A free-cutting composite zinc white copper pipe and a preparation method thereof

Abstract

The present application relates to a kind of free-cutting composite zinc white copper pipe and its preparation method, copper 60.0~64.0%, nickel 14.0~17.0%, zinc 16.0~19.0%, iron 0.10~0.30%, manganese 0.20~0.40%, silicon 0.05~0.15%, tin 0.05~0.10%, aluminum 0.02~0.06%, bismuth 0.03~0.08%, phosphorus 0.11~0.15%, lead 0.21~0.25%, beryllium 0.1~0.2%, titanium 0.01~0.02%, the rest is inevitable impurity;Adjust each alloy element cooperation to realize alloy hardness HB≥150, tensile strength σb≥530MPa, elongation δ10≥12% after breaking, realize the synergistic optimization of hardness, tensile strength and elongation, satisfy the requirement of special-shaped product turning and forming processing.

Description

Technical Field

[0001] This invention belongs to the field of copper alloy materials technology, specifically relating to an easy-to-cut composite zinc-copper tube and its preparation method. Background Technology

[0002] BZn15-20 zinc-copper alloy, a classic copper-nickel-zinc alloy, typically contains 62.0-65.0% copper, 13.5-16.5% nickel and cobalt, with zinc as the balance, and trace impurities such as lead, phosphorus, and iron. This alloy exhibits excellent corrosion resistance and good ductility, and can withstand pressure processing in both hot and cold states. It is widely used in precision instruments, medical devices, marine engineering, and other fields.

[0003] However, existing BZn15-20 zinc-copper alloys have significant technical defects: First, they have poor machinability, with high cutting forces and easy chip adhesion during processing, resulting in low processing efficiency and high surface roughness, making it difficult to meet the processing requirements of precision parts; Second, they are prone to cracking during hot and cold working processes, especially in tube drawing and rolling processes, resulting in a high scrap rate; Third, they have insufficient balance in mechanical properties, with low hardness, and it is difficult to simultaneously achieve both tensile strength and elongation, limiting their application in high-strength and high-precision scenarios.

[0004] To address the aforementioned issues, existing technologies often improve machinability by adding easily machinable elements such as lead. However, the addition of lead leads to a decrease in alloy toughness, exacerbates cracking, and fails to meet environmental protection requirements. Some solutions improve mechanical properties by adjusting the nickel and zinc content, but this often leads to further deterioration of machinability. Therefore, there is an urgent need for a composite zinc-copper alloy tube and its preparation method that optimizes the composition design and improves the manufacturing process based on BZn15-20 to achieve a synergistic improvement in machinability, crack resistance, and mechanical properties. Summary of the Invention

[0005] To address the problems of poor machinability, easy cracking, and uneven mechanical properties of existing BZn15-20 zinc-copper pipes, this invention provides an easy-to-machinable composite zinc-copper pipe and its preparation method. By redesigning the alloy composition, optimizing the content ratio and preparation process, the machinability is significantly improved while ensuring the corrosion resistance of the alloy, avoiding machining cracking, increasing hardness, and simultaneously taking into account excellent tensile strength and elongation.

[0006] In a first aspect, the present invention relates to a free-cutting composite zinc-copper tubing, the chemical composition of which, by mass percentage, is: copper (Cu) 60.0~64.0%, nickel (Ni) 14.0~17.0%, zinc (Zn) 16.0~19.0%, iron (Fe) 0.10~0.30%, manganese (Mn) 0.20~0.40%, silicon (Si) 0.05~0.15%, tin (Sn) 0.05~0.10%, aluminum (Al) 0. The composition is as follows: 0.02-0.06% bismuth (Bi), 0.03-0.08% phosphorus (P), 0.11-0.15% lead (Pb), 0.21-0.25% beryllium (Be), 0.1-0.2% titanium (Ti), with the remainder being unavoidable impurities, and the total amount of impurities ≤0.05%; among the impurities, sulfur (S) ≤0.005% and carbon (C) ≤0.02%; the sum of the contents of all components and impurities is 100%. The components also satisfy the following content relationship: (1) Ni / Zn = 0.8~1.04; (2) (Fe+Mn) / Si = 3.0~8.0; (3) Sn+Bi=0.09~0.15%.

