A heat treatment preparation and plasticity improvement process of a copper rod

Abstract

The application discloses a heat treatment preparation and plasticity improvement process of a copper rod, relates to the technical field of copper and copper alloy copper material heat treatment, and realizes targeted removal of harmful impurities at the grain boundary by constructing impurity diffusion channels through large processing rate plastic deformation and combining three-section continuous controllable atmosphere heat treatment, so that the impurity removal rate is more than 60%, the problem of intergranular brittle fracture and deep drawing cracking caused by the grain boundary impurity aggregation is solved, the copper rod grain boundary bonding strength is improved by more than 50%, and the multi-pass deep drawing forming yield is improved from about 70% to more than 99%. Meanwhile, the application does not need alloying modification in the smelting stage, avoids the problem of significant decrease in conductivity caused by the introduction of alloy elements, controls the copper rod conductivity loss to be less than or equal to 0.2% IACS through precise control of the micro-oxidation degree and the weak reduction process, and meets the dual requirements of high conductivity and high plasticity.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for electrical copper materials, specifically a heat treatment preparation and plasticity enhancement process for copper rods. Background Technology

[0002] Copper and copper alloy rods are core materials in the fields of power transmission, new energy wiring harnesses, precision electronic wires, electronic connectors, and rail transit cables. Their performance directly affects the quality and reliability of related products. In practical applications, copper rods not only need to have extremely high conductivity to meet the requirements of efficient power transmission, but also need to have excellent cold working plasticity so that they can maintain good forming performance during multiple cold drawing, deep drawing, bending and other deep processing processes, and ensure the yield of finished products.

[0003] However, during the smelting, continuous casting and rolling production of copper rods, low-melting-point harmful impurities such as S, Pb and Bi inevitably remain in the copper matrix. These impurities have almost zero solid solubility in copper at room temperature. During subsequent cold working and heat treatment, they will spontaneously segregate towards the grain boundaries, forming a continuous brittle grain boundary film. This leads to a significant decrease in grain boundary bonding energy, which in turn makes copper rods prone to intergranular brittle fracture and deep drawing cracks during deep processing, severely restricting the large-scale production of high-precision copper materials.

[0004] Existing solutions to the above problems mainly fall into two categories, but both have significant drawbacks. The first category is alloying modification methods during the smelting stage. This involves adding rare earth elements, Zr, Ag, Ti, and other alloying elements to the copper melt, causing them to react with harmful impurities such as S, Pb, and Bi to form high-melting-point intermetallic compounds, thereby suppressing the grain boundary segregation of impurities. However, the alloying elements introduced by this method inevitably cause electron scattering in the copper matrix, leading to a significant decrease in the conductivity of the copper rod. Typically, the conductivity loss is ≥2% IACS, making it difficult to meet the requirements for high-conductivity electrical copper materials. At the same time, the alloying process requires extremely high control precision, and the cost of raw materials is also significantly increased. Furthermore, it can easily generate new inclusion defects, further affecting the quality of copper rods. The second category is subsequent heat treatment modification methods. Conventional vacuum annealing and recrystallization annealing can only eliminate residual stress from cold working and control grain size, but they are powerless against harmful impurities that have already agglomerated at grain boundaries, and cannot solve the problem of brittle fracture from the root. Although a few publicly disclosed oxidation-reduction annealing processes attempt to remove grain boundary impurities to some extent, they have problems such as poor controllability of the oxidation process, insufficient targeting of grain boundary impurities, and low impurity removal efficiency. In addition, these processes can easily lead to excessive oxidation of the copper matrix, resulting in a large loss of conductivity, and poor batch stability, making it difficult to adapt to industrial continuous production and unable to be widely promoted and applied. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a heat treatment preparation and plasticity enhancement process for copper rods. This process can introduce impurity diffusion channels inside the copper rod through large deformation pretreatment, and combined with three-stage continuous controllable atmosphere heat treatment, it can achieve targeted removal of harmful impurities at grain boundaries. At the same time, with precise recrystallization heat treatment and a full-process positive pressure protection system, it can significantly improve the grain boundary bonding strength (more than 50%) and cold working plasticity while ensuring that the conductivity of the copper rod is almost unaffected (conductivity loss ≤0.2% IACS), thereby fundamentally solving the problems of intergranular brittle fracture and deep drawing cracking.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a heat treatment preparation and plasticity improvement process for copper rods, the process comprising the following specific steps:

[0007] S1. Pretreatment and crystal control: The copper rod blank is subjected to high-rate plastic deformation, and the degree of deformation is controlled at 70%-95%. The original grain size is adjusted to obtain deformed copper rod.

