A threaded copper tube and its preparation method

Abstract

This invention discloses an internally threaded copper tube, the preparation method of which includes the following steps: Step 1, drawing molten copper into an ingot through horizontal continuous casting; Step 2, stretching the ingot into a hardened mother tube blank through milling and continuous coiling; Step 3, annealing the tube blank through online induction; Step 4, forming the internal thread of the annealed soft mother tube to obtain a copper tube; Step 5, rewinding the copper tube into a coil on a horizontal finishing machine; Step 6, statically annealing the coil after finishing in Step 5 in a roller hearth furnace. The method for preparing the internally threaded copper tube of the present invention controls the average grain size of the copper tube after online annealing to be 0.01 to 0.015 mm, the internal thread forming die ratio to be controlled at 550 to 650 r / m, the rewinding cleaning agent to be a mixture of new and old cleaning agents at a mass ratio of (2 to 3):1, and the oxygen content in the finished product annealing furnace to be controlled at 50 to 100 ppm, so that the sliding friction coefficient of the outer surface of the copper tube is 0.16 to 0.40, and the tube insertion resistance of the U-tube with a length greater than 1500 mm can be reduced to below 80 N.

Description

Technical Field

[0001] This invention relates to the field of copper alloy processing technology, and in particular to an internally threaded copper tube and its preparation method. Background Technology

[0002] Internally threaded copper tubes, due to their uniformly serrated inner surface, offer a significantly increased inner surface area compared to conventional smooth copper tubes, effectively improving heat exchange efficiency. They are currently widely used in the air conditioning heat exchanger industry. The manufacturing process of internally threaded copper tubes into heat exchangers involves U-bending, tube threading, and tube expansion. The tube threading process requires manual insertion of the U-bent copper tube through the finned holes made of aluminum foil, ensuring a tight bond between the copper tube and the aluminum foil after expansion. The standard for threading force in the air conditioning industry is: ① U-tube length ≤ 1500mm, threading force ≤ 60N; ② U-tube length > 1500mm, threading force ≤ 80N. For conventionally U-bent copper tubes with a length of less than 1m, the manual threading resistance is between 40 and 60N. In recent years, with the development of commercial air conditioning, the manufacturing size of the heat exchanger and the heat exchanger has gradually increased, resulting in the length of the copper pipe after the U-bend reaching more than 2m. This leads to higher resistance when manually threading the pipe, exceeding the industry-accepted resistance of less than 80N. This not only increases the workload of employees, but also makes the copper pipe more prone to wrinkling and scrapping due to the higher resistance, thus increasing the scrap rate of copper pipes to a certain extent.

[0003] To reduce the resistance of U-tube insertion, it is necessary to further reduce the coefficient of sliding friction on the copper tube surface, thereby reducing the frictional force between the copper tube and the aluminum foil during the insertion process. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing an internally threaded copper tube and its preparation method.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] The first aspect of this invention is to provide a method for preparing an internally threaded copper tube, comprising the following steps:

[0007] Step 1: The molten copper is drawn into ingots through horizontal continuous casting;

[0008] Step 2: The ingot obtained in Step 1 is milled, rolled, and continuously drawn to form a hardened mother tube blank.

[0009] Step 3: The hardened mother tube blank obtained in Step 2 is subjected to online induction annealing;

[0010] Step four: The soft mother tube after annealing in step three is internally threaded to obtain a copper tube;

[0011] Step 5: The copper tube formed in step 4 is rewound and coiled into a roll on a horizontal finishing machine;

[0012] Step six: After finishing in step five, the coil is subjected to static annealing in a roller hearth furnace to obtain the internally threaded copper tube.

[0013] Furthermore, in step three, after the hardened mother tube blank undergoes online induction annealing, its average grain size is controlled between 0.01 and 0.015 mm. If the copper tube grain size is less than 0.01 mm, the copper tube is not sufficiently softened during annealing, resulting in a hard performance. This will lead to high drawing resistance and incomplete tooth filling during the subsequent internal thread forming process in step four. If the copper tube grain size is greater than 0.015 mm, the copper tube will have a coarse grain size after online induction annealing, resulting in poor surface quality and a rough appearance after internal thread forming and drawing in step four.

[0014] Furthermore, in step four, the spin ratio M (the ratio of motor spinning speed to stretching speed) during the internal thread forming process is controlled at 550-650 r / m.

[0015] The surface roughness of internally threaded copper tubing is related to the size and number of steel balls, the spinning speed of the motor, and the drawing speed. The surface roughness H of the tubing is calculated as: H = r - {r} 2 -[(Rr) / (M·N·R)] 2} 1 / 2 Where r represents the size of the steel ball, R represents the size of the spinning ring, N represents the number of steel balls, and M represents the die ratio. From the roughness formula, it can be seen that, with fixed tool geometry parameters, a larger die ratio M results in a smaller surface roughness of the copper tube. If the die ratio is less than 550 r / m, the gap between the thread marks on the copper tube surface is large, and the surface roughness H is high. As the die ratio gradually increases, the surface roughness of the copper tube decreases. When the die ratio exceeds 650 r / m, the surface roughness of the copper tube does not change significantly, and continuously increasing the die ratio M will sacrifice the forming and stretching speed, affecting production efficiency. Therefore, to ensure the surface roughness of the spun copper tube and the forming and stretching efficiency, the die ratio for internal thread forming should be controlled between 550 and 650 r / m.