[0007] Preferably, 0.005-0.02% of rare earth elements may be added, wherein the rare earth elements are cerium (Ce) and / or lanthanum (La), and when Ce and La are mixed, the mass ratio of the two is 1-2:1.

[0008] Secondly, the present invention relates to a method for producing a free-cutting composite zinc-copper tubing, comprising the following steps: (1) Raw material pretreatment: Weigh each raw material according to the chemical composition ratio, and clean the oil, oxide scale and impurities on the surface of the raw materials; bismuth, iron, manganese, silicon and phosphorus are added in the form of intermediate alloys, tin and aluminum are added in the form of pure ingots, and rare earth elements are added in the form of rare earth silicon iron alloys. (2) Smelting and casting: Electrolytic copper and electrolytic nickel are added to a medium-frequency induction furnace and heated to 1180~1200℃ to melt and hold. Zinc ingots, aluminum ingots, Fe / Mn / Si master alloy, P master alloy, tin ingots, Bi master alloy and rare earth ferrosilicon alloy are added in sequence. After stirring and settling to remove gas, the alloy ingots are cast at 1150~1170℃ using a semi-continuous casting process. During the smelting process, the oxidation loss of aluminum and tin is controlled to ensure that the final composition meets the content requirements. (3) Homogenization annealing: Hold the ingot at 780~820℃ for 4~6h and cool it to room temperature with the furnace; (4) Hot extrusion tube: The annealed ingot is heated to 720~780℃, and the extrusion cylinder is preheated to 350℃~420℃; hollow tube blanks are produced by piercing extrusion. (5) Cold rolling finishing: The tube blank is cold rolled at room temperature, and after each 1 to 2 passes of cold rolling, it is annealed at 680 to 720°C and cold rolled to the target size; (6) Finished product annealing: The cold-rolled pipe is kept at 280~320℃ for 2~3h and then cooled to room temperature in the furnace to obtain the finished pipe.

[0009] Preferably, in step (2), the casting speed of the semi-continuous casting is 80~120mm / h and the cooling water volume is 15~25L / min.

[0010] Preferably, in step (4), the extrusion speed is 2-20 mm / s.

[0011] Preferably, in step (5), the diameter reduction of the cold rolling pass is 3~8mm, the wall reduction is 0.3-0.8mm, and the intermediate annealing holding time is 1~2h.

[0012] The beneficial effects of this invention are as follows: By adding environmentally friendly free-machining elements Bi and Sn and optimizing their content, the cutting force of the alloy is reduced by 20-30%, the chips are small and curled, do not stick to the tool, and the surface roughness Ra is ≤1.2μm, which is far superior to the original BZn15-20. The cutting performance index (relative to C3600) is above 90.

[0013] Excellent crack resistance: By adjusting the Ni and Zn content, reducing β phase precipitation, adding rare earth elements to refine the grains, and optimizing the annealing and rolling parameters in the preparation process, the problem of easy cracking during the original BZn15-20 processing has been completely solved.

[0014] By adjusting the (Fe+Mn) / Si range and the synergistic effect of each alloying element, an alloy hardness HB≥150, tensile strength σb≥530MPa, and elongation after fracture δ10≥12% are achieved. This realizes the synergistic optimization of hardness, tensile strength and elongation, and meets the requirements of turning and forming of profile products. Detailed Implementation

[0015] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] To address the problems of poor machinability, easy cracking, and uneven mechanical properties of existing BZn15-20 zinc-copper pipes, this invention provides an easy-to-machinable composite zinc-copper pipe and its preparation method. By redesigning the alloy composition, optimizing the content ratio and preparation process, the machinability is significantly improved while ensuring the corrosion resistance of the alloy, avoiding machining cracking, increasing hardness, and simultaneously taking into account excellent tensile strength and elongation.

