High-strength high-corrosion-resistant copper alloy pipe and preparation method and application thereof

Abstract

This invention relates to the field of alloys, and more particularly to a high-strength, high-corrosion-resistant copper alloy tube, its preparation method, and its applications. The copper alloy tube, by mass percentage, comprises: Sn: 0.03%~0.25%, Mn: 0.003%~0.10%, Ni: 0.08%~0.20%, P: 0.015%~0.030%, with the balance being Cu and unavoidable impurities, and the Cu content being greater than 99.5%. The mass percentages of Sn (a) and Mn (b) satisfy: 0.20% ≤ a + 6b ≤ 0.40%. Furthermore, it achieves the same target burst pressure P as a TP2 copper tube of the same specification. m When the minimum wall thickness t required for the copper alloy tube is t≤0.9×t0, this invention introduces manganese to partially replace tin, thereby reducing the cost of the alloy and improving its corrosion resistance while maintaining the tensile strength of the copper alloy tube ≥250MPa.

Description

Technical Field

[0001] This invention relates to the field of alloys, and more particularly to a high-strength, high-corrosion-resistant copper alloy tube, its preparation method, and its applications. Background Technology

[0002] Copper and copper alloys, especially TP2 phosphorus deoxidized copper, have long been key materials for manufacturing heat exchanger piping systems due to their excellent thermal and electrical conductivity and processability. With the technological advancements in refrigeration, air conditioning, new energy vehicles, and high-end equipment manufacturing, heat exchangers are developing towards compactness, thinner walls, and higher efficiency. Furthermore, in the heat exchanger tubes, the operating pressure of the refrigerant creates greater stress in the circumferential direction of the tube than in the longitudinal direction. This requires copper tubes to maintain corrosion resistance while possessing higher strength so that the system pressure requirements can still be met after the wall thickness is reduced, and material consumption can be reduced.

[0003] Currently, the mainstream technical approach to improving the strength of copper tubes is to add solid solution strengthening elements to the copper matrix, with Sn being the most commonly used element. Sn can effectively ensure material strength through solid solution strengthening. Experiments have also shown that Sn can guarantee the plasticity of microalloyed copper tubes, but its content needs to be strictly controlled; too little Sn has limited strengthening effect, while too much also presents problems. Sn is a precious metal, and its addition directly leads to a significant increase in raw material costs. Furthermore, when the Sn content exceeds 2500 ppm, segregation problems are prone to occur. Due to uneven microstructure, segregation, and the appearance of precipitated phases, the corrosion resistance of the material deteriorates, becoming a risk point for early failure of heat exchanger tubes. Moreover, simply relying on increasing the Sn content to improve strength faces the problem of diminishing marginal returns. To achieve sufficient thinning, even more Sn is often needed, which further exacerbates the aforementioned cost and process problems, creating a vicious cycle.

[0004] In addition, existing technologies have attempted to add other strengthening elements, such as Fe, Zn, and Ni, but this often comes at the cost of sacrificing thermal conductivity, corrosion resistance, and machinability. Some process improvements, such as increasing the cold working rate or complex heat treatment, can improve strength, but they lead to longer production processes, increased energy consumption, and decreased efficiency. Moreover, existing technologies mostly focus on improving mechanical properties, and do not pay enough attention to the synergistic improvement of corrosion resistance, especially for the diverse and complex corrosive environments faced by heat exchangers in actual operation, such as organic acid condensates and chloride-containing environments. The pitting corrosion resistance and stress corrosion cracking (SCC) resistance of many high-strength alloys are even lower than those of TP2 copper tubes, becoming a weakness in their application in harsh environments.

[0005] For example, patent CN101469961B enhances tensile strength by controlling Gaussian texture and increasing the proportion of low-angle grain boundaries. However, excessively high low-angle grain boundaries are essentially dislocation accumulation, which exacerbates material brittleness and impairs processability. Patent CN101555557B improves tensile strength by adding Sn, Zn, and P, but exhibits poor bending, flaring, and tube expansion properties. Patent CN120648935A introduces a high proportion of Σ3, Σ9, and Σ27 lattice grain boundaries by adding Sn, Ni, and P in a composite manner and controlling the ratio, combined with a recrystallization process. The aim is to control the ratio to improve strength, but in actual preparation, it was found that excessive tin content affects microstructure uniformity and the number of twins. If the microstructure uniformity and twin fraction are improved by increasing the final heat treatment temperature, the solid solution strengthening effect of tin will be reduced, lowering the cost-effectiveness of alloying. In addition, high tin content causes segregation problems, thereby deteriorating the corrosion resistance of the material, increasing the sensitivity of the preparation window, and increasing the difficulty of process control.

[0006] Therefore, existing technologies lack a copper alloy tube design that can simultaneously optimize strength, corrosion resistance, and process adaptability at low cost, thereby meeting the requirements of modern heat exchangers for thin walls, high pressure resistance, long service life, and corrosion resistance in complex environments.

[0007] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the present invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0008] The first objective of this invention is to provide a high-strength, high-corrosion-resistant copper alloy pipe. By introducing manganese to partially replace tin, the cost of the alloy is reduced and the corrosion resistance is improved. While maintaining the axial tensile strength of the copper alloy pipe at ≥250MPa, the circumferential strength of the material is significantly improved, thereby achieving a reduction in pipe wall thickness and production cost while ensuring the same pressure-bearing safety.

[0009] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0010] A high-strength, high-corrosion-resistant copper alloy tube, comprising, by mass percentage: Sn: 0.03%~0.25%, Mn: 0.003%~0.10%, Ni: 0.08%~0.20%, P: 0.015%~0.030%, with the balance being Cu and unavoidable impurities, and the Cu content being greater than 99.5%;

[0011] This invention improves the yield and tensile strength of materials by utilizing the solid solution of Sn and Ni in Cu, while also enhancing the material's ability to resist cold work hardening and recovery, which is beneficial for weight reduction and resistance to softening at high temperatures. In addition, through the solid solution of a small amount of Mn and the solute dragging effect, the recovery / recrystallization of the material is controllable, comprehensively improving the strength and plasticity of the material.

[0012] Among them, the mass percentage content of Sn, a, and the mass percentage content of Mn, b, satisfy: 0.20%≤a+6b≤0.40%; by controlling the total amount of Sn and Mn, the synergistic effect of Sn and Mn is maximized.

[0013] Sn is a core element for solid solution strengthening in alloys. Due to the significant difference in atomic radius and the reasonable difference in the number of valence electrons between Sn and Cu, Sn effectively strengthens Cu through solid solution. However, the Sn content must be strictly controlled. If the Sn content exceeds 0.25%, coarse grains will form between dendrites due to solidification segregation. , , Equivalent intermetallic compounds, these hard and brittle phases are difficult to completely eliminate through subsequent homogenization annealing, and there is a significant potential difference between them and the Cu matrix. In corrosive media, they are very likely to become preferential sites for pitting and intergranular corrosion, which leads to a serious deterioration of the material's corrosion resistance. At the same time, excessively high Sn content will cause Sn supersaturation in the matrix under non-equilibrium solidification conditions, resulting in the precipitation of continuous brittle film phases along grain boundaries during hot working or heat treatment, leading to a sharp increase in stress corrosion cracking susceptibility. Conversely, if the Sn content is too low, it cannot provide sufficient solid solution strengthening effect, resulting in insufficient basic strength of the material.

[0014] This invention employs Mn to replace a portion of Sn in the alloying design, keeping Sn content below 0.25% and fundamentally avoiding tin segregation. Furthermore, through the synergistic effect of Ni, significant cost optimization is achieved while maintaining or even improving overall performance. On one hand, the market price of Mn is only 1 / 20 that of Sn and 1 / 10 that of Ni. A trace addition of 0.003% to 0.1% can exert multiple effects of strengthening, purification, and microstructure refinement, reducing alloying costs by more than 30% compared to the traditional Cu-Sn-Ni system. On the other hand, Mn's deoxidation product, MnO, is easily floated and removed, reducing the amount of copper-phosphorus deoxidizer needed in the smelting process. Simultaneously, the improved compositional uniformity resulting from the refined as-cast microstructure further reduces manufacturing costs. In addition, the increased circumferential strength brought by Mn allows for a 5% to 15% reduction in pipe wall thickness under the same pressure requirements, significantly decreasing the amount of copper used per unit length.

[0015] In this invention, the addition of manganese (Mn) can significantly improve the circumferential strength, work hardening capacity, and corrosion resistance of the pipe, while also providing metallurgical purification and refining of the as-cast microstructure. It should be noted that work hardening capacity refers to the ability of a material to increase flow stress with increasing plastic strain during plastic deformation. Copper pipes used in air conditioning units undergo further plastic deformation during subsequent secondary processing (expansion and bending, etc.), resulting in work hardening and further increased strength. The addition of Mn in this invention can increase the circumferential tensile strength of the copper pipe after bending by 2% to 3%.

