Superfine pitch internal thread copper pipe and its preparation method

Abstract

The present application relates to the field of ultra-fine pitch internal thread copper pipe preparation, and particularly relates to an ultra-fine pitch internal thread copper pipe and a preparation method thereof, comprising the following steps: S1, smelting an electrolytic copper plate and transferring the obtained copper liquid into a holding furnace; S2, wrapping rare earth intermediate alloy fragments with pure copper foil in multiple layers to form a package; S3, calculating the package adding timing according to the continuous casting traction speed and sawing position, and immersing the package into the copper liquid in the holding furnace; S4, dynamically supplementing the rare earth package according to the smelting furnace pouring copper liquid period to control the rare earth concentration; S5, obtaining a copper alloy cast blank after continuous casting and cooling; S6, milling, rolling, stretching, internal thread forming, finishing and annealing the copper alloy cast blank to obtain an ultra-fine pitch internal thread copper pipe with an outer diameter of less than or equal to 4.0 mm. In the field of ultra-fine pitch internal thread copper pipe manufacturing, the present application breaks through multiple technical bottlenecks of traditional processes.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of ultra-fine diameter inner thread copper pipe preparation, and particularly relates to an ultra-fine diameter inner thread copper pipe and a preparation method thereof. BACKGROUND

[0002] Phosphor deoxidized copper (TP2) can be used as a main heat exchange element due to its excellent electrical conductivity, thermal conductivity, corrosion resistance, weldability and cost advantage, such as a dryer, a condenser and an evaporator, and the like. In particular, the high thermal conductivity makes the TP2 copper pipe be widely used as a component of an air conditioning system. With the improvement of air conditioning energy efficiency standards and the compact design requirements, the inner thread copper pipe for air conditioning is accelerating to the direction of ultra-fine diameter and thin wall.

[0003] In the conventional technology, the 5mm caliber inner thread pipe made of TP2 copper is the limit, and it is difficult to prepare an inner thread pipe with a smaller diameter. When the outer diameter is smaller, the wall is thinner, and higher tensile strength and compressive strength are required to prevent cracking and pipe collapse. In addition, the coarse grains or uneven distribution that may occur during processing can cause cracks or deformation during fine diameter forming, limiting the reduction rate improvement. Furthermore, the material needs sufficient ductility to withstand the stress during stretching and forming. Therefore, the existing technology is controlled at 5-7mm.

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

[0005] The information disclosed in this BACKGROUND section is only for the purpose of enhancing the understanding of the general background of the application and should not be taken as admitting that such information is prior art with respect to any patentable matter disclosed in or related to the present application. SUMMARY

[0006] The first object of the present application is to provide a preparation method of an ultra-fine diameter inner thread copper pipe, which breaks through the 5.0mm fine diameter limit based on the directional regulation of rare earth elements on the microstructure of the material and the process optimization, and realizes the tensile strength ≥240MPa and the elongation rate ≥40% when the outer diameter ≤4.0mm.

[0007] The above technical object of the present application is achieved by the following technical scheme:

[0008] A preparation method of an ultra-fine diameter inner thread copper pipe, comprising 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 intermediate alloy pieces with pure copper foils in multiple layers to form a package;

[0011] S3 calculate the package adding timing according to the continuous casting pulling speed and 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 smelting furnace pouring copper liquid period to control the rare earth concentration;

[0013] S5 obtain the copper alloy cast blank after continuous casting and cooling;

[0014] S6 mill the surface of the copper alloy cast blank, roll, stretch, form internal thread, finish, and anneal to obtain the ultra-fine diameter internal thread copper pipe with an outer diameter of ≤4.0mm.

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

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

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

[0018] L1 = L0 - L2 - v × (t1 + t2);

[0019] wherein L1 is the length of the cast blank when the package is added, L0 is the set total length of a single cast blank (such as 21.5m), L2 is the distance between the sawing position and the wall of the holding furnace (such as 5.8m), v is the pulling speed, t1 is the rare earth dissolving time, and t2 is the composition homogenization time.

[0020] Specifically, L2 represents the spatial distance of the copper liquid flowing from the holding furnace to the sawing position; v×(t1+t2) represents the corresponding flow distance of the copper liquid in the dissolving and homogenization process (pulling speed v×total time t1+t2). The rare earth is added when the cast blank is pulled out to a length of L1, at which time the remaining length of the cast blank that has not been pulled out (L0-L1) is exactly equal to the spatial allowance 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 cast blank is exactly cut, thereby avoiding cross-batch mixing.

[0021] As preferred, the pulling speed v can vary due to process fluctuation. The control system collects the current value of v in real time, dynamically adjusts L1. If the speed increases by 10%, the system will automatically shorten L1 (reduce the rare earth advance), to avoid uneven composition caused by copper liquid reaching the sawing position too early.