[0016] Furthermore, in step five, online outer surface cleaning is also required during the rewinding process; the online outer surface cleaning uses a mixed cleaning agent, which includes a new cleaning agent and an old cleaning agent, wherein the old cleaning agent refers to the cleaning agent that has been used to clean 50 to 60 tons of products; the mass ratio of the new cleaning agent to the old cleaning agent is (2 to 3):1.

[0017] When the mass ratio of new cleaning agent to old cleaning agent is less than 2:1, the cleaning ability of the mixed cleaning agent is insufficient, resulting in a large amount of residual oil and a thick oil film on the copper tube surface. After subsequent annealing, the copper tube surface still has a lot of residual oil. Although this can reduce the sliding friction coefficient of the copper tube surface to some extent, it is not conducive to the customer's on-site wire laying, and slippage is likely to occur, making it impossible to lay the material normally. When the mass ratio of new cleaning agent to old cleaning agent is greater than 3:1, the mixed cleaning agent will clean the outer surface of the copper tube too thoroughly, resulting in a dry surface of the copper tube after subsequent annealing. This not only increases the sliding friction coefficient of the copper tube surface, but also makes it not conducive to the customer's on-site copper tube coil laying and is prone to material jamming.

[0018] Furthermore, the cleaning agent selected is the Kt-826m model cleaning agent from Hongrun manufacturer.

[0019] Furthermore, in step six, the oxygen content inside the roller hearth furnace is between 50 and 100 ppm.

[0020] Step six involves placing the finished copper tube in a roller hearth furnace for static annealing. During this process, some of the oil film on the copper tube surface will volatilize and decompose into long-chain hydrocarbons and aromatic compounds, preventing the oil film from fully covering the entire outer surface of the copper tube. To further reduce the sliding friction coefficient of the copper tube surface, the oxygen content in the furnace must be controlled between 50 and 100 ppm, allowing a thin oxide film of 1–3 μm thickness to form on the surface of the copper tube lacking an oil film. If the oxygen content in the annealing furnace is below 50 ppm, the surface oxidation of the copper tube during annealing will be insufficient, resulting in a thin oxide film and failing to adequately reduce the sliding friction coefficient. If the oxygen content exceeds 100 ppm, it will lead to excessive oxidation of the copper tube surface, resulting in poor discoloration and making welding difficult and prone to leakage during subsequent manufacturing processes.

[0021] A second aspect of the present invention is to provide an internally threaded copper tube prepared by the above-described preparation method, wherein the sliding friction coefficient of the outer surface of the internally threaded copper tube is 0.16 to 0.40.

[0022] The present invention adopts the above technical solution and has the following technical effects compared with the prior art:

[0023] The internally threaded copper tube of the present invention can reduce the sliding friction coefficient of the internally threaded copper tube surface, reduce the resistance of U-tubes with a subsequent length greater than 1500mm to below 80N, reduce the labor intensity of employees, improve production efficiency, and increase the yield of subsequent processing.

[0024] The method for preparing internally threaded copper tubes of the present invention controls the average grain size of the copper tube after online annealing to be 0.01 to 0.015 mm, the internal thread forming die ratio to be controlled at 550 to 650 r / m, the rewinding cleaning agent to be a mixture of new and old cleaning agents at a mass ratio of (2 to 3):1, and the oxygen content in the finished product annealing furnace to be controlled at 50 to 100 ppm, thereby reducing the tube insertion resistance of U-tubes with a length greater than 1500 mm to below 80 N. Attached Figure Description

[0025] Figure 1 This is a grain diagram of the copper tube after online annealing in Comparative Example 1 of this invention;

[0026] Figure 2 This is a grain diagram of the copper tube after online annealing in Example 1 of this invention;

[0027] Figure 3 This is a grain diagram of the copper tube after online annealing in Example 2 of this invention;

[0028] Figure 4 This is a grain diagram of the copper tube after online annealing in Comparative Example 2 of this invention. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the invention. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be combined with each other.

[0030] Examples 1-2 and Comparative Examples 1-8 adopted the following steps:

[0031] Step 1: Molten copper (Cu+Ag≥99.9%, P 0.000~0.040%) is continuously cast into a shape with an outer diameter of [missing information - likely a diameter value]. The ingots have a wall thickness of 25mm, and the horizontal continuous casting speed is 150-500mm / min, with a single ingot weighing 1-1.5 tons.

[0032] Step 2: The surface of the ingot is first milled, with a single-sided milling amount of 0.3 to 0.8 mm. Then, it is rolled by three-roll planetary rolling and double-stretched into a continuous-stretched tube blank. Finally, it is continuously stretched into a hardened mother tube blank.

[0033] Step 3: The hardened mother tube blank after being stretched is annealed on an online induction annealing equipment. After annealing, the average grain size of the copper tube is C.