[0016] Specifically, Mn exists in two main forms: one is to agglomerate in the grain boundary region and participate in the regulation of the grain boundary chemical environment and structure; the other is to combine with P to form nanoscale precipitates that are uniformly distributed in the grain and work together to strengthen and resist corrosion.

[0017] During the smelting stage, Mn preferentially reacts with dissolved oxygen in the molten copper to form MnO slag, reducing oxygen content and preventing inclusions in pores. Simultaneously, it combines with sulfur to form high-melting-point MnS, achieving deep metallurgical purification and reducing the size of interdendritic enriched phases. During the cold working stage, Mn solution can reduce the stacking fault energy of Cu to a certain extent. It can alter the slip pattern and hardening behavior through short-range ordering and solute dragging, resulting in uniform dislocation slip, moderate dynamic recovery, and a smooth rise in the work hardening curve. This avoids defects such as edge cracks and surface roughness caused by localized over-hardening or abrupt changes in hardening rate, ensuring stable deformation resistance of the tube during multiple cold drawing passes. It also possesses good strength enhancement capabilities and secondary processing forming performance.

[0018] From a performance perspective, the effect of Mn on improving circumferential strength is directly related to the actual working conditions of air conditioning heat exchange tubes under internal pressure. Conventional mechanical evaluation of copper tubes is mainly based on axial tension, but circumferential stress is the dominant failure mode during service. In this invention, Mn combines with P to form a dispersed Mn-P nanoprecipitate phase, which is uniformly distributed within the crystal and produces a strong pinning effect on dislocations. Combined with a certain proportion of Sn, it can improve the overall strength of the material. In addition, the addition of Mn will change the deformation mechanism and energy storage distribution during cold drawing, and will also affect the nucleation and growth path during recrystallization recovery, thereby affecting the proportion and evolution of the tube texture, and ultimately affecting the effective circumferential / axial slip and anisotropy, thus increasing the circumferential / axial strength ratio. This improves the deformation uniformity and fracture resistance of the tube under circumferential load, ultimately achieving the same target burst pressure P as TP2 copper tubes of the same specification. m At that time, the minimum wall thickness t required for the copper alloy tube satisfies t≤0.9×t0, where t0 is the minimum wall thickness required for the TP2 copper tube to achieve the same P. m The required minimum wall thickness reduces the weight per unit length by 5% to 15%.

[0019] In terms of microstructure regulation, manganese (Mn) has a significant inhibitory effect on the segregation behavior of sn. First, Mn has a stronger occupancy ability at grain boundaries, which changes the chemical environment of grain boundaries through solute-solute interactions, thereby reducing the driving force of sn grain boundary segregation. Second, Mn can increase solidification undercooling and refine dendritic structure, shortening the microsegregation diffusion scale of sn and changing the segregation band from continuous to diffuse distribution. In addition, the regulatory effect of Mn on grain boundary migration effectively reduces the risk of aggregation and connectivity of continuous sn-rich brittle phases at grain boundaries, making the alloy microstructure more uniform and stable.

[0020] In terms of corrosion resistance, Mn exhibits a strong affinity for O / S, and under suitable smelting purification and slag / filtration conditions, it can potentially reduce the adverse effects of certain harmful impurities on corrosion resistance, thereby reducing pitting corrosion initiation sources. Furthermore, the nano-precipitates formed by the combination of Mn and P can be fully utilized. When these precipitates are dispersed within the grains, they can stabilize the microstructure and inhibit abnormal grain boundary migration. The high thermal stability of the nano-precipitates allows for preferential passivation in chlorine- and sulfur-containing media, preventing the copper matrix from dissolving too quickly and indirectly reducing the probability of grain boundaries acting as high-velocity corrosion channels. This effectively blocks the deep propagation path of anthill corrosion and plays a positive role in inhibiting localized corrosion under the combined effects of salt spray and acid salts. In addition, the refining effect of Mn on the as-cast microstructure reduces the degree of Sn-dominated dendritic segregation, eliminating pitting corrosion and intergranular sensitivity caused by micro-area potential differences. This results in excellent performance in anthill corrosion, salt spray corrosion, and electrochemical corrosion, exhibiting superior resistance to both uniform and localized corrosion.

[0021] In addition to Sn and Mn, the copper alloy composition of this invention also includes Ni and P. Ni and Cu have similar atomic sizes and are nearly infinitely soluble in each other. Ni dissolves in the copper matrix, effectively enhancing the matrix strength while maintaining high thermal conductivity. This invention strictly controls Ni within a narrow range of 0.08% to 0.20%. Below this lower limit, the strengthening and stabilizing effects are insufficient; above this upper limit, the cost increases significantly. Regarding its contribution to corrosion, Ni has high solid solubility in Cu, which improves the structure and adhesion of the corrosion product layer. During corrosion, it participates in the construction of the surface film, filling cation vacancies in Cu₂O, reducing the defect density of the film, and making it less susceptible to corrosion by Cl. - The disturbance enhances the overall barrier effect of the film; in addition, compared with some easily segregating elements, Ni tends to form a homogeneous solid solution, which helps to reduce local galvanic differences; furthermore, Ni substitutes for Cu sites or occupies holes, reducing the electronic and ionic conductivity of the film and slowing down the Cu... 2+ dissolution and O 2-The diffusion of Ni can inhibit dealloying corrosion, avoid selective dissolution, and make corrosion proceed uniformly. Furthermore, the addition of Ni can increase polarization resistance and charge transfer resistance, and slow down anodic dissolution and cathodic reduction reactions. Therefore, the addition of Ni can affect corrosion resistance, especially in salt spray corrosion environment where the corrosion depth is significantly reduced.

[0022] This invention controls the phosphorus (P) content within the range of 0.015% to 0.030%. Below 0.015%, deoxidation is incomplete and grain boundary modification is insufficient; above 0.030%, brittle Cu3P phase is easily formed, impairing cold working plasticity. During the smelting stage, P preferentially reacts with residual oxygen in the molten copper, complementing the deoxidizing effect of Mn. This ensures a dense, porosity-free billet, helps reduce the oxygen content in the melt, and decreases oxygen-related defects / inclusions, thereby improving the number of corrosion-sensitive sources (inclusions, pores) from the source.

[0023] Preferably, the minimum achievable wall thickness of the copper alloy tube is less than 0.3 mm, and more preferably 0.2 mm. It should be noted that the minimum achievable wall thickness is not the same as the minimum wall thickness t. The minimum wall thickness is calculated based on the theoretical minimum wall thickness required under a limited burst pressure, considering the material strength and tube specifications. The minimum achievable wall thickness refers to the minimum wall thickness that can be stably manufactured using the high-strength, high-corrosion-resistant copper alloy material obtained in this invention under existing process conditions. The minimum achievable wall thickness is ≤ minimum wall thickness t. A thinner wall thickness means a lighter overall weight of the heat exchanger. For weight-sensitive applications such as air conditioner outdoor units and automotive air conditioners, the weight reduction effect is significant. Furthermore, with the same heat exchange area, the heat transfer coefficient is significantly improved, enabling a more compact heat exchanger design or increasing the heat exchange capacity for the same volume.

[0024] When the wall thickness is reduced to below 0.3 mm, the load-bearing cross section of the pipe is significantly reduced, and the circumferential stress per unit area increases significantly. The high-strength and high-corrosion-resistant copper alloy material prepared by this invention provides sufficient strength reserve for ultra-thin wall design while improving the circumferential strength of the material. At the same time, the fine-grained structure makes the plastic deformation more uniform and avoids premature local failure caused by stress concentration.

[0025] Preferably, the mass percentage content of Sn, a, and the mass percentage content of Mn, b, also satisfy: 0.1 ≤ a × b × 10 6 ≤1, to control the interaction strength between the two elements, optimize their segregation behavior and final microstructure uniformity during continuous casting, thereby simultaneously achieving a comprehensive performance improvement of high strength, high corrosion resistance, and good machinability. Where a×b represents the synergistic interaction strength factor, reflecting the total intensity or total dosage of the Sn-Mn interaction, this value is multiplied by 10. 6To obtain a numerical range that is easy to express, a low product indicates that the content of either Sn or Mn is too low, and the two cannot produce an effective synergistic effect. The improvement of the alloy's circumferential strength and corrosion resistance is limited. An excessively high product indicates that the content of both Sn and Mn is high or one of them is too high, which may lead to the precipitation of brittle phases and deterioration of workability.