[0008] S2. Furnace sealing: Place the deformed copper rod inside the furnace chamber of the horizontal vacuum resistance annealing furnace and seal the furnace body;

[0009] S3. Atmosphere replacement: Before heating, the furnace chamber is evacuated, and the vacuum pressure is controlled at -0.1MPa to -0.2MPa. Then, high-purity argon gas is introduced into the furnace chamber until the pressure inside the furnace is 0.1MPa-0.2MPa. The furnace chamber is kept under positive pressure throughout the heat treatment process.

[0010] S4. Segmented temperature control and atmosphere heat treatment: The temperature is increased at a rate of 200℃ / h-300℃ / h. When the furnace temperature reaches 400℃-450℃, the three-stage continuous controllable atmosphere heat treatment is started. After completion, the temperature is increased to the target heat treatment temperature of 500℃-600℃.

[0011] S5. Heat preservation and homogenization: After the furnace temperature is raised to the target heat treatment temperature, heat preservation treatment is carried out for 4-6 hours. During the heat preservation process, a weak reducing protective atmosphere is continuously introduced to maintain positive pressure inside the furnace.

[0012] S6. Programmed cooling: After the heat preservation is completed, the furnace is cooled at a rate of 100℃ / h-200℃ / h. High-purity argon gas is continuously introduced during the cooling process and the positive pressure inside the furnace is maintained until the furnace temperature drops to room temperature.

[0013] S7. Unloading: Open the furnace and remove the heat-treated copper rod to obtain a high-ductility, high-conductivity copper rod.

[0014] Furthermore, the specific steps of S1 are as follows:

[0015] T1 or T2 low-oxygen copper rods or TU1 or TU0 oxygen-free copper rods are selected. The billet is in a continuously cast and rolled hot-rolled state with a diameter of [missing information]. The total content of harmful impurities S, Pb and Bi is ≤500ppm, the original conductivity is ≥100%IACS, and the surface is free of cracks and oxide scale.

[0016] The process employs multi-pass cold drawing deformation, uses polycrystalline diamond wire drawing dies, and lubricates with fully synthetic copper wire drawing oil. The single-pass processing rate is controlled at 10%-30% to avoid excessive deformation in a single pass, which could lead to internal micro-cracks. The total deformation is strictly controlled at 70%-95%, preferably 80%-90%, with no intermediate annealing between passes.

[0017] When the total deformation is less than 70%, the introduced dislocation density is insufficient, the number of grain boundaries is small, the impurity diffusion channels are insufficient, and the subsequent impurity removal efficiency is ≤30%; when the total deformation is greater than 95%, the cumulative processing rate of each pass is too large, and irreparable microcracks are generated inside the copper rod, which cannot be eliminated by subsequent heat treatment, resulting in the deterioration of the final mechanical properties.

[0018] The grains of the deformed copper rod are significantly elongated, forming a continuous fibrous structure with a dislocation density ≥10. 14 m -2 The average transverse grain size is ≤2μm, the surface roughness Ra is ≤0.8μm, and there are no microcracks inside, providing a sufficient and rapid channel for the selective oxidation and diffusion removal of harmful impurities in subsequent heat treatment.

[0019] Furthermore, the specific steps of S2 are as follows:

[0020] The horizontal single-chamber / double-chamber vacuum resistance annealing furnace is selected. The furnace chamber is made of heat-resistant stainless steel and equipped with a multi-segment program temperature control system, a multi-channel high-precision atmosphere ratio system, a three-stage vacuum system, a pressure closed-loop control system, an online oxygen / hydrogen content monitoring system and an exhaust gas explosion-proof treatment system. The temperature control accuracy is ±1℃ and the furnace temperature uniformity is ±5℃.