[0022] As preferred, in step S3, the rare earth is added at the rear of the holding furnace, through the rear cover of the holding furnace.

[0023] As preferred, after the copper liquid is transferred into the holding furnace, a 200-250mm thick flaky graphite is laid on the surface of the copper liquid to isolate air and prevent the copper liquid from being oxidized. The immersion depth of the inclusions is 200-400mm. Specifically, the flaky graphite is pushed aside with a copper pipe, the inclusions are placed on the surface of the copper liquid, and the copper pipe is used to push the inclusions to a depth of 200-400mm below the surface of the copper liquid. After 5 minutes of standing, the eddy current of the molten pool is used to promote the dispersion of the inclusions, and composition fluctuation caused by surface disturbance is avoided.

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

[0025] Specifically, the addition amount of the rare earth inclusions is a combination of one-time addition and replenishment 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 pour 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 poured for the second time.

[0026] When added once, the intermediate alloy m1 is calculated according to the rare earth content a in the intermediate alloy, m1=A / a, wherein A is the rare earth amount required to be added for the design rare earth concentration, for example, the rare earth content a in the intermediate alloy is 20%, the rare earth amount A required to be added for the design rare earth concentration is 5ppm, and if the copper liquid content in the holding furnace is W kg, the required amount of the intermediate alloy m1 is W*5 / 10 6 *10 3 / 20%=0.025W kg. After pouring Wq copper liquid, the replenishment amount m2 is Wq*5 / 10 6 *10 3 / 20%=0.025Wq kg.

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

[0028] As preferred, in step S5, the copper liquid with dissolved rare earth flows into the crystallizer under static pressure, and solidifies to form the copper alloy cast blank on the inner wall of the crystallizer after meeting the cooling water.

[0029] Further preferably, the water-cooled crystallizer is cooled at a rate of 10-30℃ / s to form fine equiaxed crystals (average size 20μm), refine the grains and inhibit shrinkage holes.

[0030] As preferred, the copper alloy cast blank is phosphorus deoxidized copper, and the phosphorus content in the copper alloy cast blank is 0.015-0.040wt.% according to the mass percentage, the rare earth content is 5-10ppm, and the mass ratio of rare earth / phosphorus is 0.01-0.05. The rare earth is segregated in the grain boundary in the form of La-P-O nanoclusters, the cluster size is 5-30nm, and the grain boundary area is covered by 40-60%.

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

[0032] As preferred, the copper alloy cast blank is subjected to face milling, rolling, stretching, internal thread forming, finishing and annealing to obtain the ultra-fine diameter internal thread copper pipe with an outer diameter ≤4.0mm. The core of the application lies in improving the adaptability of the material and the forming process through material improvement to obtain the ultra-fine diameter internal thread copper pipe with an outer diameter ≤4.0mm.

[0033] The second object of the application is to obtain an ultra-fine diameter internal thread copper pipe with an outer diameter ≤4.0mm by using the above method, which breaks through the limit of 5.0mm in diameter.

[0034] As preferred, the elongation of the ultra-fine diameter internal thread copper pipe is ≥40%, the tensile strength is 220-255MPa, the grain size is 0.020-0.035mm, and the flaring rate is ≥35%. The tensile strength, elongation and grain size are simultaneously up to the standard through the synergistic regulation of rare earth-phosphorus.

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

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

[0037] As preferred, the internal threads are composed of thread grooves and spline intervals, the cross section of the spline is a triangle with an arc line at the top, and the thread groove between adjacent splines is an inverted trapezoid.

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

[0039] Through the integrated process design of "melting-wrapping-dynamic adding-precision machining", the present application breaks through the multiple technical bottlenecks of the traditional process in the field of ultra-fine diameter internal thread copper pipe manufacturing. Compared with the traditional method of relying on manual experience to judge the adding time, the present application establishes the mapping relationship between the copper liquid flow time and the billet length by real-time calculation of the continuous casting pulling speed and the sawing position, and by real-time tracking of the forming length of the billet, the time point of rare earth addition is deduced reversely. The copper liquid containing rare earth completes the two key stages of dissolution and homogenization during the physical process of flowing through the holding furnace to the sawing position, finally forming a billet with a set length (L0) and stable composition, reducing the rare earth concentration deviation of the head and tail of a single billet, and solving the problem of micro-cracks caused by composition fluctuation in ultra-fine diameter copper pipe processing.