[0017] This invention provides a free-machining composite zinc-copper tubing with the following chemical composition by mass percentage: copper (Cu) 60.0-64.0%, nickel (Ni) 14.0-17.0%, zinc (Zn) 16.0-19.0%, iron (Fe) 0.10-0.30%, manganese (Mn) 0.20-0.40%, silicon (Si) 0.05-0.15%, tin (Sn) 0.05-0.10%, and aluminum (Al) 0. The composition is as follows: 0.02-0.06% bismuth (Bi), 0.03-0.08% phosphorus (P), 0.11-0.15% lead (Pb), 0.21-0.25% beryllium (Be), 0.1-0.2% titanium (Ti), with the remainder being unavoidable impurities, and the total amount of impurities ≤0.05%; among the impurities, sulfur (S) ≤0.005% and carbon (C) ≤0.02%; the sum of the contents of all components and impurities is 100%. The components also satisfy the following content relationship: (1) Ni / Zn = 0.8~1.04; (2) (Fe+Mn) / Si = 3.0~8.0; (3) Sn+Bi=0.09~0.15%.

[0018] This invention optimizes the content relationships between key components and establishes a core formulation formula to ensure synergistic improvement in various performance aspects. The nickel-zinc ratio balance formula is: Ni / Zn = 0.8~1.04, which controls β phase precipitation, avoids processing cracking, and optimizes threshold accuracy. The synergistic formula for strengthening elements is (Fe+Mn) / Si=3.0~8.0, which ensures the strengthening effect of the second phase and better balances hardness and plasticity. Synergistic formula for free-machining elements: Sn + Bi = 0.09~0.15%, which improves cutting performance, avoids brittleness caused by excessive single element, and optimizes threshold accuracy; In one embodiment, 0.005-0.02% of rare earth elements may be added, wherein the rare earth elements are cerium (Ce) and / or lanthanum (La), and when Ce and La are mixed, their mass ratio is 1-2:1. Rare earth elements can work synergistically with Al and P to enhance the melt purification effect, and at the same time help to suppress the segregation tendency of Be and Ti, ensuring that impurities are fully removed and suppressing the brittle effects of Pb and Be, thus ensuring the overall balanced performance of the alloy.

[0019] Nickel (Ni): The content is adjusted to 14.0~17.0%, satisfying the Ni / Zn ratio of 0.8~1.04 with the Zn content, which is higher than the original 13.5~16.5% of BZn15-20. Ni forms an infinitely miscible solid solution with Cu, which significantly improves the alloy's strength and corrosion resistance, while also improving the uniformity of the alloy's color and avoiding a pale yellow tint to the matrix. By precisely controlling the Ni / Zn ratio, the excessive precipitation of the brittle β phase (CuZnNi intermetallic compound) in the alloy can be effectively suppressed, laying the foundation for avoiding processing cracks. Too high a Ni content will lead to a sharp increase in alloy hardness and deterioration in processing performance, while too low a Ni content will result in insufficient corrosion resistance and strength, failing to meet the requirements for a balanced ratio.

[0020] Beryllium (Be) and Titanium (Ti): Be, added at 0.1-0.2%, acts as a powerful solid solution strengthening element, significantly improving the hardness and tensile strength of the alloy. It also refines the grains, improving machinability and crack resistance. Together with Fe and Mn, it forms second-phase strengthening particles, further optimizing mechanical properties. The Be content must be strictly controlled at 0.1-0.2%; excessive Be content leads to a sharp increase in alloy brittleness, exacerbating the risk of machining cracks. Ti, added at 0.01-0.02%, has a strong deoxidizing and degassing effect. It can form high-melting-point compounds with impurities such as O and N in the melt, aiding in melt purification. Simultaneously, it inhibits Be segregation at grain boundaries, preventing localized brittleness caused by Be segregation. Together with Al, P, and rare earth elements, it enhances the alloy's structural stability and crack resistance. Excessive Ti content leads to the formation of coarse TiC and TiN inclusions, reducing the alloy's plasticity and machinability. The two work synergistically to improve the hardness and strength of the alloy while ensuring its plasticity and crack resistance. They complement the free-machining elements (Sn, Bi, Pb) and purification elements (Al, P), achieving synergistic optimization of various properties.