[0021] The deformed copper rods are arranged axially and horizontally, in layers with equal spacing, on a heat-resistant stainless steel special rack. The loading amount is 30%-50% of the effective volume of the furnace. The layer spacing is ≥20mm, and the spacing between copper rods in the same layer is ≥10mm. This avoids the copper rods from sticking together and causing uneven contact of the atmosphere, ensuring that the entire surface of each copper rod can be fully in contact with the heat treatment atmosphere and eliminating heat treatment dead corners.

[0022] After closing the furnace door, tighten the flange seal, start the vacuum system for pre-vacuuming, maintain pressure for 30 minutes, and ensure the furnace pressure rises to ≤5Pa. Detect the leak rate using a helium mass spectrometer leak detector, ensuring it is ≤1×10⁻⁶. -9 Pa・m 3 If the pressure rises to a certain level, the airtightness is deemed acceptable before proceeding to the next step. If the pressure rises excessively, leaks must be checked and the system resealed until it is acceptable.

[0023] Furthermore, the specific steps of S3 are as follows:

[0024] Start the three-stage vacuum system, with a pumping rate ≥50L / s, to evacuate the furnace chamber. Control the gauge pressure between -0.1MPa and -0.2MPa, preferably between -0.12MPa and -0.18MPa, with an ultimate vacuum degree ≤5Pa. The pre-evacuation time is ≥30min to completely remove residual air and water vapor from the furnace.

[0025] After evacuating to the target value, close the vacuum valve and fill the furnace with high-purity argon gas with a purity ≥99.999% until the gauge pressure inside the furnace is 0.1MPa-0.2MPa, preferably 0.12MPa-0.18MPa; repeat the evacuation-argon gas replacement process 2-3 times to ensure that the residual oxygen content inside the furnace is ≤10ppm and the water vapor content is ≤5ppm;

[0026] After the atmosphere replacement is completed, the pressure closed-loop control system is activated. Throughout the subsequent heating, three-stage heat treatment, heat preservation, and cooling process, the pressure inside the furnace is maintained at 0.1MPa-0.2MPa. When the pressure inside the furnace is lower than the set lower limit, the system automatically replenishes gas; when it is higher than the set upper limit, the system automatically and slowly depressurizes, eliminating negative pressure throughout the process and preventing outside air from seeping into the furnace and causing oxidation of the copper rod.

[0027] Furthermore, the specific steps of S4 are as follows:

[0028] After atmosphere replacement is completed, a multi-segment programmable temperature control system is started to linearly heat up at a heating rate of 200℃ / h-300℃ / h, preferably 220℃ / h-280℃ / h. If the heating rate is lower than 200℃ / h, the production cycle is too long and the efficiency is low. If the heating rate is higher than 300℃ / h, the temperature uniformity in the furnace is poor and the temperature difference between the inside and outside of the copper rod is large, resulting in uneven microstructure and properties.

[0029] When the furnace temperature reaches the target temperature range of 400℃-450℃, it is kept constant for 30 minutes. When the furnace temperature uniformity reaches within ±3℃, the heating is stopped, and the process enters a three-stage continuous controllable atmosphere heat treatment process with constant temperature. The switching time of each atmosphere is ≤10s to avoid cross-contamination of atmospheres. The selection of this temperature range is based on the fact that the oxidation reaction of low melting point impurities such as S, Pb, and Bi has a much higher thermodynamic driving force than Cu at this temperature. The saturated vapor pressure of their oxides is significantly increased, and they have strong volatility. At the same time, the oxidation reaction of the copper matrix is ​​significantly suppressed under low oxygen partial pressure, achieving selective preferential oxidation of impurities.

[0030] The specific technical solution for the three-stage continuous controllable atmosphere heat treatment is as follows:

[0031] Micro-oxidation segment: Harmful impurities at grain boundaries are selectively and preferentially oxidized;

[0032] Argon gas supply is stopped, and a micro-oxidation mixed atmosphere of N2+O2 is switched to be used, wherein the volume fraction of O2 is strictly controlled at 0.3-0.8 vol%, preferably 0.4-0.6 vol%. The mixed atmosphere is prepared online using a high-precision mass flow meter with a preparation accuracy of ±0.05 vol%. The total flow rate of the atmosphere is 1-2 times the furnace volume per hour. The oxygen content in the furnace is monitored in real time using a zirconia oxygen analyzer with a fluctuation range of ≤±0.05 vol%, ensuring a uniform atmosphere in the furnace without dead zones.