[0026] As a preferred method, the impurities include: O element controlled below 5 ppm; ensuring that the amount of deoxidation products generated by P and Mn is minimized; H element controlled below 2 ppm; avoiding porosity defects or intergranular microcracks caused by hydrogen evolution during annealing, ensuring the continuity and density of the grain boundary structure; and S element controlled below 10 ppm. Sulfur has a very strong affinity for Mn, and even trace amounts of sulfur will preferentially combine with Mn to form MnS inclusions. These inclusions have a significant potential difference with the copper matrix, and are very likely to become the preferred sites for pitting corrosion and anthill corrosion in chlorine- and sulfur-containing media. By reducing sulfur to below 10 ppm, the number density and size of MnS inclusions are suppressed to a negligible level, eliminating the source of corrosion.

[0027] Preferably, the corrosion resistance of copper alloy tubes meets the following requirements:

[0028] When suspended in a 0.04% formic acid aqueous solution and exposed to acidic vapors, the maximum pitting depth is no more than 50 μm after 35 days of corrosion.

[0029] A neutral salt spray test was conducted in a 5% NaCl solution atomized atmosphere with a neutral pH value. After 9 weeks of corrosion, the maximum pitting depth was no more than 100 μm.

[0030] In a 3.5% (w / v) NaCl aqueous solution at 25°C, the self-corrosion current density measured by potentiodynamic polarization is no higher than 5.0 × 10⁻⁶. -7 A / cm 2 ;

[0031] The high-strength, high-corrosion-resistant copper alloy tube provided by this invention, based on the barrier effect of the grain boundary modification layer, the passivation effect of the stable precipitated phase, and the source control of strictly controlling the sulfur content of impurities to within 10ppm, not only performs excellently in the formic acid solution corrosion test, but also, as verified by experiments, exhibits superior corrosion resistance to traditional TP2 copper tubes and existing high-strength copper tubes in harsh environments such as salt spray corrosion and electrochemical corrosion, demonstrating its comprehensive environmental adaptability.

[0032] Preferably, the microstructure of the copper alloy tube comprises: a face-centered cubic α-Cu matrix, and nanoscale precipitates with an average size of 2-20 nm and submicron precipitates with an average size of 50-200 nm uniformly distributed in the matrix; the microstructure does not contain hard and brittle second-phase particles larger than 1 μm. Sn mainly exists in the copper matrix in solid solution form, providing a basic solid solution strengthening effect, enabling the matrix to maintain a high degree of uniform deformation capability; Ni is partially dissolved in the matrix and partially participates in the formation of nanoscale precipitates; the Mn-P system nanoscale precipitates formed by the combination of Mn and P are uniformly distributed within the grains, producing a strong pinning effect on dislocations, thus extending the uniform deformation stage and delaying the necking initiation during circumferential stretching of the tube; simultaneously, the submicron precipitates formed by Mn and P are more uniformly distributed, and local strain concentrations are effectively dispersed. The synergistic effect of these two factors results in the tube exhibiting a continuous and uniform expansion deformation characteristic under high pressure during the bursting process, with the bursting pressure increased by 5%-10% compared to TP2 copper tubes of the same specification. This allows the same target burst pressure P as that of TP2 copper pipes of the same specification to be achieved. m This reduces the minimum wall thickness required for copper alloy tubes, resulting in a decrease in weight per unit length.

[0033] In the Sn-Mn-Ni-P system of this invention, Mn, P, and Sn are all elements that readily form brittle intermetallic compounds. Once these brittle phases precipitate at a macroscopic size, they will severely rupture the matrix, becoming crack sources and drastically deteriorating the alloy's processability and corrosion resistance. This invention controls the content of Sn and Mn and, in conjunction with a P-first, Mn-later smelting and purification process, ensures that elements such as Mn, P, and Sn exist as solid solution atoms or form nanoscale beneficial precipitates. This ensures that the alloy matrix is ​​a pure, single face-centered cubic α-Cu phase, avoiding performance degradation caused by coarse, hard, and brittle phases, and guiding elements such as Mn to form uniformly dispersed, nanoscale reinforcing phases.

[0034] Preferably, the copper alloy tube has a Vickers hardness of HV50~70 in the fully annealed state (grain size of about 0.015~0.020μm), which ensures that the tube has sufficient basic strength before subsequent processing such as fine drawing, bending, and flaring, and does not lose plasticity due to excessive solid solubility or residual undissolved precipitates.

[0035] Preferably, the circumferential tensile strength of the copper alloy tube is not less than 90% of the axial tensile strength, and the yield strength ratio is not greater than 0.35.

[0036] During subsequent service, air conditioning heat exchange tubes are subjected to alternating loads of 3-5 MPa, with the primary stress being circumferential tensile stress. From the perspective of plastic processing mechanics, the circumferential strength of the copper tube is tested more significantly. Therefore, characterizing the tube's strength through its circumferential strength is more representative. However, due to the presence of drawing texture, the circumferential strength of conventional copper tubes is typically only 60%-70% of its axial strength. The high-strength, high-corrosion-resistant copper alloy tube provided by this invention has a circumferential tensile strength of no less than 90% of its axial tensile strength. Through fine-grain strengthening and dispersed fine precipitate strengthening, along with their dual homogenization effect, the grain morphology tends to be equiaxed, the crystallographic texture strength is significantly reduced, and the deformation resistance of the tube tends to be consistent in different directions. Better work hardening capability allows for a larger work hardening tolerance during cold deformation, improving the forming limit and reducing the risk of processing cracks.

[0037] In this invention, the method for calculating the circumferential tensile strength of the pipe is as follows:

[0038] According to the principles of plasticity, copper tubing can be considered as a thin-walled cylinder during use, and its circumferential tensile strength (circumferential stress) (σ) h The relationship between the pressure inside the pipe and the pressure inside the pipe satisfies:

[0039] σ h =P·r / t;

[0040] Where P is the internal pressure of the pipe in MPa; r is the inner radius of the pipe; and t is the equivalent wall thickness of the pipe (during the service life of the pipe, the weakest point in the circumferential stress is the area without threads, and the bottom wall thickness of the internally threaded pipe is regarded as the equivalent wall thickness). Therefore, the circumferential tensile strength of the pipe at the time of bursting is the required value, which is obtained by substituting the bursting pressure of the pipe into the above formula.

[0041] Preferably, the burst pressure attenuation rate of the copper alloy tube after welding is no higher than 5%. In the welding process, Sn, Mn, Ni, and P, as solid solution elements, have a good solute dragging effect, which can effectively limit grain boundary expansion during heating. In addition, the nanophase dominated by Mn-containing precipitates has good stability under short-term heat exposure at 700℃. The combined effect of these two factors enables the tube to retain more than 95% of its yield strength and tensile strength after undergoing simulated welding thermal cycling, and suppress the burst pressure attenuation rate to within 5%.

[0042] The second objective of this invention is to provide a method for preparing high-strength, high-corrosion-resistant copper alloy tubes. By controlling the order of addition of each element and the holding time, and controlling the processing technology, a copper alloy tube with refined grains is obtained, which meets the requirements of modern heat exchangers for thin walls, high pressure resistance, long service life, and corrosion resistance in complex environments.

[0043] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0044] A method for preparing a high-strength, high-corrosion-resistant copper alloy tube includes the following steps:

[0045] S1 prepares raw materials according to mass percentage, melts the raw materials into liquid state under a protective atmosphere; and then obtains a billet through horizontal continuous casting.

[0046] Preferably, in step S1, copper material is first melted, and then P, Sn, Ni and Mn elements are added sequentially. Mn element is added through a copper-manganese master alloy. After adding the copper-manganese master alloy, the temperature is maintained at 1160~1175℃ for 20~40min to ensure that Mn is fully dissolved and initially combined with P to form Mn-P compound precursor, laying the foundation for the subsequent precipitation of nanoscale corrosion-resistant phase.

[0047] Preferably, in step S1, smelting is carried out at 1170~1190℃ under nitrogen protection. P element is added through phosphorus copper master alloy. After addition, it preferentially reacts with dissolved oxygen in the copper liquid to purify the copper liquid and adjust the composition. Sn element is added through tin ingots, Ni element is added through copper nickel master alloy, and finally copper manganese master alloy is added. The addition position is controlled to be close to the bottom of the copper liquid to prevent premature oxidation. This invention reduces the original oxygen and impurities in the copper liquid to an extremely low level by adding phosphorus copper. During the process, a protective layer of charcoal or inert gas is covered on the surface of the copper liquid to isolate air and prevent surface oxidation. Deoxidation products and slag are removed in time. After Mn is added, further deoxygenation and slag formation are achieved. The slag floats to the top with the copper liquid. After the copper liquid settles, the slag is removed to avoid it remaining in the billet and affecting subsequent processing.