[0040] In the present application, the inclusions are wrapped with copper foil to achieve delayed dissolution. By isolating the oxidizing atmosphere on the surface of the copper liquid, the yield of rare earth is ≥95%, and by freely combining the thickness and number of layers of the copper foil (such as 0.05 mm x 3 layers or 0.1 mm x 2 layers), the dynamic matching of the release rate of rare earth and the flow rate of copper liquid is achieved. Compared with the method of directly adding in advance and relying on the experience of the operator to determine the pulling 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 the phosphorus deoxidized copper can be deoxidized, excessive phosphorus is easy to form a brittle Cu3P phase, leading to phosphorus segregation at the grain boundary, and the risk of grain boundary cracking during rolling increases; the simultaneous demand for high strength and high elongation of thinner pipe diameter pipes cannot be met. The rare earth elements are uniformly distributed in the smelting stage, and the growth of the alpha-Cu grain is inhibited during continuous casting cooling, combined with dynamic rare earth concentration control and multi-pass precision stretching, realizing the limit processing of the outer pipe diameter of 3.4-4.0mm and the bottom thickness of the tooth groove of 0.17-0.23mm, the wall thickness tolerance of ±0.005mm, meeting the high-end demand of the heat dissipation copper pipe. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only some embodiments described in the present application, and for those skilled in the art, other drawings can also be obtained without creative labor based on these drawings.

[0043] Figure 1 It is a flow chart of a preparation method of an ultra-fine diameter internal thread copper pipe;

[0044] Figure 2 It is a test chart for representing the consistency of the electrochemical corrosion performance of the annealed samples of the copper pipe with the rare earth content of 5ppm;

[0045] Figure 3 It is a test chart for representing the consistency of the electrochemical corrosion performance of the annealed samples of the copper pipe with the rare earth content of 5ppm; Figure 2

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

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

[0048] In order to further illustrate the technical means and effects taken by the present application to achieve the predetermined invention purpose, the specific embodiments, features and effects of the ultra-fine diameter internal thread copper pipe and the preparation method thereof according to the present application are described in detail as follows. It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application.

[0049] As shown in Figure 1 A preparation method of an ultra-fine diameter internal thread copper pipe, comprising the following steps:

[0050] S1 smelting the electrolytic copper plate and transferring the obtained copper liquid into a holding furnace; ​

[0051] S2 wrapping the rare earth intermediate alloy pieces with pure copper foil in multiple layers to form a package;

[0052] S3 calculating the package adding timing according to the continuous casting pulling speed and sawing position, and immersing the package into the copper liquid in the holding furnace;

[0053] S4 dynamically supplementing the rare earth package according to the smelting furnace pouring copper liquid period to control the rare earth concentration;

[0054] S5 obtaining the copper alloy cast blank after continuous casting and cooling;

[0055] S6 milling, rolling, stretching, internal thread forming, finishing and annealing the copper alloy cast blank to obtain the ultra-fine diameter internal thread copper pipe with an outer diameter of ≤4.0mm.

[0056] In the prior art, although the phosphorus content in the phosphorus deoxidized copper can be deoxidized, excessive phosphorus is easy to form a brittle Cu3P phase, leading to phosphorus segregation at the grain boundary and increasing the risk of grain boundary cracking during rolling; the simultaneous demand for high strength and high elongation of thinner pipe diameter pipe materials cannot be met. In the present application, the rare earth elements are uniformly distributed in the smelting stage, and the growth of α-Cu grains is inhibited during continuous casting and cooling, combined with dynamic rare earth concentration control and multi-pass precision stretching, to realize the limit processing of the bottom thickness of 0.17-0.23mm at the tooth groove of the outer pipe diameter of 3.4-4.0mm, and the wall thickness tolerance of ±0.005mm, to meet the high-end demand of the heat dissipation copper pipe.

[0057] In the present application, high-purity copper liquid (Cu≥99.95%) is obtained by smelting electrolytic copper plate, and the rare earth intermediate alloy package is introduced after transferring to the holding furnace, and the rare earth delayed dissolution is realized by using copper foil wrapping, the copper liquid flow time is matched, and the segregation caused by excessive instantaneous concentration is avoided, wherein the package is only added to the holding furnace, and the addition is triggered based on the real-time calculation of the pulling speed and the sawing position, and the smelting furnace pouring period is dynamically supplemented to maintain the stable rare earth concentration, ensure the rare earth yield, reduce the oxidation loss, and control the grain size refinement to 0.025mm level, and improve the yield of the ultra-fine diameter internal thread copper pipe with an outer diameter of ≤4.0mm.

[0058] In step S1, the electrolytic copper plate is melted into copper liquid in the smelting furnace, and the copper liquid flows into the holding furnace through the flow channel.