[0034] Step 4: After annealing, the soft mother tube is internally threaded on a vertical plate internal thread forming machine with a die ratio of M.

[0035] Step 5: Use Kt-826m cleaning agent produced by Hongrun manufacturer to clean the outer surface of the copper tube by mixing it with a certain new and old mass ratio T, and then rewind and refine it into a coil.

[0036] Step six: After horizontal rewinding, the coil is statically annealed in a Junkers roller hearth furnace, with the oxygen content in the furnace controlled to be N, to obtain the finished soft internal thread copper tube.

[0037] The process parameters (average grain size C / mm, rotational modulus M / (r / m), mass ratio of new to old cleaning agent T, and oxygen content in the furnace N / ppm) in the preparation methods of Examples 1-2 and Comparative Examples 1-8 are shown in Table 1 below:

[0038] Table 1

[0039]

[0040]

[0041] The preparation process data and sliding friction coefficients of the finished products of Examples 1-2 and Comparative Examples 1-8 are shown in Table 2 below:

[0042] Table 2

[0043]

[0044] Note: Coefficient of sliding friction μ=F / F n Where F is the magnitude of the sliding friction force, F n This represents the magnitude of the normal force. The coefficient of sliding friction is measured using a sliding friction coefficient measuring instrument of model MXD-02.

[0045] A comparison of the data from Example 1 with Comparative Examples 1 and 2 shows that after online annealing in step 3, the average grain size of the copper tube is too small, and the copper tube is too hard, which will affect the filling degree of the internal thread forming teeth in step 4, making it impossible to form teeth; after online annealing, the average grain size of the copper tube is too large, and the grain size on the surface of the copper tube is obviously coarse after the internal thread forming and drawing, resulting in the final product's tube insertion resistance exceeding the standard by more than 80N.

[0046] A comparison of the data from Example 1 with Comparative Examples 3 and 4 shows that a smaller die ratio for internal thread forming results in a larger feed rate during copper tube forming and a wider spin stamping width, leading to a larger surface roughness of the copper tube and a larger resistance to tube insertion in the finished product. A larger die ratio results in a smaller reduction in the surface friction coefficient of the copper tube, but a significant decrease in the forming and stretching speed, leading to higher production costs and making it unsuitable for mass production.

[0047] A comparison of the data from Example 2 with Comparative Examples 5 and 6 shows that during the horizontal rewinding process of cleaning the outer surface of the copper tube in step five, the ratio of new to old cleaning agent was too small, resulting in insufficient cleaning ability and excessive residual oil on the surface of the copper tube. Although the resistance of the finished copper tube during installation met the target requirement of less than 80N, the copper tube was prone to slippage during the wire laying process at the customer's site, making it impossible to feed the material normally. Conversely, if the ratio of new to old cleaning agent was too large, the cleaning ability of the cleaning agent was too strong, and the oil on the surface of the copper tube was thoroughly cleaned, resulting in a dry surface on the finished copper tube, which increased the resistance of the copper tube during installation.

[0048] A comparison of the data from Example 2 with Comparative Examples 7 and 8 shows that if the oxygen content in the annealing furnace in step six is ​​too low, the surface of the copper tube will not be sufficiently oxidized, resulting in a thin oxide film and increased resistance to tube insertion. If the oxygen content in the furnace is too high, exceeding 100 ppm, the surface of the copper tube will undergo excessive oxidation, resulting in oxidation discoloration.

[0049] The above description is merely a preferred embodiment of the present invention and does not limit the implementation and protection scope of the present invention. Those skilled in the art should realize that any equivalent substitutions and obvious changes made based on the content and illustrations of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing an internally threaded copper tube, characterized in that, Includes the following steps: Step 1: The molten copper is drawn into ingots through horizontal continuous casting; Step 2: The ingot obtained in Step 1 is milled, rolled, and continuously drawn to form a hardened mother tube blank. Step 3: The hardened mother tube blank obtained in Step 2 is subjected to online induction annealing; after online induction annealing, the average grain size of the hardened mother tube blank is controlled at 0.01~0.015mm. Step 4: Perform internal thread forming on the soft mother tube after annealing in Step 3 to obtain a copper tube; the spin ratio M during the internal thread forming process is controlled at 550~650r / m; Step 5: The copper tube formed in Step 4 is rewound into a coil on a horizontal finishing machine. During the rewinding process, an online outer surface cleaning is also required. The online outer surface cleaning uses a mixed cleaning agent, which includes a new cleaning agent and an old cleaning agent. The old cleaning agent refers to the cleaning agent that has been used to clean 50 to 60 tons of products. The mass ratio of the new cleaning agent to the old cleaning agent is (2 to 3):

1. Step six: After finishing in step five, the coil is subjected to static annealing in a roller hearth furnace with an oxygen content of 50-100 ppm, thus obtaining the internally threaded copper tube.

2. A threaded copper tube prepared by the method described in claim 1, characterized in that, The coefficient of sliding friction on the outer surface of the internally threaded copper tube is 0.16~0.40.

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