[0048] During the smelting process, it is crucial to control the purity of the molten copper. When the manganese (Mn) content exceeds 1000 ppm, a dense manganese oxide film easily forms on the surface of the molten copper. This film has a melting point much higher than that of the molten copper, which impairs the fluidity of the copper and prevents the liquid from melting evenly during casting. This results in cold shuts on the surface of the cast billet, i.e., incompletely fused folds or grooves. This not only causes surface defects but also poses a risk of subsequent drawing cracks and corrosion initiation. This invention ensures that Mn effectively enters the matrix without forming a surface oxide film by strictly controlling the addition sequence and protective atmosphere, thereby obtaining a cast billet with excellent surface quality and internal purity, providing defect-free billets for subsequent ultra-thin-wall processing.

[0049] Preferably, the casting temperature of horizontal continuous casting is 1160~1175℃, and the casting speed is controlled at 280~450mm / min to obtain tubular billets.

[0050] S2 performs multi-pass continuous rolling on the tube blank, with a total deformation rate of >90%, to obtain tubes of the target size.

[0051] Preferably, in step S2, the tubular billet is milled to remove surface oxide scale and defects. The milled billet is then subjected to three-roll spinning at room temperature, followed by tandem drawing with a total processing rate of >70% to obtain intermediate tubing. The intermediate tubing is then subjected to disc drawing with an elongation multiple of 1.3 to 1.6 per pass. This high-deformation cold working process breaks up the as-cast structure, introduces a large number of dislocations and deformation energy, and provides a driving force for subsequent recrystallization to refine the grains. At the same time, it ensures that the Mn-P compound precursor is uniformly dispersed during the deformation process.

[0052] S3 performs online continuous annealing on the pipe during or after cold working. The annealing process achieves rapid heating and cooling of the pipe by controlling the current parameters.

[0053] As a preferred method, online continuous annealing specifically involves: using a closed-loop control system to monitor and adjust the current or induction heating power in real time, allowing the pipe to recrystallize within the target temperature window; subsequently, rapid cooling terminates the annealing process. This step aims to eliminate work hardening and control grain size. During online continuous annealing, the annealing current is controlled at 3700~5100A, the annealing temperature at 450~530℃, and the annealing time at less than 10 seconds, enabling the pipe to recrystallize in a very short time, obtaining a uniform and fine grain structure. Simultaneously, it promotes the precipitation of Mn-P compound precursors into nanoscale dispersed particles at this temperature, pinning grain boundaries to prevent grain growth. After annealing, the pipe achieves a tensile strength of over 250MPa and an elongation of ≥45%, providing a structural basis with both strength and plasticity for subsequent internal thread forming.

[0054] S4 performs internal thread forming on the annealed tube, followed by finishing, rewinding, and final annealing to obtain the finished copper alloy tube.

[0055] Preferably, after online annealing, the pipe is used to form thread teeth on its inner wall by spinning or rolling using an internal thread forming machine. The formed internal thread pipe is then finished and rewound to correct the straightness and dimensional accuracy of the pipe. Finally, the finished product is annealed to obtain a uniform recrystallized structure and suitable mechanical properties, so that the grain size of the final product is stable in the range of 15~20μm, the Mn-P particles are dispersed, and the circumferential tensile strength reaches more than 90% of the axial strength. After inspection and length cutting, a high-strength and high-corrosion-resistant copper alloy pipe is obtained.

[0056] Compared to conventional hot extrusion high-temperature forming, this invention employs a process combining horizontal continuous casting, cold working, and rapid online annealing. This avoids the high-temperature deformation stage, freeing alloy design from the constraints of high Sn content requirements during hot working. This enables the design of low-Sn alloy systems. Furthermore, cold working breaks up the as-cast structure and introduces a large amount of deformation energy storage. Rapid annealing completes recrystallization in a very short time, effectively inhibiting grain growth and resulting in a uniform fine-grained structure of 15-20 μm. During annealing, nano-scale Mn-P dispersed strengthening phases precipitate simultaneously. Combined with deep purification control during the casting stage to maintain O≤5ppm, H≤2ppm, and S≤10ppm in the matrix, the minimum achievable wall thickness of the copper alloy tube can reach below 0.3mm, with circumferential tensile strength exceeding 90% of the axial strength. Excellent internal pressure resistance can be achieved without relying on complex texture control. Moreover, it features high material utilization, low energy consumption, no surface oxidation, and high dimensional accuracy. Combined with the raw material cost advantages of the low-Sn alloy system, the overall manufacturing cost is significantly reduced.

[0057] The third objective of this invention is to provide a method for applying high-strength, high-corrosion-resistant copper alloy tubes, using these tubes as heat pipes in air conditioning heat exchangers.

[0058] Compared with the prior art, the present invention has the following beneficial effects:

[0059] This invention replaces expensive Sn with a portion of inexpensive Mn, combined with a Cu content greater than 99.5% and strict control over unavoidable impurities. While ensuring strength reaches or even surpasses that of traditional high-tin copper alloys, it effectively reduces raw material costs. Furthermore, by simultaneously controlling the Sn and Mn content and interaction strength within a specific critical window, it achieves synergistic optimization of strength and corrosion resistance. Sn is the main solid solution strengthening and precipitation strengthening element, while Mn can form a stable Mn-P nanoscale corrosion-resistant phase, refine grain boundary structure, and enhance work hardening ability. By replacing Sn with Mn and controlling the total amount of Sn and Mn, the tendency for dendritic segregation and brittle grain boundary phase precipitation, common in high-tin copper alloys during continuous casting and rolling, is fundamentally suppressed. This results in significantly improved ductility, processability, and microstructure uniformity of the material while achieving high strength.

[0060] This invention achieves excellent circumferential strength by controlling the pure matrix during the casting stage, introducing energy storage through high deformation during the cold working stage, obtaining 15-20μm fine grains and nano-Mn-P particles through rapid recrystallization during the intermediate annealing stage, and stabilizing the microstructure during the finished product annealing stage. This allows the manufactured pipe to have a significantly reduced wall thickness while meeting the same burst pressure requirements as TP2 copper pipes, thereby reducing the weight per unit length (grams per meter). This enables lightweighting for applications such as air conditioning heat exchangers, improving energy efficiency while reducing material consumption and manufacturing costs.

[0061] In this invention, the narrow-range control of Ni and P further enhances the alloy's structural stability and inherent corrosion resistance. Ni dissolves in the copper matrix, effectively improving the matrix strength while maintaining high thermal conductivity. Regarding its contribution to corrosion, Ni's high solubility in Cu improves the structure and adhesion of the corrosion product layer. During corrosion, it participates in the construction of the surface film, filling cation vacancies in Cu2O, reducing the defect density of the film, and enhancing the overall barrier effect of the film. In addition, its uniform solid solution reduces the tendency for local galvanic corrosion, inhibits selective dissolution in the dealloying process, increases polarization resistance and charge transfer resistance, and significantly reduces the salt spray corrosion rate. P, on the one hand, acts as a deoxidizer, preferentially reacting with dissolved oxygen in the molten copper to generate P2O5, which escapes or enters the slag, ensuring a dense, non-porous ingot. On the other hand, it combines with Mn to form the thermodynamically highly stable Mn-P phase, giving the alloy corrosion resistance. Attached Figure Description

[0062] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0063] Figure 1 This is a microstructure diagram of the copper alloy tube in Example 4;

[0064] Figure 2 This is a graph showing the trend of the ratio of circumferential strength to axial tensile strength in Example 2;

[0065] Figure 3 The electrochemical corrosion potential polarization curves for Examples 4-3, 4-4, and 4-5 are shown.

[0066] Figure 4 This is a diagram showing the circumferential strength-axial strength relationship of the pipe in Example 4;

[0067] Figure 5 The Σ3 twin distribution of the copper tubing in Comparative Example 1 is shown.

[0068] Figure 6 Summary of twinning fractions for the copper tubing in Comparative Example 1;

[0069] Figure 7 The distribution of Σ3 twins in the copper tubing of Examples 1-5 is shown.

[0070] Figure 8 This is a summary of the twinning fractions of the copper tubing in Examples 1-5;

[0071] Figure 9The distribution of Σ3 twins in the copper tubing of Example 4-1;

[0072] Figure 10 Here is a summary of the twinning fraction of the copper tubing in Example 4-1;

[0073] Figure 11 The distribution of Σ3 twins in the copper tubing of Examples 1-6 is shown.

[0074] Figure 12 This is a summary of the twinning fractions of the copper tubing in Examples 1-6;

[0075] Figure 13 The distribution of Σ3 twins in the copper tubing of Example 4-3;

[0076] Figure 14 Here is a summary of the twinning fraction of the copper tubing in Example 4-3;

[0077] Figure 15 Curves showing the effect of annealing at different high temperatures on the axial strength of copper tubes;

[0078] Figure 16 The graph shows the effect of annealing at different high temperatures on the circumferential strength of copper tubes. Detailed Implementation

[0079] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the specific implementation methods, features, and effects of a high-strength, high-corrosion-resistant copper alloy tube, its preparation method, and its application according to the present invention are described in detail below. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0080] The testing methods and standards in this invention include:

[0081] (1) Axial tensile properties (axial tensile strength, yield strength, elongation):

[0082] According to the GB / T 228.1-2021 test standard, at room temperature, a 50mm standard tensile specimen is cut along the axial direction of the pipe and subjected to uniaxial tensile testing on a universal testing machine at a tensile rate of 50mm / min until the specimen breaks. The stress-strain curve is recorded, and the tensile strength, yield strength, and elongation after fracture are calculated.

