Ultrasmall-diameter internal thread copper pipe and preparation method thereof

Abstract

The invention relates to the field of superfine-diameter internal-thread copper pipe preparation, in particular to a superfine-diameter internal-thread copper pipe and a preparation method thereof.The preparation method comprises the following steps that S1, an electrolytic copper plate is smelted, and obtained copper liquid is transferred into a heat preservation furnace; s2, the rare earth intermediate alloy fragments are wrapped with pure copper foil in a multi-layer mode to form a wrapping body; s3, the ladle adding time is calculated according to the continuous casting traction speed and the saw cutting position, and the ladle is immersed in molten copper of the heat preservation furnace; s4, rare earth inclusion is dynamically supplemented according to the molten copper pouring period of the smelting furnace, and the rare earth concentration is controlled; s5, cooling after continuous casting to obtain a copper alloy casting blank; and S6, the copper alloy casting blank is subjected to face milling, rolling, stretching, internal thread forming, finishing and annealing, and the superfine internal thread copper pipe with the outer diameter smaller than or equal to 4.0 mm is obtained. And multiple technical bottlenecks of a traditional process are broken through in the field of manufacturing of the ultra-thin-diameter internal-thread copper pipe.

Description

Technical Field

[0001] The present invention relates to the field of preparation of ultra - fine diameter internally threaded copper tubes, and particularly to an ultra - fine diameter internally threaded copper tube and a preparation method thereof. Background Art

[0002] Phosphorus - deoxidized copper (TP2) can be used as a main heat - exchange element, such as a dryer, a condenser, and an evaporator, etc., due to its excellent electrical conductivity, thermal conductivity, corrosion resistance, weldability, and cost advantages. Especially, the high thermal conductivity makes TP2 copper tubes widely used as components of air - conditioning systems. With the improvement of air - conditioning energy - efficiency standards and the demand for compact design, internally threaded copper tubes for air - conditioners are accelerating towards ultra - fine diameter and thin - wall directions.

[0003] In conventional technology, it is already the limit to produce an internally threaded tube with a 5 - mm diameter from TP2 copper. It is difficult to prepare an internally threaded tube with a smaller diameter. The reasons are as follows: when the outer diameter is smaller, the tube wall is thinner, and higher tensile strength and compressive strength are required to prevent rupture and tube collapse. Moreover, the grain coarsening or uneven distribution that may occur during the processing leads to cracks or deformation easily during the formation of a fine diameter, restricting the increase of the reduction ratio. Furthermore, the material needs sufficient ductility to withstand the stress during the stretching and forming processes. Therefore, in the prior art, it is mostly controlled within 5 - 7 mm.

[0004] To break through the above bottleneck, the industry has tried the substitution scheme of oxygen - free copper, that is, reducing the oxygen content (O≤5 ppm) through vacuum melting to increase the strength to more than 250 MPa, but the cost increases by 50% - 70%, and inert gas protection is required for welding, which is difficult to promote. Therefore, how to break through the 5.0 - mm fine - diameter limit and produce an internally threaded tube with a smaller outer diameter and excellent performance while retaining the low - cost and easy - processing advantages of TP2 copper is a technical problem that needs to be solved urgently.

[0005] The information disclosed in this background - art section is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or any form of implication that this information constitutes the prior art known to those skilled in the art. Summary of the Invention

[0006] The first object of the present invention is to provide a preparation method of an ultra - fine diameter internally threaded copper tube. Based on the directional regulation of the microstructure of the material by rare - earth elements and the synergistic optimization of the process, the 5.0 - mm fine - diameter limit is broken through, and when the outer diameter is ≤4.0 mm, the tensile strength is ≥240 MPa and the elongation is ≥40%.

[0007] The above technical object of the present invention is achieved through the following technical solutions:

[0008] A preparation method of an ultra - fine diameter internally threaded copper tube includes the following steps:

[0009] S1 Smelt the electrolytic copper plate and transfer the obtained copper liquid into a holding furnace;

[0010] S2 Wrap the rare earth master alloy fragments with multiple layers of pure copper foil to form a package;

[0011] S3 Calculate the addition timing of the package according to the continuous casting traction speed and the sawing position, and immerse the package into the copper liquid in the holding furnace;

[0012] S4 Dynamically supplement the rare earth package according to the pouring cycle of the melting furnace to control the rare earth concentration;

[0013] S5 Cool after continuous casting to obtain a copper alloy billet;

[0014] S6 Milling, rolling, stretching, internal thread forming, finishing, and annealing the copper alloy billet to obtain an ultra-fine diameter internal thread copper tube with an outer diameter ≤ 4.0 mm.

[0015] Preferably, in step S1, the electrolytic copper plate is melted into copper liquid in the melting furnace, and the copper liquid flows into the holding furnace through a launder.

[0016] Preferably, in step S2, the package is wrapped with ≥ 3 layers of copper foil, the diameter of the package is 30 ± 2 mm, the purity of the outer layer of copper foil is ≥ 99.9%, the thickness of the copper foil is 0.05 - 0.1 mm, and the width is 300 mm. When wrapping, the layers are misaligned by 20% to enhance the sealing performance and reduce the introduction of impurities such as Fe and S.

[0017] Preferably, in step S3, the addition timing of the package satisfies:

[0018] L1 = L0 - L2 - v × (t1 + t2);

[0019] Wherein, L1 is the length of the billet when the package is added, L0 is the set total length of a single billet (such as 21.5 m), L2 is the distance between the sawing position and the wall of the holding furnace (such as 5.8 m), v is the traction speed, t1 is the rare earth dissolution time, and t2 is the component homogenization time.