[0033] The furnace temperature is stabilized at 400℃-450℃ and held at that temperature for 15-45 minutes, preferably 20-30 minutes. If the holding time is less than 15 minutes, the oxidation of grain boundary impurities is insufficient, resulting in low impurity removal efficiency. If the holding time is more than 45 minutes, the copper matrix undergoes excessive oxidation, making subsequent reduction difficult and leading to a loss of conductivity.

[0034] Utilizing the difference in oxidation thermodynamics between low-melting-point impurities such as S, Pb, and Bi segregated at grain boundaries and the Cu matrix, under conditions of 400-450℃ and low oxygen partial pressure, the oxidation Gibbs free energy of S, Pb, and Bi is much lower than that of Cu, preferentially reacting with O2 to form volatile oxides such as SO2, PbO, and Bi2O3. Among them, SO2 is gaseous and directly enters the atmosphere, while PbO and Bi2O3 have saturated vapor pressures of over 133 Pa and 80 Pa respectively at this temperature, exhibiting strong volatility and rapidly diffusing from the grain boundaries to the surface of the copper rod and entering the furnace atmosphere. At the same time, the low oxygen partial pressure strictly inhibits the oxidation of the Cu matrix, forming only a trace Cu2O oxide layer with a thickness ≤50 nm on the surface of the copper rod, thus avoiding excessive oxidation of the matrix.

[0035] Gas phase impurity removal section: Dynamic purging thoroughly removes volatile impurities;

[0036] After the micro-oxidation stage is completed, immediately close the micro-oxidation mixed atmosphere valve and quickly switch to a high-purity N2 atmosphere with a purity ≥99.999% within 10 seconds. Perform dynamic purging with a large flow rate. The purging gas flow rate is 2-4 times the furnace volume per hour, preferably 2.5-3.5 times per hour. A straight-through flow field design with annular air inlet at the front end and central air outlet at the rear end is adopted to ensure rapid atmosphere renewal inside the furnace without any dead zones.

[0037] Maintain a constant furnace temperature of 400℃-450℃, and a dynamic purging time of 10-30 minutes, preferably 15-25 minutes. If the purging time is less than 10 minutes, volatile impurities and oxides cannot be completely discharged from the furnace and are prone to secondary condensation and fall back to the surface of the copper rod during the cooling process, or even diffuse back to the grain boundary. If the purging time is more than 30 minutes, the production efficiency will decrease and the oxide layer on the surface of the copper rod will be thickened.

[0038] During the purging process, the gauge pressure inside the furnace is kept stable at 0.1MPa-0.2MPa. A dynamic balance mode of continuous air intake and continuous micro-pressure relief is adopted to ensure that the volatilized impurity oxides are continuously carried out. A condensation collection device is equipped at the gas outlet to condense and collect the impurity oxides, thus avoiding exhaust gas pollution.

[0039] By using high-flow-rate dynamic purging, volatile impurity oxides generated in the micro-oxidation section are rapidly and thoroughly removed from the furnace body, achieving targeted removal of harmful impurities at grain boundaries, avoiding secondary pollution, and ensuring a total removal rate of S, Pb, and Bi at grain boundaries ≥60%.

[0040] Controlled reduction section: The surface oxide layer is precisely reduced to avoid loss of conductivity;

[0041] After the gas phase impurity removal section is completed, the high-purity N2 valve is immediately closed, and the atmosphere is rapidly switched to a weakly reducing mixed atmosphere of N2 and H2 within 10 seconds. The volume fraction of H2 is strictly controlled at 2-4 vol%, preferably 2.5-3.5 vol. The mixed atmosphere is prepared online using a high-precision mass flow meter with a mixing accuracy of ±0.1 vol%. The total atmosphere flow rate is 1-2 times the furnace volume per hour. The hydrogen content in the furnace is monitored in real time using a thermal conductivity hydrogen analyzer, with a fluctuation range of ≤±0.1 vol%.

[0042] After maintaining the furnace temperature at 400℃-450℃ for 5-10 minutes, continue to linearly increase the temperature at a rate of 200℃ / h-300℃ / h until the target recrystallization heat treatment temperature of 500℃-600℃ is reached. During the heating process, the weakly reducing mixed atmosphere is continuously introduced to simultaneously complete the reduction and heating process of the surface oxide layer.