[0083] (2) Circumferential tensile strength:

[0084] The circumferential tensile strength (σh) is calculated based on the relationship between the circumferential tensile strength (σh) and the internal pressure of the pipe: σh = P·r / t, where P is the internal pressure of the pipe in MPa; r is the inner radius of the pipe; and t is the equivalent wall thickness of the pipe (the bottom wall thickness of the internally threaded pipe is considered as the equivalent wall thickness). Substituting the burst pressure of the pipe into the above formula, the circumferential tensile strength of the pipe can be obtained.

[0085] (3) Average hardness:

[0086] According to the GB / T 4340.1-2009 test standard, a Vickers hardness tester is used with a test force of 0.5 kgf. A diamond indenter in the shape of a square pyramid is pressed into the smooth cross-section or outer surface of the pipe. After holding for 3 seconds, the test force is removed, the diagonal length of the indentation is measured, and the hardness is calculated. Then, the average hardness is calculated by taking the average value of 3 points.

[0087] (4) Bursting pressure:

[0088] According to the GB / T 241-2007 test standard, the two ends of a straight pipe section of the specified length are sealed and placed in a hydraulic burst test bench. The pressure is increased to 6.5 MPa at a rate of 0.5 MPa / s, and after holding the pressure for 120 seconds, the burst is carried out directly. The highest pressure value at the moment of rupture is recorded as the burst pressure.

[0089] (5) Post-weld burst pressure:

[0090] The welding process refers to GB / T 11363-2020 standard; the post-weld burst test method is the same as item 4. Weld the pipe and then conduct the above burst pressure test on the pipe section containing the weld. The attenuation rate is the ratio of the difference between the post-weld burst pressure and the burst pressure of the base material (before welding) to the burst pressure of the base material, expressed as a percentage.

[0091] (6) Mass per unit length (grams):

[0092] According to GB / T 31059-2014, a 1-meter section of pipe sample should be cut and weighed using a precision electronic balance, accurate to 0.01 grams.

[0093] (7) Formic acid corrosion test:

[0094] Following industry-standard methods, the test copper tube was suspended above a 0.04% formic acid aqueous solution and exposed to acidic vapors. After 35 days of corrosion, the maximum pitting depth on the surface was observed and measured under a metallographic microscope.

[0095] (8) Neutral salt spray test:

[0096] Following industry-standard methods, the sample was placed in a salt spray chamber, and a neutral 5% NaCl solution was continuously or intermittently sprayed onto its surface. The laboratory temperature of the salt spray chamber was maintained at 35°C, the saturation tank temperature at 47°C, and the spray pressure at 0.8~1 kgf / cm². 2 Nine weeks after corrosion, the maximum pitting depth on the surface was observed and measured under a metallographic microscope.

[0097] (9) Electrochemical corrosion test (self-corrosion current density):

[0098] Referring to GB / T 24196-2009 standard, an electrochemical workstation was used with a three-electrode system (the working electrode was the test sample, the reference electrode was a saturated calomel electrode or a silver / silver chloride electrode, and the counter electrode was a platinum sheet). Potentiodynamic polarization scanning was performed in a 3.5% NaCl solution at 25℃. The polarization curve was analyzed by linear polarization method, and the cross-sectional area was calculated from the pipe's per-mcg weight. The self-corrosion current density (icorr) was calculated by dividing the current by the cross-sectional area.

[0099] (10) Expansion rate:

[0100] According to the GB / T 242-2007 test standard, the cone is 60°, and the opening is flared to the specified flaring ratio to check for cracks.

[0101] Comparative Example 1: Conventional TP2 Copper Pipe

[0102] TP2 copper tubes (code C12200) were manufactured according to the national standard GB / T1527-2017, with a nominal composition of Cu ≥ 99.9%, P: 0.025%, and the balance being unavoidable impurities. Internally threaded tubes with an outer diameter of 7.0 mm and a wall thickness of 0.25 mm were produced using conventional continuous casting, rolling, drawing, and annealing processes. Performance tests were conducted on the above tubes, and the results are shown in Table 1 below.

[0103] Table 1: Performance test results in Comparative Example 1

[0104]

[0105] Example 1: Comparative test of mechanical properties of Cu-Sn-P alloy with tin added alone

[0106] This embodiment aims to verify the effect of adding tin alone on the mechanical properties of copper alloys, in order to verify the synergistic effect of the multi-element microalloying design of the present invention. The experimental design covered tin content gradients of 500ppm, 1500ppm, 2000ppm, 3000ppm, 3500ppm, and 5000ppm (i.e., 0.05%, 0.15%, 0.2%, 0.3%, 0.35%, and 0.5%), with an appropriate amount of phosphor bronze added for deep deoxidation, but without the addition of Mn and Ni elements.

[0107] Experimental materials and preparation methods:

[0108] S1 prepares raw materials according to mass percentage, using 99.99% high-purity electrolytic copper as the base material, adding pure tin blocks and phosphorus copper master alloy according to the designed composition. Melting is carried out in a medium-frequency induction furnace under nitrogen protection, with the melting temperature controlled at 1180℃. During the melting process, the copper material is melted first. After the copper liquid is completely melted, phosphorus copper master alloy is added for deep deoxidation. The phosphorus addition is controlled at 250ppm. After deoxidation is completed, tin blocks are added and stirred evenly. The surface of the copper liquid is covered with charcoal for protection, and slag is removed in time. After settling, the billet is obtained by horizontal continuous casting. The casting temperature of horizontal continuous casting is 1170℃, and the casting speed is controlled at 300mm / min.

[0109] S2 performs multi-pass continuous rolling on the tube blank. Specifically, the tubular casting blank is milled to remove surface oxide scale and defects. The milled casting blank is then subjected to three-roll rotary rolling at room temperature, followed by continuous drawing. The total processing rate is controlled at 75% to obtain intermediate tubes. The intermediate tubes are then coiled and drawn, with an elongation multiple of 1.5 per pass, to obtain tubes of the target size.

[0110] During the cold working process, the S3 performs online continuous annealing on the pipe, controlling the annealing current to 4800A, to achieve rapid heating and cooling of the pipe; the annealing time is less than 10 seconds.

[0111] S4 performs internal thread forming on the annealed tube, followed by finishing, rewinding, and final annealing to obtain a finished copper alloy tube with an outer diameter of 7.0 mm and a wall thickness of 0.25 mm.

[0112] The specific component ratios are shown in Table 2 below.

[0113] Table 2 Comparison of elemental additions in Cu-Sn-P alloys in Example 1

[0114]

[0115] The main performance data is summarized in Table 3 below:

[0116] Table 3 Performance data of each alloy combination in Example 1

[0117]

[0118] The test results show that, in the range of 500~5000ppm, the tensile strength, yield strength, hardness and burst pressure of Cu-Sn binary alloy tubes all increase monotonically with the increase of tin content. The elongation and flaring rate of the material generally show a downward trend. When the Sn content exceeds 3500ppm, the decrease in elongation intensifies and the flaring rate also drops to 51%. This indicates that high Sn content leads to a reduction in the plasticity reserve of the material, a deterioration in cold working formability (such as flaring and bending), and a greater risk of cracking in subsequent processing or assembly.

[0119] Example 2: Comparative Test of Mechanical Properties and Corrosion Resistance of Cu-Mn-P Alloy with Manganese Added Alone

[0120] This embodiment aims to investigate the effect of adding manganese alone on the mechanical properties of copper alloys, particularly the circumferential strength and the circumferential / axial strength ratio, to verify the irreplaceable synergistic effect of manganese with tin, nickel, and phosphorus in the multi-element microalloying design of this invention. The experimental design included manganese content gradients covering 30ppm, 50ppm, 100ppm, 200ppm, 500ppm, and 1000ppm (i.e., 0.003%, 0.005%, 0.01%, 0.02%, 0.05%, and 0.1%), without the addition of Sn or Ni.

[0121] The experimental materials and preparation methods are the same as in Example 1, except that a copper-manganese master alloy is added after the phosphorus copper master alloy. Other details are not described here.

[0122] The specific component ratios are shown in Table 4 below.

[0123] Table 4 Comparison of elemental additions in Cu-Mn-P alloys in Example 2

[0124]

[0125] The test results are summarized in Table 5.

