 and 400 °C for a duration between 1 and 6 h, and obtaining the CuNiSn alloy material whose metallurgical structure, on the one hand, is devoid of porosity whose largest dimension is greater than 20 pm at any point in the volume of said alloy, on the other hand, has residual discontinuous precipitation below a very low defined threshold, the proportion of the total volume of discontinuous precipitation representing less than 1% of the total volume of material and, finally, has a grain size MC less than 60 pm.

10. A method for obtaining a material based on a spinodal decomposition CuNiSn alloy according to claim 8 or claim 9 characterized in that, during the direct cooling of step 1) of continuous vertical melting and casting, the temperature of the external surface of the non-round section bar decreases continuously from the exit of the ingot mold to an external surface temperature less than or equal to 300 °C.

11. A process for obtaining a material based on a spinodal decomposition CuNiSn alloy according to any one of claims 8 to 10 characterized in that step 3) of first hot processing is carried out by forging, blooming, hammering, rolling, extrusion.

12. A process for obtaining a material based on a spinodal decomposition CuNiSn alloy according to any one of claims 8 to 11 characterized in that step 4) of the second hot transformation, referred to as "near to shape" or "close to the final shape", is carried out at a temperature of 850 to 900 °C, with a preheating time before step 4) of the second hot transformation for a duration greater than or equal to 2h, and a reduction in cross-section greater than 70%, for obtaining a grain size MC of between 40 and 60 pm, measured according to the standard method ASTM E112 revision 2024.

13. A process for obtaining a material based on a spinodal decomposition CuNiSn alloy according to any one of claims 8 to 11 characterized in that step 4) of the second hot transformation, referred to as "near to shape" or "close to the final shape", is carried out at a temperature of 800 to 850 °C, with a preheating time before step 4) of the second hot transformation for a duration greater than or equal to 2h, and a reduction in cross-section greater than 70%, for obtaining a grain size MC of between 20 and 40 pm, measured according to the standard method ASTM E112 revision 2024.

14. A process for obtaining a material based on a spinodal decomposition CuNiSn alloy according to any one of claims 8 to 11 characterized in that step 4) of the second hot transformation, referred to as "near to shape" or "close to the final shape", is carried out at a temperature between 750 and 800 °C, with a preheating time before step 4) of the second hot transformation for a duration greater than or equal to 2h, and a reduction in cross-section greater than 70%, for obtaining a grain size MC less than 20 pm, measured according to the standard method ASTM E112 revision 2024.

15. A process for obtaining a material based on a spinodal decomposition CuNiSn alloy according to any one of claims 8 to 14 characterized in that step 5) of the solution treatment is carried out in a static furnace at a temperature between 780 and 795 °C, for a holding time at this temperature of between 1 and 2 h, before quenching carried out by immersion in water of the hot-processed and solution-treated bar, with a maximum time of 2 min between the moment when said bar is removed from the furnace and the end of its immersion in water.

16. A method for obtaining a material based on a spinodal decomposition CuNiSn alloy according to any one of claims 8 and 10 to 15 characterized in that step 6) of heat treatment by spinodal decomposition is carried out for a duration of between 3 and 6 h.

17. A vertical continuous casting installation (1) for carrying out step 1) of the vertical continuous casting process for obtaining a material based on a spinodal decomposition CuNiSn alloy according to any one of claims 8 to 16, said installation (1) being designed to allow the formation of a casting bar (4) from the molten CuNiSn alloy and comprising, at its upper part (11), through which the molten alloy (2) is introduced from a feed hopper located upstream of said installation (1), a bottomless mold (3) of cross-section S1 and height h, for the production of said casting bar (4) of cross-section S2 and length LB greater than h, said mold (3) being associated with first cooling means (5) to form an indirect cooling zone (6) of the molten alloy (2) at the upper part (11) of said installation (1),the latter being characterized in that it further comprises, downstream of said direct cooling zone (6), a direct cooling zone (7) at which the outer surface (41) of said casting bar (4) is in direct contact with a cooling liquid, for example water, and in that section S1 of said ingot mold (3), and consequently section S2 of said casting bar (4), consist of a quadrilateral with right angles, preferably rounded with a large radius, and with flat or convex faces.

Images