[0059] The package in step S2 is wrapped by copper foils of ≥3 layers, the package diameter is 30±2mm, the purity of the outer layer copper foil is ≥99.9%, the thickness of the copper foil is 0.05-0.1mm, and the width is 300mm. The misalignment between layers is 20% during wrapping to enhance the sealing property and reduce the introduction of impurities such as Fe and S. In the present application, the rare earth package wrapped by copper foils realizes gradient dissolution at the deep copper liquid by using the high thermal conductivity of copper, avoids high-temperature oxidation, and the yield is ≥95%. After the copper foils contact the copper liquid in the holding furnace, the outer layer melts in 30 seconds, a small amount of rare earth is released to initiate nucleation; the middle layer dissolves in 2-3 minutes, and the rare earth diffuses in the form of Cu-RE eutectic liquid phase (melting point 950℃); gradient dissolution is realized, and the interlayer gap (0.1-0.3mm) of the copper foil promotes turbulent mixing and reduces the segregation index.

[0060] In the present application, the package adopts the delayed dissolution setting of copper foil wrapping, which isolates the surface of the copper liquid from the oxidizing atmosphere, so that the yield of rare earth is ≥95%, and through the free combination of the thickness and the number of layers of the copper foil (such as 0.05mm×3 layers or 0.1mm×2 layers), the dynamic matching of the rare earth release rate and the copper liquid flow rate is realized, compared with the direct advance addition and the way of relying on the experience of the operator 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 package addition timing meets:

[0062] L1=L0-L2-v×(t1+t2);

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

[0064] Specifically, L2 represents the spatial distance of the copper liquid flowing from the holding furnace to the sawing position; v×(t1+t2) represents the corresponding flow distance of the copper liquid in the dissolution and homogenization process (traction speed v×total time t1+t2). When the rare earth is added when the casting blank is pulled out to a length L1, the remaining length of the casting blank (L0-L1) is exactly equal to the spatial allowance required for the copper liquid to flow to the sawing position (i.e. L2+v×(t1+t2)), so that when the copper liquid containing rare earth flows to the sawing position, the remaining length of the whole casting blank is exactly cut, so as to avoid cross-batch mixing.

[0065] The formula is applicable to batch production, generally, the setting length of the casting blank is 21.5 meters, the distance from the sawing machine to the surface of the holding furnace is about 5.8 meters, the pulling speed is set according to the actual value of the pulling machine in the experiment process, the time required for melting the package body is initially set as 10 minutes, and the time required for homogenizing the package body is set as 5 minutes, and the known values are substituted into the above formula, so that the real-time length of the casting blank corresponding to the rare earth to be added can be calculated and obtained. The present application calculates the adding time of the package body, is oriented to batches, accurately matches the rare earth content of a single casting blank, forcibly aligns the rare earth adding nodes of each casting blank through L0, so that the rare earth distribution in the same casting blank is more uniform, and the rare earth concentration deviation of the head and tail of the same casting blank is reduced.

[0066] The pulling speed v can change 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 the composition being uneven due to the copper liquid reaching the sawing position too early.

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

[0068] After the copper liquid is transferred into the holding furnace, 200-250mm thick flaky graphite is laid on the surface of the copper liquid to isolate air and prevent the copper liquid from being oxidized, and the immersion depth of the package body is 200-400mm. Specifically, the flaky graphite is pushed away by a copper pipe, the package body is placed on the surface of the copper liquid, and the copper pipe is used to push the package body to a depth of 200-400mm below the surface of the copper liquid, and the package body is left to stand for 5 minutes. The vortex of the molten pool is used to promote the dispersion of the package body, and composition fluctuation caused by surface disturbance is avoided. The immersion depth (200-400mm) can avoid the about 50mm thick oxidation layer on the surface of the copper liquid; the temperature gradient of the copper liquid (for example, the surface is 980℃ and the deep layer is 1000℃) is used to accelerate the dissolution of the package body; and the deeper the immersion, the stronger the convection of the copper liquid (Reynolds number Re=5000-8000), which improves the mixing efficiency.

[0069] In step S4, the rare earth package body is only added to the holding furnace, and the dynamic replenishment of the rare earth package body according to the pouring cycle of the smelting furnace means that after the smelting furnace pours the copper liquid into the holding furnace, the replenishment amount of the rare earth master alloy in the holding furnace is dynamically adjusted, so that the rare earth concentration in the holding furnace is always stably within the target value ±5%, and the target rare earth concentration is 5-10ppm.

[0070] As Figure 2As shown, two groups of electrochemical experiments were carried out on the annealed samples without adding rare earth and after adding rare earth, the electrochemical test selected a three-electrode system, with a saturated calomel electrode as the reference electrode, a platinum electrode as the auxiliary electrode, and the sample as the working electrode, and the potential dynamic polarization curve of the copper pipe in 1% formic acid solution was tested. According to the corrosion potential, the consistency of the corrosion potential of the two groups of samples is poor; 373-4 (the rare earth content is 5 ppm) is relatively good. On this basis, repeated electrochemical experiments were carried out, Figure 3 The 373-4 in the figure indicates that the copper pipe with a rare earth content of 5 ppm has a relatively concentrated corrosion potential. It should be noted that the Figure 2 The 369, 371-1, 373-4, etc. in the figure are the numbers of ingots.