[0126] Table 5. Performance data of each alloy combination in Example 2

[0127]

[0128] Experimental results show that Mn content has a significant impact on the circumferential / axial strength ratio of copper tubes. The axial tensile strength of the tube generally increases with increasing Mn content, ranging from 240 MPa to 256 MPa; simultaneously, the circumferential strength also increases, ranging from 172 MPa to 238 MPa. During this process, the ratio of circumferential strength to axial tensile strength... It also shows an overall increasing trend, with a range of approximately 0.72 to 0.93. The results are as follows... Figure 2As shown, the circumferential / axial strength ratio increases continuously with increasing Mn content: reaching 0.79 at 200 ppm Mn content, 0.84 at 500 ppm Mn content, and further increasing to 0.93 at 1000 ppm Mn content. It is noteworthy that a circumferential / axial strength ratio of 0.93 was previously only observed in high-tin alloys with approximately 5000 ppm Sn added alone. This example demonstrates that adding approximately 1000 ppm Mn alone can achieve the same circumferential strengthening effect. This indicates that Mn is far more efficient than Sn in improving the circumferential isotropy of copper tubes. The mechanism lies in Mn significantly refining recrystallized grains, promoting grain equiaxation, and suppressing the formation of undesirable textures. However, the cost of adding Mn alone is a significant decrease in material plasticity, making it difficult to meet the stringent plasticity requirements of secondary processing such as flaring and bending of air conditioning heat exchanger tubes. Furthermore, even with the addition of 1000 ppm Mn, the strength increase is limited and cannot meet the strength requirements of high-strength tubes.

[0129] The effect of manganese (Mn) on improving corrosion resistance was verified in this embodiment. With increasing Mn content, the pitting depth in formic acid corrosion showed a trend of first improving and then worsening. Adding an appropriate amount of Mn helps refine the grains, making corrosion more uniform and slightly reducing the pitting depth. However, when the Mn content is too high, due to the negative electrode potential of Mn, it is more likely to selectively dissolve as an anolyte in the formic acid environment, thus exacerbating localized corrosion and increasing the pitting depth. The neutral salt spray corrosion depth also showed an inflection point of first decreasing and then increasing. An appropriate amount of Mn helps form a denser initial oxide film, improving corrosion resistance in the short term. However, excessive Mn leads to increased internal stress and decreased stability of the oxide film, and may form a Mn-rich cathodic second phase, accelerating the corrosion of the matrix under chloride ion penetration, thus reducing salt spray resistance. The self-corrosion current density in electrochemical corrosion showed a trend of first decreasing and then significantly increasing. Low Mn content slightly reduced the corrosion current by refining the microstructure. However, as the Mn content continues to increase, its solid solubility in the copper matrix is ​​limited, and it tends to segregate at grain boundaries or defects, forming micro-couples, which significantly accelerates the cathodic reaction process and leads to a sharp increase in the overall electrochemical corrosion rate.

[0130] Example 3: Performance Comparison Test of Cu-Sn-Ni-P Multi-element Copper Alloy Pipes

[0131] This embodiment aims to investigate the effect of a manganese-free Cu-Sn-Ni-P quaternary alloy on the comprehensive performance of copper alloy pipes. This verifies the irreplaceable key role of manganese in the multi-element synergistic system of this invention. Specifically, three sets of manganese-free comparative alloys were designed, with Sn, Ni, and P contents all falling within the scope of the claims of this invention. Their compositions are shown in Table 6 below. Using the exact same experimental materials and preparation process as in Example 1, finished pipes of the same specifications were obtained.

[0132] Table 6. Comparison of elemental additions in Cu-Sn-Ni-P alloys in Example 3.

[0133]

[0134] The performance was evaluated according to the same test criteria as in Example 1, and the test results are summarized in Table 7.

[0135] Table 7 Performance data of each alloy combination in Example 3

[0136]

[0137] Data shows that in Example 3 of this application, the tensile strength of the three Cu-Sn-Ni-P alloys ranged from 254 to 257 MPa, increasing with increasing Sn content, indicating that Sn is the main strengthening element. Specifically, regarding circumferential strength, in Example 3-1, even with Sn increased to 0.20%, the circumferential / axial ratio failed to exceed 0.85, indirectly demonstrating that manganese has an independent functional dimension in texture control. In terms of post-weld performance, the burst pressure decay rate of the three alloys was as high as 6.1% to 6.9%, indicating insufficient post-weld structural stability and significant loss of load-bearing capacity. Corrosion resistance also showed significant differences: the maximum pitting depth in formic acid corrosion was 69–74 μm, and the maximum pitting depth in neutral salt spray corrosion was 130–135 μm, with a self-corrosion current density of 4.8 × 10⁻⁶. -7 ~5.1×10 -7 A / cm 2 All of these values ​​are higher than the values ​​specified in this application; this indicates that alloy systems relying solely on Sn, Ni, and P without the participation of Mn have significant shortcomings in multiple dimensions such as circumferential isotropy, post-weld stability, and corrosion resistance, and cannot simultaneously meet the comprehensive material requirements of high-performance air conditioning heat exchange tubes.

[0138] Example 4: Performance Comparison Test of Cu-Sn-Mn-Ni-P Multi-element Copper Alloy Pipes

[0139] This embodiment aims to verify the influence of different proportions of tin, manganese, nickel, and phosphorus on the comprehensive performance of copper alloy tubing within the composition range of this invention. Six alloys were designed, all with compositions falling within the protection range of this invention. Examples 4-1, 4-2, 4-3, 4-4, 4-5, and 4-6 cover different contents of Sn and Mn, respectively. Using the exact same preparation process as in Example 1, finished tubing of the same specifications was obtained. A full performance comparison was conducted with Examples 1-6, Examples 2-6, Example 3-1, and Comparative Example 1 to verify the nonlinear synergistic effect of the multi-component microalloying system.

[0140] The experimental materials and preparation methods are the same as in Example 1, except that tin blocks, copper-nickel master alloys, and finally copper-manganese master alloys are added sequentially after the phosphorus copper master alloy. Other details are not described here.

[0141] The specific component ratios are shown in Table 8 below.

[0142] Table 8. Comparison of elemental additions in Cu-Sn-Mn-Ni-P alloys in Example 4

[0143]

[0144] The performance was evaluated according to the same test criteria as in Example 1, and the test results are summarized in Table 9.

[0145] Table 9. Performance data of each alloy combination in Example 4

[0146]

[0147] The data shows that the alloy properties in Example 4 of this application are far superior to any binary comparative sample, fully demonstrating that the Sn-Mn-Ni-P quaternary system has a strong positive interactive strengthening effect, such as... Figure 1 The image shown is a microstructure diagram of the copper alloy tube in Example 4. Figure 3 The figures shown are electrochemical corrosion test diagrams for Examples 4-3, 4-4, and 4-5.

[0148] Compared to Examples 1-6, Example 4 achieves the same or even higher circumferential strength while significantly optimizing all corrosion resistance indicators. In Examples 1-6, the high tin content of 0.5% leads to severe grain boundary segregation and precipitation of brittle Cu3Sn phase, which is the reason for poor plasticity and deterioration of corrosion resistance, especially anthill corrosion and electrochemical corrosion at grain boundaries. In Example 4 of this invention, the addition of an appropriate amount of Mn, as in Example 4-2, preferentially segregates at grain boundaries, suppressing Sn grain boundary segregation and allowing Sn to be fully retained within the grains to exert a solid solution strengthening effect. At the same time, Mn refines the grains and regulates the grain boundary structure, eliminating the risk of continuous brittle phase precipitation at grain boundaries, thereby significantly improving plasticity and toughness while maintaining high strength. Furthermore, Ni, in synergy with elements such as P and Sn, promotes the formation of a denser and more stable composite passivation film on the pipe surface, effectively improving the pipe's resistance to salt spray corrosion and electrochemical corrosion.

[0149] Compared to Examples 2-6, Example 4 achieved higher circumferential strength and significantly improved corrosion resistance with a lower total alloy element content. The fundamental reason lies in the synergistic strengthening effect of Sn-Mn. Although the solid solution strengthening effect of Mn in copper is weaker than that of Sn, and the strengthening efficiency relying solely on Mn is relatively low, Mn has a grain-refining effect, causing a grain-refining strengthening effect; furthermore, Mn can effectively suppress Sn segregation. Therefore, in Example 4, Mn was used to replace part of the Sn, thereby achieving higher strength with a lower element content. However, excessive Mn can impair plasticity and cause corrosion problems. When Mn exceeds a certain content, due to its negative potential, it becomes the anode of the corrosion galvanic cell, accelerating electrochemical corrosion and localized corrosion. In Example 4, the presence of Sn, Ni, and P altered the alloy's potential and surface film properties, placing Mn in a more stable chemical environment to fully exert its role in purifying grain boundaries and refining grains, while its negative electrochemical activity was effectively suppressed, thus achieving a comprehensive improvement in corrosion resistance.