[0020] Specifically, L2 represents the spatial distance for the copper liquid to flow from the holding furnace to the sawing position; v × (t1 + t2) represents the corresponding flow distance of the copper liquid during the dissolution and homogenization processes (traction speed v × total time t1 + t2). When the billet is pulled out to a length of L1, the remaining length of the billet that has not been pulled out (L0 - L1) is exactly equal to the spatial margin required for the copper liquid to flow to the sawing position (i.e., L2 + v × (t1 + t2)), thereby ensuring that when the rare earth-containing copper liquid flows to the sawing position, the remaining length of the entire billet is exactly cut completely, thus avoiding cross-batch mixing of materials.

[0021] Preferably, the pulling speed v may vary due to process fluctuations. The control system collects the current value of v in real time and dynamically adjusts L1. If the speed suddenly increases by 10%, the system will automatically shorten L1 (reducing the rare earth lead) to avoid uneven composition caused by the copper liquid reaching the sawing position too early.

[0022] Preferably, in step S3, the rare earth is added to the breech of the insulation furnace through the breech cover of the insulation furnace.

[0023] Preferably, after the copper melt is transferred to the holding furnace, a 200-250mm thick layer of graphite flakes is laid on the surface of the copper melt to isolate it from air and prevent oxidation. The inclusions are immersed at a depth of 200-400mm. Specifically, the graphite flakes are spread apart with a copper tube, and the inclusions are placed on the surface of the copper melt. The inclusions are then pushed into the copper melt to a depth of 200-400mm below the surface of the copper melt using a copper tube. The inclusions are allowed to stand for 5 minutes, using eddy currents in the molten pool to promote dispersion of the inclusions and avoid surface disturbances that may cause composition fluctuations.

[0024] Preferably, in step S4, the rare earth inclusions are only added to the holding furnace, and the dynamic replenishment of the rare earth inclusions according to the period of pouring the molten copper from the smelting furnace means that after the smelting furnace pours the molten copper into the holding furnace, the amount of rare earth intermediate alloy replenished in the holding furnace is dynamically adjusted so that the rare earth concentration in the holding furnace is always stable within the range of ±5% of the target value, and the target rare earth concentration is 5 to 10 ppm.

[0025] Specifically, the amount of rare earth inclusions added is a combination of one-time addition and supplementary addition to ensure that the composition is stable within half an hour. After more than half an hour, due to the process requirements of the weight of the copper liquid in the holding furnace, the smelting furnace needs to dump and replenish the copper liquid. At this time, the rare earth content is reduced due to dilution; therefore, a certain amount of rare earth is added when the smelting furnace is dumped for the second time.

[0026] When adding once, the amount of master alloy m1 is calculated as A / a according to the rare earth content a in the master alloy, where A is the amount of rare earth required to be added according to the designed rare earth concentration. For example, if the rare earth content a in the master alloy is 20%, the amount of rare earth required to be added according to the designed rare earth concentration is 5ppm. If the copper content in the holding furnace is W kg, the amount of master alloy m1 required to be added is W*5 / 10 6 *10 3 / 20% = 0.025Wkg. After each Wq of copper liquid is poured, the inclusion is replenished, and the replenishment amount m2 = Wq*5 / 10 6 *10 3 / 20%=0.025Wqkg.

[0027] Preferably, the rare earth content in the rare earth master alloy accounts for 15-20 wt.%, and the balance is copper and other impurities. The rare earth metals include at least one of lanthanum, cerium, praseodymium, and neodymium. Preferably, it is lanthanum, or a mixture of lanthanum and other rare earth metals.

[0028] Preferably, in step S5, the molten copper solution dissolved with rare earth flows into the mold under static pressure. After encountering the cooling water, it solidifies on the inner wall of the mold to form a copper alloy billet.

[0029] More preferably, the water-cooled mold is cooled at a rate of 10-30 °C / s to form fine equiaxed grains (average size 20 μm), refining the grains and suppressing shrinkage cavities at the same time.

[0030] Preferably, the copper alloy billet is phosphor-deoxidized copper. Calculated by mass percentage, the phosphorus content in the copper alloy billet is 0.015-0.040 wt.%, the rare earth content is 5-10 ppm, and the rare earth / phosphorus mass ratio is 0.01-0.05. The rare earth segregates at the grain boundaries in the form of La-P-O nanoclusters, and the cluster size is 5-30 nm, covering 40-60% of the grain boundary area.

[0031] Preferably, the content of cu+Ag in the copper alloy billet is ≥99.9%, the phosphorus content is 0.015-0.030 wt.%, the content of H in the continuous casting billet does not exceed 1 ppm, the content of O does not exceed 3 ppm, and the remaining impurities include Fe, Bi, Zn, S, Sn, Ni, Pb, Sb, As, etc.

[0032] Preferably, the copper alloy billet is subjected to milling, rolling, stretching, internal thread forming, finishing, and annealing to obtain an ultra-fine diameter internal thread copper tube with an outer diameter ≤4.0 mm. Specifically, conventional processes are used. The core of this application lies in improving the adaptability of the material and the forming process through material improvement to obtain an ultra-fine diameter internal thread copper tube with an outer diameter ≤4.0 mm.

[0033] The second object of the present invention is to obtain an ultra-fine diameter internal thread copper tube with an outer diameter ≤4.0 mm by the above method, breaking through the limit of 5.0 mm fine diameter.

[0034] Preferably, the elongation of the ultra-fine diameter internal thread copper tube is ≥40%; the tensile strength is 220-255 MPa; the grain size is 0.020-0.035 mm; the flaring rate is ≥35%. The tensile strength, elongation, and grain size are synchronized to meet the standards through rare earth-phosphorus synergistic regulation.