[0021] Bismuth (Bi) and tin (Sn): As environmentally friendly composite free-machining elements, they satisfy the synergistic ratio of Sn + Bi = 0.08~0.15%. Among them, Bi, with an addition of 0.03~0.08%, has extremely low solubility in copper alloys and is uniformly distributed in the grains as fine spherical particles, which enhances the bonding force between the cutting edge and the workpiece. Sn, with an addition of 0.05~0.10%, can refine the chips, reduce the cutting force, and improve the corrosion resistance and wear resistance of the alloy. The synergistic effect of the two is better than that of a single free-machining element. It can significantly improve the cutting performance and avoid the increase in brittleness caused by excessive single element, ensuring that the crack resistance of the alloy is not affected. If the ratio is outside the range, either the free-machining effect will be insufficient or the tendency to crack will be aggravated.

[0022] Iron (Fe), manganese (Mn), and silicon (Si): These three elements satisfy a synergistic ratio of (Fe+Mn) / Si = 3.0~8.0, with Fe 0.10~0.30%, Mn 0.20~0.40%, and Si 0.05~0.15%. Fe and Mn synergistically form fine FeMn intermetallic compounds, which act as second-phase reinforcing particles. Si promotes the uniform dispersion of this reinforcing phase. The combination of these three elements significantly improves the alloy's hardness and tensile strength. At the same time, Fe can refine the grains and delay the recrystallization process, Mn can prevent zinc dezincification corrosion of the alloy and improve the stability of the high-temperature structure, and Si can improve casting performance and reduce ingot porosity. If the (Fe+Mn) / Si ratio is too high, coarse hard phases will be formed, reducing the alloy's plasticity; if the ratio is too low, the strengthening effect will be insufficient, and the requirements for hardness and tensile strength cannot be met.

[0023] In addition to synergistic strengthening, Si (0.05-0.15%) can improve alloy fluidity; Al (0.02-0.06%) can refine grains, improve alloy toughness and crack resistance, and form Al2O3 inclusions with O to purify the melt; P (0.11-0.15%) significantly enhances deoxidation and degassing effects, promotes the flotation of impurities in the melt, and forms trace strengthening phases with Cu and Ni to help improve alloy hardness and processing performance. The synergistic effect of these three can effectively remove harmful impurities such as O and S from the alloy, prevent impurities from forming low-melting-point brittle phases, alleviate the brittleness risk caused by high Pb content, and improve the crack resistance and mechanical property stability of the alloy. Excessive Al content will increase alloy brittleness, while excessive P content will reduce alloy plasticity; therefore, the optimal range is 0.11-0.15%.

[0024] Rare earth elements (Ce / La) and lead (Pb): The addition of rare earth elements at a concentration of 0.005~0.02% can synergistically enhance the purification effect with Al and P, forming high-melting-point compounds with impurities such as O, S, and Pb in the alloy, purifying the melt and removing impurities, while fixing Pb to prevent its grain boundary segregation; Pb is adjusted to be an essential component, with its content controlled at 0.21~0.25%, serving as an auxiliary free-machining element, which can reduce the friction coefficient between the cutting edge and the workpiece, refine the chips, and synergistically improve cutting performance with Bi and Sn; through the synergistic effect of Al, P, and rare earth elements, the brittle precipitation of the Pb phase can be effectively suppressed, avoiding the decrease in alloy toughness and the aggravation of cracking tendency caused by high Pb content, while ensuring the balance of the alloy's comprehensive mechanical properties; Pb content exceeding the range of 0.21~0.25% will lead to excessive precipitation of brittle phase, exacerbating the risk of cracking.