[0150] Compared to Example 3-1, Examples 4-1 and 4-4 of the present invention are similar in proportion, the difference being that Example 4-1 contains a higher amount of manganese, meaning that the strength level of the copper tube increases significantly after the addition of manganese. Furthermore, the circumferential / axial strength ratio is significantly improved, indicating that manganese plays a role in suppressing recrystallization texture and promoting equiaxed grains that cannot be replaced by tin or nickel. Regarding corrosion performance, although the corrosion resistance of the manganese-free alloy in Example 3 is better than that of TP2 copper, it still falls significantly short of the material properties described in this application. This is because the manganese-free system cannot form a Mn-P corrosion-resistant phase, thus failing to achieve the effects described in this application.

[0151] Compared with Comparative Example 1, the performance of Example 4 of the present invention stems from the optimization of the microstructure and surface state brought about by the Sn, Mn, Ni, and P multi-element microalloying system. Sn atoms are dissolved in the copper lattice, producing strong lattice distortion, which significantly improves the matrix strength. Mn not only has a slight solid solution strengthening effect, but more importantly, it can refine grains and purify grain boundaries. This suppresses the grain boundary segregation of Sn, making the strengthening more uniform, thereby significantly improving the circumferential / axial strength ratio, making it close to 1, and improving plasticity. Ni can be infinitely dissolved in copper, further stabilizing the solid solution and improving the thermal stability of the alloy, which helps to maintain the performance after processing. In the present invention, the grain boundary purification and grain refinement effect of Mn reduces the chemical composition segregation and microscopic defects at the grain boundaries, making corrosion more uniform and greatly suppressing the occurrence and development of local pitting corrosion.

[0152] The corresponding circumferential strength-axial strength relationship for the pipe in Example 4 is as follows: Figure 4As shown, the circumferential / axial strength ratio is a core indicator of the pipe's internal pressure bearing capacity. In Examples 4-1 to 4-6, this ratio consistently exceeds 0.90. This invention achieves the isotropic level achievable by adding only 1000 ppm manganese or 5000 ppm tin with a small amount of manganese combined with tin, fully demonstrating the synergistic effect of multi-element synergy in texture suppression and grain equiaxation. Observations revealed that the overall circumferential strength of the pipe can exceed 251 MPa, with the circumferential strength to axial strength ratio ranging from 0.97 to 1.04; while the corresponding axial strength ranges from 256 MPa to 265 MPa. This indicates that with high element content, if properly controlled, although the increase in axial strength is relatively small, the circumferential strength of the pipe can be significantly improved, resulting in a significant improvement in the pipe's pressure resistance.

[0153] Example 5: Wall Thickness Limit Test

[0154] The alloy was prepared according to the component ratio of Examples 4-3, wherein the alloy comprises: Sn: 0.03%, Mn: 0.06%, Ni: 0.1%, P: 0.025%, with the balance being Cu and unavoidable impurities, and the circumferential tensile strength is 273 MPa.

[0155] TP2 copper tubes of the same specifications as Comparative Example 1 were selected as the comparison benchmark. Their nominal composition is: Cu≥99.9%, P:0.025%, with the balance being unavoidable impurities, and the circumferential tensile strength is 174MPa.

[0156] According to σ h =P·r / t Calculate the minimum pipe wall thickness under the same burst pressure:

[0157] Where, σ h For circumferential strength, calculated according to the above axial tensile strength, P is the internal pressure of the pipe, set to 13MPa; r is the inner radius of the pipe, set to 5mm; and t is the equivalent wall thickness of the pipe.

[0158] Table 10 Minimum wall thickness required for both materials to achieve this target pressure.

[0159]

[0160] Calculation results show that when the same burst pressure of 13MPa is achieved, the minimum wall thickness required for the high-strength pipe of this invention is only 62% of that of TP2 copper pipe, which meets the requirement of t≤0.9×t0.

[0161] To further verify this, alloy pipes of Examples 4-3 with different wall thicknesses were prepared and subjected to hydrostatic burst tests, as shown in Table 11 below:

[0162] Table 11. Hydrostatic burst tests on alloy pipes of different wall thicknesses (Examples 4-3).

[0163]

[0164] The test results show that the alloy in Example 4-3 can still achieve a burst pressure of over 13.7 MPa when the wall thickness is 0.25 mm, which verifies the reliability of the calculation results.

[0165] To determine the minimum achievable wall thickness of the alloy in Examples 4-3, batch trials were conducted with the wall thickness gradually reduced, while maintaining a yield rate of ≥90%, with each batch consisting of 1000 meters.

[0166] Table 12 Minimum achievable wall thickness of alloys in Examples 4-3

[0167]

[0168] Taking into account both process stability and economy, the minimum achievable wall thickness of this alloy is determined to be 0.20 mm.

[0169] Example 6: Copper content control test

[0170] The present invention will be described in detail below with reference to specific embodiments. The high-strength and high-corrosion-resistant copper alloy tube provided by the present invention has the following basic composition: Sn: 0.03%~0.25%, Mn: 0.003%~0.10%, Ni: 0.08%~0.20%, P: 0.015%~0.030%, with the balance being Cu and unavoidable impurities, and the Cu content being greater than 99.5%. In order to optimize performance, the impurity content is strictly controlled, so that the O element is controlled within 5ppm; the H element is controlled within 2ppm; and the S element is controlled within 10ppm.

[0171] The alloy raw materials were prepared according to the following mass percentages: Sn: 0.03%, Mn: 0.06%, Ni: 0.1%, P: 0.025%, with the balance being Cu. High-purity electrolytic copper raw materials were used, and a strict impurity control process was implemented. Deep deoxidation and multiple slag removal processes were employed during the smelting process, and the protective atmosphere was strictly controlled. The final product, after testing, had the following impurity contents: O: 5ppm, H: 2ppm, S: 9ppm, and approximately 15ppm of associated impurities such as As, Bi, Sb, Pb, Se, and Te. The copper content was 99.78%. Using the exact same preparation process as in Example 1, finished pipes of the same specifications were obtained, designated as Example 6-1.

[0172] Using the same Sn, Mn, Ni, and P elements, but with the Sn content increased to 0.50%, specifically Sn: 0.50%, Mn: 0.06%, Ni: 0.1%, P: 0.025%, ordinary electrolytic copper raw materials were used. No strict impurity control process was employed, and deep deoxidation and multiple slag removal were not used during smelting. The protective atmosphere control was relatively lenient. The final product contained the following impurities: O: 85ppm, H: 12ppm, S: 35ppm, and approximately 180ppm of associated impurities such as As, Bi, Sb, Pb, Se, and Te. The copper content was 99.28%. Using the exact same preparation process as Example 1, finished pipes of the same specifications were obtained, designated as Example 6-2.

[0173] Table 13 Performance Comparison Test

[0174]

[0175] As the data above shows, the increased impurity content in Example 6-2 significantly reduced the purity compared to Example 6-1. This indicates that impurity elements tend to segregate at grain boundaries, reducing the activation energy for grain boundary migration and promoting grain growth during recrystallization. The high-purity matrix eliminates the grain boundary lubrication effect, effectively suppressing grain growth. Combined with the pinning effect of Mn-P particles, a finer and more uniform grain structure can be obtained. In ultra-thin-walled tubes (≤0.3mm), any micron-sized inclusion can become a crack source, leading to processing cracking or service failure. The high-purity matrix eliminates this hidden danger at the source, providing support for achieving ultra-thin walls.

[0176] It can also be seen that when the Sn content is too high, although the axial strength is improved, it comes at the cost of sacrificing circumferential performance, corrosion resistance, and processability, ultimately leading to a deterioration in the overall performance of the material. This is because when the Sn content exceeds 0.25%, micro-segregation occurs during solidification, accumulating at grain boundaries and between dendrites, resulting in weakened grain boundaries, reduced circumferential load-bearing capacity, uneven deformation, localized stress concentration, and increased anisotropy. Mn has a significant inhibitory effect on Sn segregation, but at a Sn content of 0.5%, the compensating effect of Mn is offset by the segregation effect, failing to fully restore circumferential performance. Furthermore, Sn segregation leads to compositional inhomogeneity, and the potential difference between the segregated region and the matrix accelerates electrochemical corrosion, disrupting the uniformity of the Mn-P corrosion-resistant phase and preventing the formation of a continuous and dense protective film, thus reducing corrosion performance compared to Example 6-1. In contrast, comparing Examples 1-6, the corrosion performance and circumferential strength of Example 6-2 are superior to those of Examples 1-6, fully demonstrating the effect of manganese.