[0035] Preferably, the outer diameter of the copper tube is 3.4 - 4.0 mm. The copper tube is provided with internal threads, and the thickness between the bottom of the tooth groove of the internal threads and the outer diameter of the copper tube is 0.17 - 0.23 mm. The internal threads have a spiral angle, and the value range of the spiral angle is 13° - 17°. The height of the thread tooth is 0.10 - 0.14 mm, the number of teeth is 38, and the weight per meter is 21 - 24 g / m. Zero eddy current defects and a pressure resistance of 24.5 MPa are achieved, reflecting the optimization effect of the internal thread flow field.

[0036] Preferably, the outer diameter of the copper tube is 3.6 ± 0.3 mm. The copper tube is provided with internal threads, and the thickness between the bottom of the tooth groove of the internal threads and the outer diameter of the copper tube is 0.19 - 0.22 mm. The height of the thread tooth is 0.10 - 0.13 mm.

[0037] Preferably, the internal threads are composed of alternating tooth grooves and teeth. The cross-section of the teeth is a triangle with an arc-shaped top, and the adjacent teeth have an inverted trapezoid-shaped tooth groove.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] Through the integrated process design of "smelting - wrapping - dynamic addition - precision machining", the present invention breaks through multiple technical bottlenecks of traditional processes in the field of manufacturing ultra-fine diameter internal thread copper tubes. Compared with the traditional method that relies on manual experience to judge the addition timing, this method establishes the mapping relationship between the copper liquid flow time and the billet length through the real-time calculation of the continuous casting traction speed and the sawing position. By real-time tracking the forming length of the billet, the time point of rare earth addition is inversely deduced, so that the copper liquid containing rare earth just completes the two key stages of dissolution and homogenization during the physical process of flowing through the holding furnace to the sawing position, and finally forms a billet that meets the set length (L0) and has a stable composition, reducing the deviation of rare earth concentration at the head and tail of a single billet and solving the micro-crack problem caused by composition fluctuations in the processing of ultra-fine diameter copper tubes.

[0040] In the present invention, the inclusion adopts a delayed dissolution setting wrapped with copper foil. By isolating the oxidizing atmosphere on the surface of the copper liquid, the recovery rate of rare earth is ≥95%. Through the free combination of the copper foil thickness and the number of layers (such as 0.05 mm × 3 layers or 0.1 mm × 2 layers), the dynamic matching of the rare earth release rate and the copper liquid flow rate is achieved. Compared with the method of directly adding in advance and relying on the operator's experience to determine the traction time, the concentration fluctuation is reduced, and the high-temperature oxidation and segregation loss of rare earth are avoided.

[0041] In the prior art, although the phosphorus content in phosphorus-deoxidized copper can deoxidize, excessive phosphorus is likely to form a brittle Cu3P phase, resulting in phosphorus segregation at grain boundaries and an increased risk of grain boundary cracking during rolling; it cannot meet the synchronous requirements for high strength and high elongation of thinner-diameter pipes. In the present invention, rare earth elements are evenly distributed during the melting stage, inhibiting the growth of α-Cu grains during continuous casting and cooling. Combining dynamic rare earth concentration control and multi-pass precision stretching, the ultimate processing with a bottom thickness of the tooth groove of 0.17 - 0.23 mm under an outer pipe diameter of 3.4 - 4.0 mm is achieved, with a wall thickness tolerance of ±0.005 mm, meeting the high-end requirements of heat dissipation copper pipes. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0043] Figure 1 It is a flowchart of a preparation method for an ultra-fine diameter inner-thread copper pipe;

[0044] Figure 2 It is a test chart for characterizing the consistency of the electrochemical corrosion performance of two groups of annealed specimens without adding rare earth and after adding rare earth;

[0045] Figure 3 For Figure 2 Based on the characterization, it is a test chart for characterizing the consistency of the electrochemical corrosion performance of the annealed specimens of copper pipes with a rare earth content of 5 ppm;

[0046] Figure 4 It is the change rate of P ions with the change of rare earth content;

[0047] Figure 5 It is the change rate of Si ions with the change of rare earth content. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0048] To further elaborate on the technical means and effects adopted by the present invention to achieve the intended invention purpose, for an ultra-fine diameter inner-thread copper pipe and its preparation method proposed according to the present invention, its specific implementation manners, features and effects are described in detail as follows. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0049] As Figure 1 shown, a preparation method for an ultra-fine diameter inner-thread copper pipe includes the following steps:

[0050] S1 Melting electrolytic copper plates and transferring the obtained copper liquid into a holding furnace;

[0051] S2 wraps the rare earth master alloy fragments with multiple layers of pure copper foil to form a package body;

[0052] S3 calculates the addition timing of the package body according to the continuous casting traction speed and the sawing position, and immerses the package body into the copper liquid in the holding furnace;

[0053] S4 dynamically supplements the rare earth package body according to the pouring cycle of the melting furnace to control the rare earth concentration;

[0054] S5 cools the continuous casting to obtain a copper alloy billet;

[0055] S6 mills, rolls, stretches, forms internal threads, finishes, and anneals the copper alloy billet to obtain an ultra-fine diameter internal threaded copper tube with an outer diameter ≤ 4.0 mm.