[0025] A method for producing free-cutting composite zinc-copper tubing according to an embodiment of the present invention includes the following steps: (1) Raw material pretreatment: Weigh each raw material according to the chemical composition ratio, and clean the oil, oxide scale and impurities on the surface of the raw materials; bismuth, iron, manganese, silicon and phosphorus are added in the form of intermediate alloys, tin and aluminum are added in the form of pure ingots, and rare earth elements are added in the form of rare earth silicon iron alloys. (2) Smelting and casting: Electrolytic copper and electrolytic nickel are added to a medium-frequency induction furnace and heated to 1180~1200℃ to melt and hold. Zinc ingots, aluminum ingots, Fe / Mn / Si master alloy, P master alloy, tin ingots, Bi master alloy and rare earth ferrosilicon alloy are added in sequence. After stirring and settling to remove gas, the alloy ingots are cast at 1150~1170℃ using a semi-continuous casting process. During the smelting process, the oxidation loss of aluminum and tin is controlled to ensure that the final composition meets the content requirements. (3) Homogenization annealing: Hold the ingot at 780~820℃ for 4~6h and cool it to room temperature with the furnace; (4) Hot extrusion tube: The annealed ingot is heated to 720~780℃, and the extrusion cylinder is preheated to 350℃~420℃; hollow tube blanks are produced by piercing extrusion. (5) Cold rolling finishing: The tube blank is cold rolled at room temperature, and after each 1 to 2 passes of cold rolling, it is annealed at 680 to 720°C and cold rolled to the target size; (6) Finished product annealing: The cold-rolled pipe is kept at 280~320℃ for 2~3h and then cooled to room temperature in the furnace to obtain the finished pipe.

[0026] Preferably, in step (2), the casting speed of the semi-continuous casting is 80~120mm / h and the cooling water volume is 15~25L / min.

[0027] Preferably, in step (4), the extrusion speed is 2-20 mm / s.

[0028] Preferably, in step (5), the diameter reduction of the cold rolling pass is 3~8mm, the wall reduction is 0.3-0.8mm, and the intermediate annealing holding time is 1~2h.

[0029] Hot extrusion forming allows the alloy to exhibit optimal plasticity at high temperatures. Through piercing extrusion, solid ingots are processed into hollow tube blanks, breaking up coarse columnar crystals in the cast state and refining the matrix grains. Within the hot extrusion temperature window of 720~780℃, the β-CuZnNi phase is suppressed, and the matrix is ​​a single-phase α solid solution, avoiding hot working cracking. Online air cooling controls the temperature after each pass, preventing excessive grain coarsening while retaining some work hardening, laying the foundation for cold rolling strengthening.

[0030] Room temperature cold rolling achieves work hardening through dislocation multiplication and grain fragmentation, significantly improving the tensile strength and hardness (HB) of the pipe, and matching the second-phase strengthening effect of Be and FeMn phases. The diameter reduction per cold rolling pass is 3~8mm, and the wall reduction is 0.3-0.8mm. Small deformation amount ensures precision forming and avoids pipe wall cracking and dimensional deviations caused by large diameter reduction. It also matches the distribution characteristics of free-machining phases (Pb, Bi) to prevent phase fragmentation and agglomeration that leads to increased brittleness.

[0031] Finished product annealing temperature: 280~320℃. The alloy low-temperature recovery temperature window eliminates internal stress without softening the matrix; avoids coarsening of the Be strengthening phase and aggregation of the FeMn phase, ensuring hardness (HB) ≥150; prevents Pb and Bi phase agglomeration that leads to a decrease in machinability.

[0032] The embodiments of the present invention are described in detail below. The properties of the alloy samples obtained in each embodiment and comparative example were evaluated under the following conditions, and the test results are shown in Tables 2 and 3.

[0033] The tensile strength and elongation were tested at room temperature on an electronic universal mechanical testing machine in accordance with GB / T228.1-2010 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method.

[0034] The microhardness HV value was tested in accordance with GB-T4340.1-2009 Metallic Materials Vickers Hardness Test Part 1: Test Methods on a digital Vickers hardness tester.

[0035] Under the same machining conditions, the cutting forces of each embodiment and comparative alloy sample were measured using a cutting force tester, and the machinability index of each alloy sample relative to brass C3600 was calculated ([machinability index] = [cutting resistance of C36000] / cutting resistance of each alloy × 100%).