[0177] Other performance analysis:

[0178] 1. Twin analysis:

[0179] Based on this, such as Figures 5-14 As shown, the twin distribution of the pipes in Comparative Example 1, Examples 1-5, Examples 1-6, Examples 4-1, Examples 4-3, and Control Example 1 was examined. In the bar chart, red represents Σ3 lattice grain boundaries (commonly referred to as Σ3 twin boundaries), and purple represents Σ9 lattice grain boundaries. Figure 5 and Figure 6 It can be seen that in Comparative Example 1, the Sn content is 0 ppm, the matrix grains are dominant, and the Σ3 twin fraction is relatively high, reaching 55% overall, with relatively uniform grain size. Figure 7 and Figure 8 It can be seen that the proportion of Σ3 twin boundaries in Examples 1-5 decreased to 29%, from Figure 9 and Figure 10 It can be seen that the proportion of Σ3 twin boundaries in Example 4-1 is reduced to 28%, from Figure 11 and Figure 12 It can be seen that the proportion of Σ3 twin boundaries in Examples 1-6 is reduced to 25%, from Figure 13 and Figure 14 It can be seen that the proportion of Σ3 twin boundaries in Examples 4-3 decreased to 39%. This indicates that pure copper easily forms a large number of annealing twins during recrystallization annealing, resulting in uniform grain size. In contrast, the proportion of Σ3 twin boundaries in all examples with added Sn element showed a significant decrease: this indicates that the addition of Sn element inhibited the formation of annealing twins. This is because Sn, after dissolving in the copper matrix, alters the grain boundary migration behavior during recrystallization, causing an evolution in the distribution of grain boundary types. The reduction in the proportion of Σ3 twin boundaries means a decrease in conventional annealing twins and a more complex grain boundary structure. This change in the distribution of grain boundary characteristics helps to reduce the electrochemical difference between twin boundaries and the matrix, improving the corrosion resistance of the material. This invention, by reducing the Sn content and increasing the Mn content, achieves the effect equivalent to pure high-Sn content copper pipes, and the overall proportion of Σ3 twin boundaries in the pipe is lower than that of ordinary TP2 copper pipes, thus achieving synergistic optimization of mechanical properties and corrosion resistance.

[0180] 2. High-temperature softening performance analysis:

[0181] The effect of annealing at different high temperatures on the performance of copper tubes was tested. Specifically, the alloy was instantaneously heated to the annealing temperature, held at that temperature for 10 minutes, and then air-cooled. The circumferential tensile strength and circumferential compressive strength of copper tubes made from TP2 copper alloy and the copper alloy prepared in Example 4-1 were tested at 450℃, 700℃, 800℃, and 900℃, respectively, as well as at the same temperature. Tubes annealed at 450℃ were used as normal finished products for performance comparison with tubes under other high-temperature conditions. The performance under softening at 700℃, 800℃, and 900℃ was then analyzed. The results are as follows: Figures 15 to 16 As shown.

[0182] Among them, such as Figure 15 As shown, throughout the entire tested temperature range (450℃~900℃), the circumferential tensile strength of the copper tube of the present invention was consistently higher than that of the TP2 copper tube. For example, near the critical brazing temperature (700~800℃), the strength of the copper tube of the present invention remained at 215~225MPa, while that of the TP2 copper tube had dropped to 175~185MPa, a strength advantage of over 40MPa. As the annealing temperature increased, the strength of both materials decreased, but the rate and magnitude of the decrease in the TP2 copper tube were much greater than those of the copper tube of the present invention. At a high temperature of 900℃, the strength of the TP2 copper tube plummeted by about 27% compared to the 450℃ benchmark, while the strength of the copper tube of the present invention only decreased by about 13%. This demonstrates that the alloy of the present invention has a higher recrystallization temperature and extremely strong resistance to high-temperature softening.

[0183] like Figure 16 As shown, after annealing at different temperatures, the circumferential pressure resistance of the copper tube of this invention is consistently higher than that of the TP2 copper tube. This indicates that even after high-temperature brazing or accidental overheating, components made from the copper tube of this invention can withstand higher system pressures or provide a larger safety factor for the same pressure requirements. The pressure resistance performance of the TP2 copper tube is more sensitive to temperature, and its pressure drop curve is steeper. For example, at higher temperatures, the percentage decrease in pressure resistance of the copper tube of this invention is significantly lower than that of the TP2 copper tube. This means that under complex operating conditions or processing, the performance of the copper tube of this invention is more stable and predictable, thereby improving the long-term reliability of the entire heat exchanger system.

[0184] Therefore, it can be seen that the high-strength copper tube of this invention not only has high room temperature strength, but its high-temperature thermal stability is also far superior to that of traditional TP2 copper tubes. This high-temperature softening resistance is mainly due to two aspects: the synergistic effect of Sn and Mn elements plays a role in solute dragging and pinning, preventing grain boundary migration and limiting grain growth; the synergistic effect of Mn and P forms a certain distribution of nano-precipitates within the grains, which has excellent high-temperature stability, restricts dislocation movement, etc., ensuring that the tube can withstand high temperatures and limiting excessive grain growth. At the same time, the restriction effect of the precipitates on dislocation movement ensures material strength and guarantees high-temperature softening resistance, thus achieving lightweight, high reliability and long service life of heat exchanger products.

[0185] Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to this invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A high-strength, high-corrosion-resistant copper alloy pipe, characterized in that, By weight percentage It includes: Sn: 0.03%~0.25%, Mn: 0.003%~0.10%, Ni: 0.08%~0.20%, P: 0.015%~0.030%, with the balance being Cu and unavoidable impurities, and the Cu content is greater than 99.5%; Wherein, the mass percentage content a of Sn and the mass percentage content b of Mn satisfy: 0.20% ≤ a + 6b ≤ 0.40%; The copper alloy tube has a grain size of 0.015~0.020mm in the fully annealed state, and achieves the same target burst pressure P as the TP2 copper tube of the same specification. m At that time, the minimum wall thickness t required for the copper alloy tube satisfies t≤0.9×t0, where t0 is the minimum wall thickness t required for the TP2 copper tube to achieve the same P. m Minimum wall thickness required.

2. The high-strength, high-corrosion-resistant copper alloy pipe according to claim 1, characterized in that, The minimum achievable wall thickness of the copper alloy tube is less than 0.3 mm.

3. The high-strength, high-corrosion-resistant copper alloy pipe according to claim 1, characterized in that, The corrosion resistance of the copper alloy tube meets the following requirements: When suspended in a 0.04% formic acid aqueous solution and exposed to acidic vapors, the maximum pitting depth is no more than 50 μm after 35 days of corrosion. A neutral salt spray test was conducted in a 5% NaCl solution atomized atmosphere with a neutral pH value. After 9 weeks of corrosion, the maximum pitting depth was no more than 100 μm. In a 3.5% (w / v) NaCl aqueous solution at 25°C, the self-corrosion current density measured by potentiodynamic polarization is no higher than 5.0 × 10⁻⁶. -7 A / cm 2 .

4. The high-strength, high-corrosion-resistant copper alloy pipe according to claim 1, characterized in that, The mass percentage content a of Sn and the mass percentage content b of Mn also satisfy: 0.1 ≤ a × b × 10 6 ≤1.

5. A high-strength, high-corrosion-resistant copper alloy pipe according to claim 1, characterized in that, Of the impurities, the O element is controlled to be less than 5 ppm; the H element is controlled to be less than 2 ppm; and the S element is controlled to be less than 10 ppm.

6. A high-strength, high-corrosion-resistant copper alloy pipe according to claim 1, characterized in that, The copper alloy tube has a Vickers hardness of HV50~70 in the fully annealed state, and its circumferential tensile strength is not less than 90% of its axial tensile strength, and its yield strength ratio is not greater than 0.

35.

7. A high-strength, high-corrosion-resistant copper alloy pipe according to claim 1, characterized in that, The burst pressure attenuation rate of the copper alloy tube after welding is no higher than 5%.

8. A method for preparing a high-strength, high-corrosion-resistant copper alloy tube according to any one of claims 1 to 7, characterized in that, Includes the following steps: S1 prepares raw materials according to mass percentage, melts the raw materials into liquid state under a protective atmosphere; and then obtains a billet through horizontal continuous casting. S2 performs three-roll rotary rolling on the billet, followed by cold working of combined continuous drawing and disc drawing; S3 involves online continuous annealing of the pipe during or after cold working, wherein the annealing is achieved by controlling the current parameters to realize rapid heating and cooling of the pipe; S4 performs internal thread forming on the annealed tube, followed by finishing, rewinding, and final annealing to obtain the finished copper alloy tube.

9. The method for preparing a high-strength, high-corrosion-resistant copper alloy tube according to claim 8, characterized in that, In step S1, copper material is first melted, and then P, Sn, Ni and Mn elements are added in sequence. The Mn element is added through a copper-manganese master alloy. After adding the copper-manganese master alloy, the mixture is kept at 1160~1175℃ for 20~40 minutes.

10. The high-strength, high-corrosion-resistant copper alloy tube according to any one of claims 1 to 7 is used in an air conditioning heat exchanger.

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