[0071] As shown in Figure 4 and Figure 5 , with the change of the addition concentration of rare earth, the overall content of P element is in a reasonable range and has 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 and does not exceed 2.22 ppm; when the designed content of rare earth is 25 ppm, the content of Si decreases first and then increases, 1.30-3.85 ppm; when the designed content of rare earth is 50 ppm, the content of Si is relatively high, 7.12-9.85 ppm; it is shown that the rare earth content of 5-10 ppm in the present application has a good optimization effect. Under the conditions of adding and not adding rare earth, there are 10-75 μm size hole defects in the cast blank sample without adding rare earth, and no hole with a size of more than 20 μm is found after adding rare earth, which shows that the addition of rare earth can effectively inhibit the generation of hole defects.

[0072] Specifically, the addition amount of the rare earth inclusions is a combination of one-time addition and supplementary addition, which ensures that the composition is stable within half an hour. After more than half an hour, due to the process requirement of the weight of the copper liquid in the holding furnace, the smelting furnace needs to be poured and supplemented with copper liquid. At this time, the content of rare earth is reduced due to dilution; therefore, a certain amount of rare earth is supplemented when the second smelting furnace is poured.

[0073] When one-time addition is carried out, the amount m1 of the intermediate alloy is calculated as A / a, wherein A is the amount of rare earth to be added according to the designed rare earth concentration, and a is the content of rare earth in the intermediate alloy. For example, if the content of rare earth in the intermediate alloy is 20% and the amount A of rare earth to be added according to the designed rare earth concentration is 5 ppm, and the content of the copper liquid in the holding furnace is W kg, then the amount of the intermediate alloy m1 to be added is W*5 / 10 6 *10 3 / 20% = 0.025W kg. After pouring Wqt of copper liquid, the amount m2 of the inclusions to be supplemented is Wq*5 / 10 6 *10 3 / 20% = 0.025Wq kg.

[0074] The rare earth content in the rare earth intermediate alloy accounts for 15-20 wt.%, and the balance is copper and other impurities. When the rare earth content is less than 15%, the melting point of the alloy is greater than 1000℃, and the dissolution time is prolonged to more than 15 minutes. When the rare earth content is greater than 20%, brittle RE-Cu intermetallic compounds (such as Cu6La) are easily formed, which reduces the processability. The rare earth metal includes at least one of lanthanum, cerium, praseodymium and neodymium. Preferably, it is lanthanum or a mixture of lanthanum and other rare earth metals.

[0075] In step S5, the copper liquid with dissolved rare earth flows into the crystallizer under static pressure and solidifies into copper alloy cast blank on the inner wall of the crystallizer after encountering cooling water. Further preferably, the water-cooled crystallizer is cooled at a rate of 10-30℃ / s to form fine equiaxed crystals (average size 20μm), which refines the grains and inhibits shrinkage holes.

[0076] The copper alloy cast blank is phosphorus deoxidized copper, the content of H in the continuous casting blank is not more than 1 ppm, and the content of O is not more than 3 ppm. According to the mass percentage, the phosphorus content in the copper alloy cast blank is 0.015-0.040 wt.%, the rare earth content is 5-10 ppm, and the balance is copper and unavoidable impurities. The mass ratio of rare earth to phosphorus is 0.01-0.05. When the ratio is greater than 0.05, LaPO3 is coarsened, and when the ratio is less than 0.01, deoxidation is insufficient. In the present application, phosphorus deoxidation forms P2O5 volatilization deoxidation, residual P reacts with rare earth La to form LaPO4 nanoparticles, so that the rare earth is segregated in the grain boundary in the form of La-P-O nanoclusters, the cluster size is 5-30 nm, and the cluster covers 40-60% of the grain boundary area.

[0077] The copper alloy cast blank is milled, rolled, stretched, internally threaded, finished and annealed to obtain an ultra-fine diameter internal threaded copper pipe with an outer diameter of ≤4.0mm. The specific process is conventional, and the core of the present application is to improve the adaptability of the material to the forming process by improving the material, so as to obtain an ultra-fine diameter internal threaded copper pipe with an outer diameter of ≤4.0mm.

[0078] The elongation of the ultra-fine diameter internal threaded copper pipe is ≥40%; the thread top filling rate is ≥95% when the thread is formed, the tensile strength is 220-255MPa; the deformation of the rack is ≤0.5% in the 10MPa pressure test; the limit wall thickness of the support tooth groove bottom is 0.17mm, which avoids the bulging failure caused by fluid pressure.