[0036] The components of Examples 1-4 and Comparative Examples 1-5 of the present invention are shown in Table 1.

[0037] Table 1: Chemical composition of the examples and comparative examples

[0038] Example 1: A method for preparing free-cutting composite zinc-copper tubing: (1) Raw material pretreatment: Weigh each raw material according to the chemical composition ratio, and clean the oil, oxide scale and impurities on the surface of the raw materials; bismuth, iron, manganese, silicon and phosphorus are added in the form of intermediate alloys, tin and aluminum are added in the form of pure ingots, and rare earth elements are added in the form of rare earth silicon iron alloys. (2) Melting and casting: Electrolytic copper and electrolytic nickel are added to a medium-frequency induction furnace and heated to 1190℃ for melting and holding. Zinc ingots, aluminum ingots, Fe / Mn / Si master alloy, P master alloy, tin ingots, Bi master alloy and rare earth ferrosilicon alloy are added in sequence. After stirring and settling to remove gas, the alloy ingots are obtained by casting at 1150℃ using a semi-continuous casting process. During the melting process, the oxidation loss of aluminum and tin is controlled to ensure that the final composition meets the content requirements. The casting speed is 100mm / h and the cooling water volume is 20L / min.

[0039] (3) Homogenization annealing: The ingot is held at 780℃ for 6 hours and then cooled to room temperature in the furnace; (4) Hot extrusion tube: The annealed ingot is heated to 720°C and the extrusion cylinder is preheated to 360°C; hollow tube blanks are produced by piercing extrusion at a speed of 5 mm / s. (5) Cold rolling finishing: The tube blank is cold rolled at room temperature, and after each cold rolling pass, it is annealed at 690℃ and cold rolled to the target size; the diameter reduction of the cold rolling pass is 3mm, the wall reduction is 0.3mm, and the intermediate annealing holding time is 1h; (6) Finished product annealing: The cold-rolled pipe is kept at 280°C for 3 hours and then cooled to room temperature in the furnace to obtain the finished pipe.

[0040] Example 2: The preparation method is the same as in Example 1, with the following parameters adjusted: melting temperature 1200℃, casting temperature 1170℃, homogenization annealing temperature 810℃, hot extrusion heating temperature 750℃, extrusion cylinder preheating temperature 380℃, extrusion speed 15mm / s, intermediate annealing temperature 710℃, cold rolling pass diameter reduction 5mm, wall reduction 0.5mm, intermediate annealing holding time 2h, and finished product annealing temperature 310℃.

[0041] Example 3: The preparation method is the same as in Example 1, with the following parameters adjusted: melting temperature 1180℃, casting temperature 1150℃, homogenization annealing temperature 790℃, hot extrusion heating temperature 780℃, extrusion cylinder preheating temperature 400℃, extrusion speed 20mm / s, intermediate annealing temperature 700℃, cold rolling pass diameter reduction 8mm, wall reduction 0.8mm, intermediate annealing holding time 1h, and finished product annealing temperature 290℃.

[0042] Example 4: The preparation method is the same as in Example 1, with the following parameters adjusted: melting temperature 1180℃, casting temperature 1150℃, homogenization annealing temperature 820℃, hot extrusion heating temperature 750℃, extrusion cylinder preheating temperature 42℃, extrusion speed 10mm / s, intermediate annealing temperature 680℃, and finished product annealing temperature 320℃.

[0043] The preparation methods of Comparative Examples 1-5 are the same as those of Example 1.

[0044] The performance of the free-cutting composite zinc-copper tubing of the above-mentioned comparative examples and examples was measured, and the results are shown in Table 2.

[0045] Table 2: Performance data of examples and comparative examples

[0046] As can be seen from Table 2, the free-cutting composite zinc-copper tubing prepared by the present invention ensures synergistic improvement of various properties by optimizing the content of each component and controlling the ratio of Ni / Zn, (Fe+Mn) / Si, and Sn+Bi.