[0056] In the prior art, although the phosphorus content in phosphor-deoxidized copper can deoxidize, excessive phosphorus is likely to form a brittle Cu3P phase, resulting in phosphorus segregation at grain boundaries and an increased risk of grain boundary cracking during rolling; it cannot meet the synchronous requirements of higher strength and higher elongation for thinner diameter tubes. In the present invention, rare earth elements are evenly distributed during the melting stage, inhibit the growth of α-Cu grains during continuous casting and cooling, and combine dynamic rare earth concentration control and multi-pass precision stretching to achieve ultimate processing with a bottom thickness of the tooth groove of 0.17 - 0.23 mm under an outer diameter of 3.4 - 4.0 mm, a wall thickness tolerance of ±0.005 mm, meeting the high-end requirements of heat dissipation copper tubes.

[0057] The present invention obtains high-purity copper liquid (Cu ≥ 99.95%) by melting electrolytic copper plates, transfers it to the holding furnace and then introduces rare earth master alloy package bodies. The use of copper foil wrapping realizes the delayed dissolution of rare earths, matches the copper liquid flow time, and avoids segregation caused by excessively high instantaneous concentration. Among them, the package body is only added to the holding furnace, and the addition is triggered based on the real-time calculation of the traction speed and the sawing position, and dynamically supplemented according to the pouring cycle of the melting furnace to maintain the stability of the rare earth concentration, ensure the rare earth recovery rate, reduce the oxidation loss, and control the grain size to be refined to the 0.025 mm level, improving the yield rate of ultra-fine diameter internal threaded copper tubes with an outer diameter ≤ 4.0 mm.

[0058] In step S1, the electrolytic copper plates are melted into copper liquid in the melting furnace, and the copper liquid flows into the holding furnace through the launder.

[0059] In step S2, the inclusion is wrapped by ≥3 layers of copper foil. The diameter of the inclusion is 30±2 mm. The purity of the outer copper foil is ≥99.9%, the thickness of the copper foil is 0.05 - 0.1 mm, and the width is 300 mm. During wrapping, there is a 20% offset between layers to enhance the sealing performance and reduce the introduction of impurities such as Fe and S. In the present invention, the rare-earth inclusion wrapped by copper foil realizes gradient dissolution at the deep part of the copper liquid by utilizing the high thermal conductivity of copper, avoiding high-temperature oxidation, and the recovery rate is ≥95%. After the copper foil contacts the copper liquid in the holding furnace, the outer layer melts within 30 seconds, releasing a small amount of rare earth to initiate nucleation; the intermediate layer dissolves within 2 - 3 minutes, and the rare earth diffuses in the form of a Cu-RE eutectic liquid phase (melting point 950 °C); gradient dissolution is achieved, and the gap between copper foil layers (0.1 - 0.3 mm) promotes turbulent mixing, reducing the segregation index.

[0060] In the present invention, the inclusion adopts a delayed dissolution setting wrapped by copper foil. By isolating the oxidizing atmosphere on the surface of the copper liquid, the recovery rate of rare earth is ≥95%. Through the free combination of the copper foil thickness and the number of layers (such as 0.05 mm × 3 layers or 0.1 mm × 2 layers), the dynamic matching between the rare-earth release rate and the copper liquid flow rate is achieved. Compared with the method of directly adding in advance and relying on the operator's experience to determine the traction time, the concentration fluctuation is reduced, and the high-temperature oxidation and segregation loss of rare earth are avoided.

[0061] In step S3, the timing of adding the inclusion satisfies:

[0062] L1 = L0 - L2 - v×(t1 + t2);

[0063] Wherein, L1 is the length of the continuous casting billet when the inclusion is added, L0 is the set total length of a single continuous casting billet (such as 21.5 m), L2 is the distance between the sawing position and the furnace wall of the holding furnace (such as 5.8 m), v is the traction speed, t1 is the rare-earth dissolution time, and t2 is the component homogenization time.

[0064] Specifically, L2 represents the spatial distance that the copper liquid flows from the holding furnace to the sawing position; v×(t1 + t2) represents the corresponding flow distance of the copper liquid during the dissolution and homogenization processes (traction speed v×total time t1 + t2). When the continuous casting billet is pulled out to the length L1 and the rare earth is added, the remaining length of the continuous casting billet that has not been pulled out (L0 - L1) is exactly equal to the spatial margin required for the copper liquid to flow to the sawing position (i.e., L2 + v×(t1 + t2)), thereby ensuring that when the copper liquid containing rare earth flows to the sawing position, the remaining length of the entire continuous casting billet is exactly cut completely, thus avoiding cross-batch mixing of materials.

[0065] This formula is applicable to batch production. Generally, the set length of the continuous casting billet is 21.5 meters, the distance from the sawing machine to the surface of the holding furnace is about 5.8 meters, the traction speed is set according to the actual value of the traction machine during the experiment, the time required for the inclusion to melt is initially set at 10 minutes, the time required for the inclusion to be homogenized is set at 5 minutes. Substituting the known values into the above formula, the real-time length of the continuous casting billet corresponding to the time when rare earth needs to be added can be calculated. By calculating the addition timing of the inclusion, the present invention is batch-oriented, accurately matches the rare earth content of each single continuous casting billet, and makes the rare earth distribution in the same continuous casting billet more uniform by forcibly aligning the rare earth addition nodes of each continuous casting billet through L0; reduces the deviation of rare earth concentration at the head and tail of the same continuous casting billet.

[0066] The traction speed v may vary due to process fluctuations. The control system collects the current value of v in real time and dynamically adjusts L1. If the speed suddenly increases by 10%, the system will automatically shorten L1 (reduce the rare earth advance amount) to avoid uneven composition caused by the premature arrival of the molten copper at the sawing position.

[0067] In step S3, the addition position of the rare earth is the rear chamber of the holding furnace, and it is added through the cover plate of the rear chamber of the holding furnace.