[0043] The weakly reducing mixed atmosphere uses N2 as the carrier gas, and the volume fraction of H2 is strictly controlled below the explosion limit. The entire process ensures that no air seeps into the furnace. The tail gas end is equipped with a flame arrester and a combustion treatment device. Before starting the furnace, the residual hydrogen in the furnace must be replaced with nitrogen until the hydrogen content is ≤0.1% to ensure production safety.

[0044] By utilizing the weak reducing properties of H2, the trace Cu2O oxide layer generated on the surface of the copper rod in the micro-oxidation section is precisely reduced to pure Cu without damaging the copper substrate. The generated H2O is carried out of the furnace with the atmosphere. By controlling the H2 concentration, reduction temperature and time, only the surface oxide layer is reduced, avoiding over-reduction and ensuring that the conductivity loss of the copper rod is ≤0.2% IACS.

[0045] Furthermore, the specific steps of S5 are as follows:

[0046] The target recrystallization heat treatment temperature is raised to 500℃-600℃, preferably 530℃-570℃, with a temperature control accuracy of ±1℃ and a furnace temperature uniformity controlled within ±5℃.

[0047] The furnace should be kept at a constant temperature for 4-6 hours, preferably 4.5-5.5 hours. The furnace temperature should be recorded every hour during the holding process, and the temperature fluctuation should be ≤±2℃. If the holding time is less than 4 hours, recrystallization will be insufficient, residual stress will not be completely eliminated, and the plasticity improvement effect will be poor. If the holding time is more than 6 hours, abnormal grain growth will occur, resulting in a decrease in the strength of the copper rod and a deterioration in the deep processing performance.

[0048] During the heat preservation process, a weak reducing mixed atmosphere of N2 + 2-4 vol% H2 is continuously introduced to keep the gauge pressure inside the furnace stable at 0.1 MPa-0.2 MPa, to avoid oxidation of the copper rod, and at the same time to ensure uniform atmosphere inside the furnace and consistent microstructure and properties of the copper rods throughout the furnace.

[0049] After the heat preservation is completed, the deformed fibrous structure is completely transformed into uniform, equiaxed recrystallized grains with an average grain size controlled between 10μm and 30μm. This completely eliminates residual stress from cold deformation. At the same time, recrystallization further optimizes the grain boundary structure, reduces the degree of grain boundary segregation of residual impurities, and significantly improves the plasticity and processing performance of the copper rod.

[0050] Furthermore, the specific steps of S6 are as follows:

[0051] After the heat preservation is completed, heating is stopped, and the temperature is programmed by adjusting the heating power and cooling fan speed at a linear cooling rate of 100℃ / h-200℃ / h, preferably 120℃ / h-180℃ / h. If the cooling rate is lower than 100℃ / h, the production cycle is too long, and the grains are prone to slow growth. If the cooling rate is higher than 200℃ / h, the temperature difference between the inside and outside of the copper rod is too large, and thermal stress is generated inside, resulting in an increase in residual stress and a decrease in plasticity.

[0052] During the cooling process, high-purity argon gas with a purity of ≥99.999% is continuously introduced into the furnace to maintain the furnace pressure at 0.1MPa-0.2MPa until the furnace temperature drops below 100℃; positive pressure protection is maintained throughout the process to prevent outside air from seeping in and causing oxidation and discoloration on the copper rod surface during the cooling process.

[0053] When the furnace temperature drops to ≤40℃, stop the argon gas supply and prepare to depressurize and start the furnace.

[0054] Furthermore, the specific steps of S7 are as follows:

[0055] High-purity nitrogen is used to replace the residual hydrogen in the furnace until the hydrogen content in the furnace is ≤0.1%. Then, the pressure is slowly released until the pressure inside the furnace is equal to the external atmospheric pressure. After confirming that there is no safety risk, the furnace door is unlocked.

[0056] Open the furnace door, remove the copper rod from the rack, and check the surface quality. Qualified finished products should have a bright metallic color, without oxidation discoloration, scratches, or adhesion. Then, conduct performance tests, including conductivity, grain boundary impurity content, room temperature tensile properties, grain boundary bonding strength, and deep drawing performance. Qualified products are stored in vacuum packaging to prevent oxidation.