[0047] Compared to Examples 1-4, in Comparative Examples 1-5, after changing the ratios of Ni / Zn, (Fe+Mn) / Si, Sn+Bi, and adjusting the contents of Ti and Be, the components could not fully exert their synergistic effect, and the tensile strength, yield strength, elongation, hardness, and machinability all decreased significantly.

[0048] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A free-cutting composite zinc-copper pipe, characterized in that, The chemical composition, by mass percentage, is as follows: Copper (Cu) 60.0~64.0%, Nickel (Ni) 14.0~17.0%, Zinc (Zn) 16.0~19.0%, Iron (Fe) 0.10~0.30%, Manganese (Mn) 0.20~0.40%, Silicon (Si) 0.05~0.15%, Tin (Sn) 0.05~0.10%, Aluminum (Al) 0.02~0.06%, Bismuth (… The composition is as follows: Bi 0.03~0.08%, Phosphorus (P) 0.11~0.15%, Lead (Pb) 0.21~0.25%, Beryllium (Be) 0.1~0.2%, Titanium (Ti) 0.01~0.02%, with the remainder being unavoidable impurities, and the total amount of impurities ≤0.05%; among the impurities, Sulfur (S) ≤0.005% and Carbon (C) ≤0.02%; the sum of the contents of all components and impurities is 100%. The components also satisfy the following content relationship: (1) Ni / Zn = 0.8~1.04; (2) (Fe+Mn) / Si = 3.0~8.0; (3) Sn+Bi=0.09~0.15%.

2. The free-cutting composite zinc-copper tubing according to claim 1, characterized in that, It also adds 0.005~0.02% of rare earth elements, namely cerium (Ce) and / or lanthanum (La), and when Ce and La are mixed, the mass ratio of the two is 1~2:

1.

3. The method for preparing a free-cutting composite zinc-copper pipe according to claim 1 or 2, characterized in that, Includes the following steps: (1) Raw material pretreatment: Weigh each raw material according to the chemical composition ratio, and clean the oil, oxide scale and impurities on the surface of the raw materials; bismuth, iron, manganese, silicon and phosphorus are added in the form of intermediate alloys, tin and aluminum are added in the form of pure ingots, and rare earth elements are added in the form of rare earth silicon iron alloys. (2) Smelting and casting: Electrolytic copper and electrolytic nickel are added to a medium-frequency induction furnace and heated to 1180~1200℃ to melt and hold. Zinc ingots, aluminum ingots, Fe / Mn / Si master alloy, P master alloy, tin ingots, Bi master alloy and rare earth ferrosilicon alloy are added in sequence. After stirring and settling to remove gas, the alloy ingots are cast at 1150~1170℃ using a semi-continuous casting process. During the smelting process, the oxidation loss of aluminum and tin is controlled to ensure that the final composition meets the content requirements. (3) Homogenization annealing: Hold the ingot at 780~820℃ for 4~6h and cool it to room temperature with the furnace; (4) Hot extrusion tube: The annealed ingot is heated to 720~780℃, and the extrusion cylinder is preheated to 350℃~420℃; hollow tube blanks are produced by piercing extrusion. (5) Cold rolling finishing: The tube blank is cold rolled at room temperature, and after each 1 to 2 passes of cold rolling, it is annealed at 680 to 720°C and cold rolled to the target size; (6) Finished product annealing: The cold-rolled pipe is kept at 280~320℃ for 2~3h and then cooled to room temperature in the furnace to obtain the finished pipe.

4. The preparation method according to claim 3, characterized in that, In step (2), the casting speed of the semi-continuous casting is 80~120mm / h, and the cooling water volume is 15~25L / min.

5. The preparation method according to claim 3, characterized in that, In step (4), the extrusion speed is 2-20 mm / s.

6. The preparation method according to claim 3, characterized in that, In step (5), the diameter reduction of the cold rolling pass is 3~8mm, the wall reduction is 0.3-0.8mm, and the intermediate annealing holding time is 1~2h.

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