[0068] After the molten copper is transferred to the holding furnace, flake graphite with a thickness of 200 - 250 mm is laid on the surface of the molten copper to isolate the air and prevent the molten copper from being oxidized. The immersion depth of the inclusion is 200 mm - 400 mm. Specifically, use a copper pipe to push aside the flake graphite, place the inclusion on the surface of the molten copper, and use a copper pipe to push it into the molten copper surface to a depth of 200 mm - 400 mm, and let it stand for 5 minutes. Utilize the eddy current in the molten pool to promote the dispersion of the inclusion and avoid composition fluctuations caused by surface disturbance. The immersion depth (200 - 400 mm) can avoid the oxide layer with a thickness of about 50 mm on the surface of the molten copper; utilize the temperature gradient of the molten copper (for example, 980 °C on the surface → 1000 °C in the deep layer) to accelerate the dissolution of the inclusion; and, the deeper the immersion, the stronger the convection of the molten copper (Reynolds number Re = 5000 - 8000), which improves the mixing efficiency.

[0069] In step S4, the rare earth inclusion is only added to the holding furnace. Dynamically supplementing the rare earth inclusion according to the pouring cycle of the molten copper from the melting furnace means that after the melting furnace pours the molten copper into the holding furnace, dynamically adjust the supplementary amount of the rare earth master alloy in the holding furnace to keep the rare earth concentration in the holding furnace always stable within the range of ±5% of the target value, and the target rare earth concentration is 5 - 10 ppm.

[0070] Such as Figure 2As shown, two groups of electrochemical experiments were carried out on the annealed specimens without and with rare earth addition. The three-electrode system was used for the electrochemical test, with the saturated calomel electrode as the reference electrode, the platinum electrode as the auxiliary electrode, and the specimen as the working electrode. The anodic polarization curve of the copper tube was tested in a 1% formic acid solution. According to the corrosion potential, it can be seen that the corrosion potentials of the two groups of specimens are poorly consistent; 373-4 (rare earth content is 5 ppm) is relatively better. On this basis, multiple repeated electrochemical experiments were carried out. Figure 3 It indicates that for the copper tube with a rare earth content of 5 ppm, the corrosion potential is relatively concentrated. It should be noted here that Figure 2 369, 371-1, 373-4, etc. in it are the numbers of the ingots.

[0071] Such as Figure 4 and Figure 5 As shown, with the change of the rare earth addition concentration, the overall content of P element is within a reasonable range and there is no obvious change rule. When the rare earth is expected to be about 0 ppm and 10 ppm, the content of Si element is relatively low, not exceeding 2.22 ppm; when the designed rare earth content is 25 ppm, the Si content first decreases and then increases, 1.30 - 3.85 ppm; when the designed rare earth content is 50 ppm, the Si content is relatively high, 7.12 - 9.85 ppm; it shows that the rare earth content of 5 - 10 ppm in this application has a better optimization effect. Under the conditions of adding rare earth and not adding rare earth, there are hole defects with a size of 10 - 75 μm in the cast billet specimens without adding rare earth, and no holes with a size exceeding 20 μm are seen after adding rare earth, indicating that the addition of rare earth can effectively inhibit the generation of hole defects.

[0072] Specifically, the addition amount of rare earth inclusions is a combination of primary addition and supplementary addition to ensure the component stability within half an hour. After more than half an hour, due to the technological requirements of the weight of the copper liquid in the holding furnace, the melting furnace needs to pour and supplement the copper liquid. At this time, the rare earth content decreases due to dilution; so a certain amount of rare earth is supplemented when the melting furnace is poured for the second time.

[0073] For the primary addition, according to the rare earth content a in the master alloy, the usage amount m1 of the master alloy is calculated as A / a, where A is the amount of rare earth to be added according to the designed rare earth concentration. For example, if the rare earth content a in the master alloy is 20%, and the amount of rare earth to be added according to the designed rare earth concentration A is 5 ppm, if the copper liquid content in the holding furnace is W kg, then the usage amount m1 of the master alloy to be added is W * 5 / 10 6 *10 3 / 20% = 0.025W kg. After pouring W qt of copper liquid each time, the inclusion is supplemented, and the supplementary amount m2 = Wq * 5 / 10 6 *10 3 / 20% = 0.025Wq kg.

[0074] The proportion of rare earth in the rare earth master alloy is 15-20 wt.%, and the balance is copper and other impurities. When the proportion of rare earth is less than 15%, the melting point of the alloy > 1000 °C, and the dissolution time is extended to more than 15 min; when the proportion of rare earth is higher than 20%, brittle RE-Cu intermetallic compounds (such as Cu6La) are easily formed, which instead reduces the workability. The rare earth metal includes at least one of lanthanum, cerium, praseodymium, and neodymium. It is preferably lanthanum, or a mixture of lanthanum and other rare earth metals.

[0075] In step S5, the molten copper solution dissolved with rare earth flows into the mold under static pressure. After encountering the cooling water, it solidifies on the inner wall of the mold to form a copper alloy billet. Further preferably, the water-cooled mold is cooled at a rate of 10-30 °C / s to form fine equiaxed crystals (average size 20 μm), refining the grains and suppressing shrinkage cavities at the same time.

[0076] The copper alloy billet is phosphor-deoxidized copper. The content of H in the continuous casting billet does not exceed 1 ppm, and the content of O does not exceed 3 ppm. Calculated by mass percentage, the phosphorus content in the copper alloy billet is 0.015-0.040 wt.%, the rare earth content is 5-10 ppm, and the rest is copper and inevitable impurities. The rare earth / phosphorus mass ratio is 0.01-0.05. When the ratio is greater than 0.05, LaPO3 coarsens, and when the ratio is less than 0.01, deoxidation is insufficient. In the present invention, phosphor deoxidation forms P2O5 volatilization deoxidation, and the residual P reacts with rare earth La to generate LaPO4 nanoparticles, so that the rare earth segregates at the grain boundary in the form of La-P-O nanoclusters, and the cluster size is 5-30 nm, covering 40-60% of the grain boundary area.