[0079] The grain size is 0.020-0.035mm; by inhibiting grain boundary sliding during cold rolling, reducing work hardening, and improving the residual compressive stress on the tooth side surface, the bulging rate is ≥35%, the copper pipe with an outer diameter of ≤4.0mm and a wall thickness of 0.17-0.23mm is stably formed, and the rolling cracking rate is reduced.

[0080] The application forms nanoscale RE-P-O complex phase at the grain boundary through the synergistic effect of rare earth elements (La, Ce, etc.) and phosphorus, which not only realizes grain refinement, but also avoids the generation of Cu3P brittle phase in traditional phosphorus deoxidized copper. This microstructure regulation makes the product have high strength and high elongation at the same time, perfectly overcomes the technical problem that strength and plasticity are in conflict with each other after the ultra-fine copper pipe is miniaturized, and meets the demand of thin-walled high-strength copper pipe for air conditioner miniaturization.

[0081] The prepared copper pipe has an outer diameter of 3.4-4.0 mm, and is provided with internal threads formed by tooth grooves and tooth bars, the cross section of the tooth bar is triangular with an arc line at the top, the tooth grooves between adjacent tooth bars are inverted trapezoidal, the thickness between the bottom of the tooth groove of the internal thread and the outer diameter of the copper pipe is 0.17-0.23 mm. The ultra-fine diameter is realized by multi-pass stretching and rare earth grain refinement, the diameter reduction rate of each pass is ≤25%, necking fracture is avoided, and the cold working deformation limit of the copper pipe is broken through.

[0082] The internal thread has a spiral angle, and the spiral angle is in the range of 13°-17°. The thread height is 0.10-0.14 mm, the corner radius R of the thread top is ≤0.02 mm, the thread top angle is in the range of 8°-22°, the number of tooth bars is 38, and the inner diameter of the copper pipe is 2.93-2.99 mm, and the mickey weight is 21-24 g / m. In the application, the internal thread structure combined with the design of the corner radius R of the thread top ≤0.02 mm can eliminate stress concentration, and the 38 tooth number distribution makes the copper pipe have a pressure resistance of 24.5 MPa and zero defect in eddy current flaw detection, which reflects the flow field optimization effect of the internal thread.

[0083] The application solves the contradiction between the thin-walled requirement and the processing brittleness in the production of the ultra-fine internal thread copper pipe through the material-structure synergistic design. The grain is refined by rare earth, and the brittle phase is inhibited by phosphorus, so that the structure limit and the performance limit are unified, and an irreplaceable technical barrier is formed.

[0084] Example 1

[0085] A preparation method of an ultra-fine internal thread copper pipe, comprising the following steps:

[0086] S1 electrolytic copper plate is melted into copper liquid in a smelting furnace, and the copper liquid flows into a holding furnace through a flow channel.

[0087] S2 a specified amount of rare earth intermediate alloy is pressed into small pieces by a press, and the piece is preferably in a block shape with a diameter of about 30 mm. The rare earth intermediate alloy pieces are wrapped with multiple pure copper foils to form a package; the package is wrapped with 3 layers of copper foils, 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 mm, and the width of the copper foil is 300 mm. The layers are misaligned by 20% during wrapping to enhance the sealing performance.

[0088] S3 calculates the inclusions adding timing according to the continuous casting pulling speed and sawing position, and immerses the inclusions into the copper liquid in the holding furnace through the rear bore cover plate of the holding furnace; the inclusions adding timing satisfies: L1 = 21.5 m - 5.8 m - 0.3 m / min x (10 min + 5 min) = 11.2 m;

[0089] S4 dynamically supplements the rare earth inclusions into the holding furnace according to the smelting furnace pouring copper liquid period, and controls the rare earth concentration to be stably within the range of ±5% of 5 ppm.

[0090] S5 the copper liquid temperature of the holding furnace is 1080±10℃, the crystallizer cooling water flow is 20 m 3 / h (water temperature 25±2℃); the pulling speed v = 0.3 m / min, the cast blank diameter Φ82 mm, the surface roughness Ra≤6.3μm. After the cast blank goes out of the crystallizer, it enters the water mist cooling section (water pressure 0.5 MPa, atomized particle diameter 50-100μm); the cooling rate is 50-80℃ / min, and the final cooling temperature is ≤200℃, so as to avoid the deformation caused by residual stress.

[0091] The prepared copper alloy cast blank is phosphorus deoxidized copper, and the phosphorus content in the copper alloy cast blank is 0.0231wt.% according to the mass percentage calculation, the rare earth content is 5 ppm, and the rare earth / phosphorus mass ratio is 0.0216.