[0077] The copper alloy billet is subjected to surface milling, rolling, stretching, internal thread forming, finishing, and annealing to obtain an ultra-fine diameter internal thread copper tube with an outer diameter ≤ 4.0 mm. Specifically, conventional processes are used. The core of this application is to improve the adaptability of the material to the forming process through material improvement to obtain an ultra-fine diameter internal thread copper tube with an outer diameter ≤ 4.0 mm.

[0078] The elongation of the ultra-fine diameter internal thread copper tube ≥ 40%; ensure that the tooth tip filling rate ≥ 95% during thread forming, and the tensile strength is 220-255 MPa; ensure that the deformation of the rack ≤ 0.5% in the 10 MPa pressure resistance test; support the limit wall thickness of 0.17 mm at the bottom of the tooth groove to avoid bulging failure caused by fluid pressure.

[0079] The grain size is 0.020-0.035 mm; by suppressing grain boundary slip during cold rolling, reducing work hardening, increasing the residual compressive stress on the tooth side surface, and the flaring rate ≥ 35%, the stable forming of a copper tube with an outer diameter ≤ 4.0 mm and a wall thickness of 0.17-0.23 mm is realized, and the rolling cracking rate is reduced.

[0080] Through the synergistic effect of rare earth elements (such as La and Ce) and phosphorus, a nano-scale RE-P-O composite phase is formed at the grain boundaries, which not only realizes grain refinement but also avoids the generation of brittle Cu3P phase in traditional phosphorus-deoxidized copper. This microscopic structure regulation enables the product to have both high strength and high elongation, perfectly overcoming the technical problem of the mutual trade-off between strength and plasticity commonly seen after the ultra-fine diameter reduction of copper tubes, and meeting the requirements of air-conditioning miniaturization for thin-wall high-strength copper tubes.

[0081] The outer diameter of the produced copper tube is 3.4 - 4.0 mm, and the copper tube is provided with internal threads. The internal threads are composed of alternating tooth grooves and racks. The cross-section of the rack is a triangle with an arc-shaped top. There is an inverted trapezoidal tooth groove between adjacent racks to improve fluid turbulence. The thickness between the bottom of the tooth groove of the internal thread and the outer diameter of the copper tube is 0.17 - 0.23 mm. Specifically, through multi-pass drawing and cooperating with rare earth to refine grains, ultra-fine diameter is achieved. The reduction rate per pass is ≤25%, avoiding necking fracture and breaking through the cold working deformation limit of copper tubes.

[0082] The internal thread has a helix angle, and the value range of the helix angle is 13° - 17°. The thread tooth height is 0.10 - 0.14 mm, the radius of the tooth tip fillet R ≤ 0.02 mm, the value range of the tooth tip angle is 8° - 22°, the number of racks is 38, the inner diameter of the copper tube is 2.93 - 2.99 mm, and the weight per meter is 21 - 24 g / m. In the present invention, the design of the internal thread structure combined with the tooth tip fillet R ≤ 0.02 mm can eliminate stress concentration. With the distribution of 38 teeth, the pressure resistance of the copper tube reaches 24.5 MPa, and there are zero defects in eddy current flaw detection, reflecting the optimized effect of the internal thread flow field.

[0083] Through the collaborative design of material-structure, the contradiction between "thin-wall requirement" and "processing brittleness" in the production of ultra-fine diameter internal thread copper tubes is solved. By refining grains with rare earth and inhibiting brittle phases with phosphorus, the unity of structural limit and performance limit is finally achieved, forming an irreplaceable technical barrier.

[0084] Example 1

[0085] A preparation method of an ultra-fine diameter internal thread copper tube includes the following steps:

[0086] S1 The electrolytic copper plate is melted into copper liquid in a melting furnace, and the copper liquid flows into a holding furnace through a launder.

[0087] S2 Use a press to press a specified amount of rare earth master alloy into small pieces, preferably in the form of blocks with a diameter of about 30 mm. Wrap the rare earth master alloy fragments with multiple layers of pure copper foil to form a package; the package is wrapped by 3 layers of copper foil, the diameter of the package is 30 ± 2 mm, the purity of the outer copper foil is ≥99.9%, the thickness of the copper foil is 0.05 mm, and the width is 300 mm. When wrapping, the layers are misaligned by 20% to enhance the sealing performance.

[0088] S3 calculates the inclusion addition timing based on the continuous casting traction speed and sawing position, and immerses the inclusion into the molten copper in the holding furnace through the rear hearth cover of the holding furnace; the inclusion addition timing meets the requirement: L1 = 21.5m - 5.8m - 0.3m / min × (10min + 5min) = 11.2m;

[0089] S4 dynamically supplements rare earth inclusions into the holding furnace according to the tilting cycle of the melting furnace, and controls the rare earth concentration to be stable within the range of ±5% of 5ppm.

[0090] The temperature of the molten copper in the holding furnace is 1080 ± 10 °C, and the cooling water flow rate of the mold is 20m 3 / h (water temperature 25 ± 2 °C); the traction speed v = 0.3m / min, the diameter of the billet is Φ82mm, and the surface roughness Ra ≤ 6.3μm. After the billet exits the mold, it enters the water mist cooling section (water pressure 0.5MPa, atomized particle diameter 50 - 100μm); the cooling rate is 50 - 80 °C / min, and the final cooling temperature ≤ 200 °C to avoid deformation caused by residual stress.

[0091] The obtained copper alloy billet is phosphor-deoxidized copper. Calculated by mass percentage, the phosphorus content in the copper alloy billet is 0.0231wt.%, the rare earth content is 5ppm, and the rare earth / phosphorus mass ratio is 0.0216.

[0092] S6 removes the surface oxide layer of the copper alloy billet (unilateral cutting amount 0.5 - 1.0mm), and the surface finish Ra ≤ 1.6μm; through a three-roll planetary rolling mill (rolling force 800 - 1000kN), the total reduction ratio is 85%, and the pass deformation amount ≤ 25%; the stretching is divided into tandem drawing and coil drawing, and the final outer diameter is Φ4.0mm after multi-pass cold drawing; during internal thread forming, the copper tube is annealed online, and then the spinning and reducing method (core head helix angle 15°, rotation speed 1200rpm) is used, and the tooth top filling rate ≥ 95%; then it is annealed again and packaged to form a finished product.

[0093] The elongation of the obtained ultra-fine diameter internal thread copper tube is 53%; the tensile strength is 242MPa; the grain size is 0.0221mm; the flaring rate is 36.5%. The outer diameter of the copper tube is 3.63mm, the copper tube is provided with internal threads, and the thickness between the bottom of the tooth groove of the internal thread and the outer diameter of the copper tube is 0.19 - 0.22mm. The internal thread has a helix angle, and the value range of the helix angle is 14.5°. The internal thread is composed of alternating tooth grooves and racks, the cross-section of the rack is a triangle with an arc-shaped top, and the adjacent racks have an inverted trapezoidal tooth groove.

[0094] Testing revealed clean interior and exterior surfaces, free of harmful defects such as pinholes, cracks, peeling, bubbles, inclusions, blackening, green rust, severe oxide film, copper powder, water droplets, flattening, bruises, and mold marks. There were no scratches, pits, or spots. Eddy current testing revealed zero defects per disc. A flattening test revealed no visible microcracks. With a bend diameter of 1.5 times the nominal outer diameter, the copper tube was bent 180° without wrinkles or cracks. The sample was filled with water and slowly pressurized to 6.5 MPa for 5 minutes without macroscopic deformation or leakage. After the pressure test, the pressure was slowly increased to 10 MPa and maintained for 1 minute. No cracks were observed (actually measured at 24.5 MPa without explosion).

[0095] Example 2

[0096] A method for preparing an ultra-fine diameter internally threaded copper tube comprises the following steps:

[0097] The S1 electrolytic copper plate is melted into copper liquid in the smelting furnace, and the copper liquid flows into the holding furnace through the launder.

[0098] S2 uses a press to compress a specified amount of rare earth master alloy into small pieces, preferably approximately 30mm in diameter. The rare earth master alloy pieces are then wrapped in multiple layers of pure copper foil to form a package. The package consists of three layers of copper foil, with a diameter of 30±2mm. The outer layer of copper foil has a purity of ≥99.9%, a thickness of 0.05mm, and a width of 300mm. The layers are staggered by 20% to enhance sealing.

[0099] S3 calculates the timing for adding the ladle based on the continuous casting traction speed and sawing position, and immerses the ladle into the copper liquid in the holding furnace through the rear cover of the holding furnace; the timing for adding the ladle satisfies: L1 = 21.5m-5.8m-0.3m / min×(10min+5min)=11.2m; ensuring that the two batches are not mixed.

[0100] S4 is based on the preparation of the first batch of ingots in Example 1, and rare earth inclusions are added to the holding furnace according to the periodic dynamics of pouring molten copper from the smelting furnace, and the rare earth concentration is controlled to be stable within the range of ±5% of 10ppm.

[0101] Since the experiment is continued based on the previous one, the influence of the rare earth content in the previous experiment needs to be considered. The amount of addition required to supplement the increased rare earth concentration is calculated as follows:

[0102]

[0103] Wherein, y represents the weight of the intermediate alloy to be replenished (in kg), W represents the real-time mass of the copper liquid in the holding furnace, b represents the design value of the rare earth concentration of the second group of experiments (in ppm), and a represents the design value of the rare earth concentration of the first group of experiments (in ppm);

[0104] The molten copper temperature in the S5 holding furnace is 1080 ± 10 °C, and the cooling water flow rate in the mold is 20 m 3 / h (water temperature 25 ± 2 °C); the drawing speed v = 0.3 m / min, the billet diameter Φ82 mm, and the surface roughness Ra ≤ 6.3 μm. After the billet exits the mold, it enters the water mist cooling section (water pressure 0.5 MPa, atomized particle diameter 50 - 100 μm); the cooling rate is 50 - 80 °C / min, and the final cooling temperature ≤ 200 °C to avoid deformation caused by residual stress. The obtained copper alloy billet is phosphor-deoxidized copper. Calculated by mass percentage, the phosphorus content in the copper alloy billet is 0.0229 wt.%, the rare earth content is 10 ppm, and the rare earth / phosphorus mass ratio is 0.0433.

[0105] S6 removes the surface oxide layer of the copper alloy billet (single-side cutting amount 0.5 - 1.0 mm), and the surface finish Ra ≤ 1.6 μm; through a three-roll planetary rolling mill (rolling force 800 - 1000 kN), the total diameter reduction rate is 85%, and the pass deformation amount ≤ 25%; the drawing is divided into tandem drawing and spool drawing, and after multi-pass cold drawing, the final outer diameter is Φ4.0 mm; when forming the internal thread, the copper tube is annealed online, and then by the rotary compression and reduction method (core head spiral angle 15°, rotation speed 1200 rpm), the tooth top filling rate ≥ 95%; then annealed again and packaged to form the finished product.