[0092] S6 removes the surface oxide layer of the copper alloy cast blank (single side cutting amount 0.5-1.0 mm), and the surface finish Ra≤1.6μm; through the three-roll planetary rolling mill (rolling force 800-1000 kN), the total diameter reduction rate is 85%, and the pass deformation amount is ≤25%; the stretching is divided into joint stretching and disc stretching, and the final outer diameter Φ4.0 mm is obtained through multiple pass cold drawing; when the internal thread is formed, the copper tube is annealed online, and then the spinning diameter reduction method is used (core head spiral angle 15°, rotating speed 1200 rpm), and the tooth top filling rate is ≥95%; then the copper tube is annealed again, and is packaged to form the finished product.

[0093] The prepared ultra-fine diameter internal thread copper tube has an elongation of 53%, a tensile strength of 242 MPa, a grain size of 0.0221 mm, and a flaring rate of 36.5%. The outer tube diameter of the copper tube is 3.63 mm, the copper tube is provided with an internal thread, and the thickness between the tooth groove bottom 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 spiral angle has a value range of 14.5°. The internal thread is composed of tooth grooves and tooth bars, and the cross section of the tooth bar is a triangle with an arc line at the top, and the tooth grooves between adjacent tooth bars have an inverted trapezoidal shape.

[0094] The inner and outer surfaces are clean, and there are no harmful defects such as pinholes, cracks, peeling, bubbles, inclusions, blackening, green rust, severe oxide film, copper powder, water droplets, flattening, scratches, and mold marks. There are no scratches, pits, and spot defects. After eddy current testing, the number of single disc defects is 0, and after the flattening experiment, no visible microcracks are observed. Under the condition that the bending core diameter is 1.5 times the nominal outer diameter, the copper pipe is bent 180° once without wrinkles and cracks. Fill the test sample with water, slowly pressurize to 6.5 MPa, and hold for 5 minutes without macroscopic deformation and leakage. After the pressure test is completed, slowly pressurize to 10 MPa and hold for 1 minute. The test is not broken (the actual measurement is 24.5 MPa without breaking).

[0095] Example 2

[0096] A method for preparing a super-fine inner thread copper pipe, comprising the following steps:

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

[0098] S2 a specified amount of rare earth intermediate alloy is pressed into small pieces by a press, and the pieces are preferably in the form of blocks with a diameter of about 30 mm. The rare earth intermediate alloy pieces are wrapped with pure copper foil in multiple layers to form a package; the package is wrapped with 3 layers of copper foil, the package has a diameter of 30±2 mm, the purity of the outer layer of copper foil is ≥99.9%, the thickness of the copper foil is 0.05 mm, and the width of the copper foil is 300 mm. The layers are misaligned by 20% during wrapping to enhance the sealing performance.

[0099] S3 the timing of adding the package is calculated according to the continuous casting pulling speed and the sawing position, and the package is immersed into the copper liquid in the holding furnace through the rear bore plate of the holding furnace; the timing of adding the package satisfies: L1=21.5m-5.8m-0.3m / min×(10min+5min)=11.2m; and the two batches are ensured not to be mixed.

[0100] S4 based on the preparation of the first batch of cast slabs in example 1, the rare earth package is supplemented into the holding furnace according to the pouring period of the smelting furnace to control the rare earth concentration to be stably within the range of ±5% of 10 ppm.

[0101] Since the experiment is continuous on the basis of the previous experiment, the influence of the previous rare earth content needs to be considered. At this time, the amount of the rare earth concentration needed to be added is calculated:

[0102]

[0103] wherein y represents the weight of the intermediate alloy needed to be supplemented (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] S5 The temperature of the copper liquid in the holding furnace is 1080±10°C, and the cooling water flow rate of the crystallizer is 20 m 3 / h (water temperature 25±2°C); the pulling speed v=0.3 m / min, the diameter of the cast blank Φ82 mm, and the surface roughness Ra≤6.3 μm. After the cast blank leaves the crystallizer, it enters a water mist cooling section (water pressure 0.5 MPa, mist particle diameter 50-100 μm); the cooling rate is 50-80°C / min, and the final cooling temperature is ≤200°C, so as to avoid deformation caused by residual stress. The prepared copper alloy cast blank is phosphorus deoxidized copper, and the phosphorus content in the copper alloy cast blank is 0.0229 wt.%, the rare earth content is 10 ppm, and the mass ratio of rare earth to phosphorus is 0.0433 according to the mass percentage.

[0105] S6 The surface oxide layer of the copper alloy cast blank is removed (single-side cutting amount 0.5-1.0 mm), and the surface smoothness Ra≤1.6 μm; the copper pipe is processed by a three-roller planetary rolling mill (rolling force 800-1000 kN), the total diameter reduction rate is 85%, and the pass deformation amount is ≤25%; the stretching is divided into joint stretching and disc stretching, and the final outer diameter Φ4.0 mm is obtained after multiple passes of cold drawing; when the internal thread is formed, the copper pipe is annealed on line, and then is compressed in diameter by spinning (core head spiral angle 15°, rotating speed 1200 rpm), the tooth top filling rate is ≥95%; then the copper pipe is annealed again, and is packaged to form a finished product.