[0106] The elongation of the obtained ultra-fine diameter internal thread copper tube is 52%; the tensile strength is 243 MPa; the grain size is 0.0225 mm; the flaring rate is 36.8%. The outer diameter of the copper tube is 3.62 mm, the copper tube is provided with internal threads, and the thickness between the bottom of the tooth groove of the internal thread and the outer diameter of the copper tube is 0.19 - 0.22 mm. The internal thread has a spiral angle, and the value range of the spiral angle is 14.7°. The internal thread is composed of tooth grooves and racks spaced apart, the cross-section of the rack is a triangle with an arc-shaped top, and there is an inverted trapezoidal tooth groove between adjacent racks.

[0107] After inspection, the inner and outer surfaces are clean, without harmful defects such as pinholes, cracks, peeling, bubbles, inclusions, blackening, green rust, severe oxide films, copper powder, water droplets, flattening, bruising, and die marks. There are no scratch, pit, and spot defects. After eddy current flaw detection, the number of defects per single spool is 0. After the flattening test, no micro-cracks visible to the naked eye are observed. Under the condition that the bending core diameter is 1.5 times the nominal outer diameter, the copper tube is bent 180° once without wrinkles and cracks. The specimen is filled with water, and the test pressure is slowly increased to 6.5 MPa and kept for 5 min without macroscopic deformation and leakage. After the pressure test is completed, the pressure is slowly increased to 10 MPa and kept for 1 min, and the test has no rupture (the actual measurement is 24.8 Mpa without explosion).

[0108] Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principle of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of the present invention claimed is defined by the appended claims and their equivalents.

Claims

1. A method for preparing an ultra-fine diameter internally threaded copper tube, characterized in that, It includes the following steps: S1 Melting the electrolytic copper plate and transferring the obtained copper liquid into a holding furnace; S2 Wrapping the rare earth master alloy fragments with multiple layers of pure copper foil to form a package; S3 Calculating the addition timing of the package according to the continuous casting traction speed and the sawing position, and immersing the package into the copper liquid in the holding furnace; S4 Dynamically supplementing the rare earth package according to the pouring cycle of the melting furnace to control the rare earth concentration; S5 Cooling after continuous casting to obtain a copper alloy billet; S6 Milling, rolling, stretching, internal thread forming, finishing, and annealing the copper alloy billet to obtain an ultra-fine diameter internal thread copper tube with an outer diameter ≤ 4.0 mm.

2. The preparation method of an ultra-fine diameter internally threaded copper tube according to claim 1, characterized in that, In step S2, the package is wrapped by ≥ 3 layers of copper foil, the diameter of the package is 30 ± 2 mm, the purity of the outer layer of copper foil is ≥ 99.9%, the thickness of the copper foil is 0.05 - 0.1 mm, and the width is 300 mm.

3. The preparation method of an ultra-fine inner-threaded copper tube according to claim 1, characterized in that, In step S3, the addition timing of the package satisfies: L1 = L0 - L2 - v×(t1 + t2); wherein, L1 is the length of the billet when the package is added, L0 is the set total length of a single billet, L2 is the distance between the sawing position and the furnace wall of the holding furnace, v is the traction speed, t1 is the rare earth dissolution time, and t2 is the composition homogenization time.

4. The manufacturing method of an ultra-fine inner-threaded copper tube according to claim 1, characterized in that After the copper liquid is transferred into the holding furnace, scale graphite with a thickness of 200 - 250 mm is laid on the surface of the copper liquid, and the immersion depth of the package is 200 mm - 400 mm below the copper liquid surface.

5. The preparation method of an ultra-fine inner-threaded copper tube according to claim 1, characterized in that, After the melting furnace pours the copper liquid into the holding furnace, dynamically adjust the supplement amount of the rare earth master alloy to make the rare earth concentration in the holding furnace stable within the range of ± 5% of the target value, and the target value is 5 - 10 ppm.

6. The preparation method of an ultra-fine diameter internally threaded copper tube according to claim 1, characterized in that, The proportion of rare earth content in the rare earth master alloy is 15 - 20 wt.%, and the balance is copper and other impurities. The rare earth metals include at least one of lanthanum, cerium, praseodymium, and neodymium.

7. The preparation method of an ultra-fine diameter internally threaded copper tube according to claim 1, characterized in that, The copper alloy billet is phosphor-deoxidized copper. Calculated by mass percentage, the phosphorus content in the copper alloy billet is 0.015 - 0.040 wt.%, the rare earth content is 5 - 10 ppm, and the rare earth / phosphorus mass ratio is 0.01 - 0.

05.

8. A method for preparing an ultra-fine inner-threaded copper tube according to any one of claims 1 to 7, characterized in that, The elongation of the ultra-fine diameter internal thread copper tube ≥ 40%; the tensile strength is 220 - 255 MPa; the grain size is 0.020 - 0.035 mm; the flaring rate ≥ 35%.

9. An ultra-fine diameter internally threaded copper tube prepared by the method according to claim 8, characterized in that, The outer diameter of the copper tube is 3.4 - 4.0 mm. The copper tube is provided with internal threads, and the thickness between the bottom of the tooth groove of the internal thread and the outer diameter of the copper tube is 0.17 - 0.23 mm.

10. The ultra-fine diameter internally threaded copper tube according to claim 9, wherein The internal thread has a helix angle, and the value range of the helix angle is 13° - 17°.

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