[0106] The prepared ultra-fine diameter internal thread copper pipe has an elongation of 52%, a tensile strength of 243 MPa, a grain size of 0.0225 mm, and a flaring rate of 36.8%. The outer diameter of the copper pipe is 3.62 mm, the copper pipe is provided with an internal thread, and the thickness between the tooth groove bottom of the internal thread and the outer diameter of the copper pipe is 0.19-0.22 mm. The internal thread has a spiral angle, and the spiral angle has a value range of 14.7°. The internal thread is composed of tooth grooves and tooth bars, and the cross section of the tooth bar is a triangle with an arc line at the top, and the tooth grooves between adjacent tooth bars have an inverted trapezoidal shape.

[0107] After detection, the internal and external surfaces are clean, and have no harmful defects such as pinholes, cracks, peeling, bubbles, inclusions, blackening, green rust, serious oxide film, copper powder, water droplets, flattening, bruising, and die marks. There are no scratches, pits, and spot defects. After eddy current flaw detection, the number of single disc defects is 0, and after the flattening experiment, no visible micro cracks are observed. Under the condition that the bending core diameter is 1.5 times the nominal outer diameter, the copper pipe is bent 180° once without wrinkles and cracks. The test sample is filled with water, and the test pressure is slowly increased to 6.5 MPa, and no macro deformation and leakage occur after pressure maintaining for 5 min. After the pressure test is completed, the test pressure is slowly increased to 10 MPa again, and pressure maintaining for 1 min, and the test does not break (the actual measured value is 24.8 MPa without breaking).

[0108] Those skilled in the art should understand that the present application is not limited to the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only to illustrate the principles of the present application, and various changes and improvements can be made without departing from the spirit and scope of the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method of manufacturing a super slim internal thread copper pipe, characterized by, The method comprises the following steps: S1 smelting electrolytic copper plate and transferring the obtained copper liquid into a holding furnace; S2 wrapping the rare earth intermediate alloy pieces with ≥3 layers of pure copper foil to form a package; the package has a diameter of 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; S3 calculating the package adding time according to the continuous casting traction speed and sawing position, and immersing the package into the copper liquid in the holding furnace; the package adding time satisfies L1=L0-L2-v×(t1+t2); wherein, L1 is the length of the casting blank at the time of adding the package, L0 is the set total length of a single casting blank, L2 is the distance between the sawing position and the wall of the holding furnace, v is the traction speed, t1 is the rare earth dissolution time, and t2 is the composition homogenization time; S4 dynamically supplementing the rare earth package according to the pouring cycle of the smelting furnace to control the rare earth concentration; after the smelting furnace pours the copper liquid into the holding furnace, the amount of rare earth intermediate alloy is dynamically adjusted to stabilize the rare earth concentration in the holding furnace within the range of ±5% of the target value, and the target value is 5-10 ppm; S5 obtaining a copper alloy casting blank after continuous casting and cooling; S6 milling, rolling, stretching, internal thread forming, finishing and annealing the copper alloy casting blank to obtain an ultra-fine diameter internal thread copper pipe with an outer diameter ≤4.0 mm; the elongation of the ultra-fine diameter internal thread copper pipe is ≥40%; the tensile strength is 220-255 MPa; the grain size is 0.020-0.035 mm; and the flaring rate is ≥35%.

2. The method of manufacturing a super slimline copper pipe according to claim 1, wherein, After the copper liquid is transferred into the holding furnace, 200-250 mm thick flake graphite is laid on the surface of the copper liquid, and the immersion depth of the package is 200-400 mm below the surface of the copper liquid.

3. The method of manufacturing a super slimline copper pipe according to claim 1, wherein, The rare earth content in the rare earth intermediate alloy accounts for 15-20 wt.%, and the balance is copper and other impurities; the rare earth includes at least one of lanthanum, cerium, praseodymium and neodymium.

4. The method of manufacturing a super slimline copper pipe according to claim 1, wherein, The copper alloy casting blank is phosphorus deoxidized copper, and the phosphorus content in the copper alloy casting blank is 0.015-0.040 wt.% and the rare earth content is 5-10 ppm according to the mass percentage, and the mass ratio of rare earth / phosphorus is 0.01-0.

05.

5. A copper pipe with extra-fine internal thread produced by the method according to any one of claims 1 to 4, characterized in that, The outer diameter of the copper pipe is 3.4-4.0 mm, the copper pipe is provided with an internal thread, and the thickness between the tooth groove bottom of the internal thread and the outer diameter of the copper pipe is 0.17-0.23 mm.

6. A copper pipe with extra small pitch internal thread according to claim 5, characterized in that, The internal thread has a spiral angle, and the value of the spiral angle is 13°-17°.

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