[0057] Compared with existing technologies, this heat treatment preparation and plasticity enhancement process for copper rods has the following advantages:

[0058] This invention constructs impurity diffusion channels through high-processing-rate plastic deformation, combined with three-stage continuous controllable atmosphere heat treatment, to achieve targeted removal of harmful impurities at grain boundaries, achieving an impurity removal rate of over 60%. This solves the problems of intergranular brittle fracture and deep-drawing cracking caused by impurity segregation at grain boundaries, increasing the grain boundary bonding strength of copper rods by over 50%, and raising the yield of multi-pass deep-drawing forming from around 70% to over 99%. Furthermore, this invention eliminates the need for alloying modification during the smelting stage, avoiding the significant decrease in conductivity caused by the introduction of alloying elements. By precisely controlling the degree of micro-oxidation and weak reduction processes, the conductivity loss of copper rods is controlled to ≤0.2% IACS, balancing the dual requirements of high conductivity and high plasticity. In addition, this process employs full-process programmed temperature control, high-precision atmosphere ratio, and closed-loop pressure control, ensuring precise and controllable process parameters, high batch stability, and a performance deviation of ≤3% for the entire batch of copper rods.

[0059] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0060] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0061] Figure 1 A flowchart of a heat treatment preparation and plasticity improvement process for a copper rod;

[0062] Figure 2 This is a flowchart of a three-stage continuous controlled atmosphere heat treatment process for the preparation and plasticity enhancement of copper rods. Detailed Implementation

[0063] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0064] Example 1

[0065] This embodiment describes a heat treatment method for preparing copper rods to suppress grain boundary impurity segregation and a plasticity enhancement process. The specific technical solutions for each step are as follows:

[0066] Pretreatment for crystal control: φ8mm T2 low-oxygen copper rod billet was selected, hot-rolled via continuous casting and rolling, with an initial conductivity of 101.3% IACS and a total S, Pb, and Bi content of 128ppm at grain boundaries. A 10-pass cold drawing deformation process was employed, using polycrystalline diamond drawing dies and fully synthetic copper drawing oil for lubrication. The single-pass processing rate was [not specified]. The total deformation rate was 80%, resulting in a deformed copper rod with a diameter of 3.57 mm. No intermediate annealing was performed between passes. The dislocation density of the deformed copper rod was 1.2 × 10⁻⁶. 14 m -2 The average transverse grain size is 1.8 μm, the surface roughness Ra=0.6 μm, and there are no microcracks inside.

[0067] S2. Loading and Airtightness Testing: Arrange the deformed copper rods axially horizontally and in equal layers on the 310S stainless steel rack. The loading amount is 40% of the effective volume of the furnace chamber, with a layer spacing of 25mm and a spacing of 15mm between copper rods in the same layer. Close the furnace door and lock the flange sealing structure. After pre-vacuuming, maintain pressure for 30 minutes. After the pressure rises by 3Pa, use a helium mass spectrometer leak detector to detect a leak rate of 8×10⁻⁶. -10 Pa・m 3 / s, airtightness is qualified.

[0068] S3. Vacuum-Argon Atmosphere Replacement: Start the three-stage vacuum system to evacuate the furnace chamber at a pumping rate of 60 L / s until the gauge pressure is -0.15 MPa and the ultimate vacuum is 3 Pa. The pre-vacuuming time is 40 min. Then, high-purity argon gas with a purity of 99.999% is introduced until the gauge pressure is 0.15 MPa. Repeat the vacuum-argon replacement process twice. The final residual oxygen content in the furnace is 6 ppm and the water vapor content is 3 ppm. Start the pressure closed-loop control system to maintain the gauge pressure in the furnace at 0.12 MPa-0.18 MPa throughout the process.

[0069] S4. Segmented Temperature Control and Atmosphere Heat Treatment: The furnace temperature is linearly increased at a rate of 250℃ / h. After reaching 420℃, the temperature is stabilized for 30 minutes, with a furnace temperature uniformity of ±2℃. Three-stage heat treatment is then initiated, with each stage's atmosphere switching time ≤8s.

[0070] Micro-oxidation section: Argon gas supply is stopped, and the atmosphere is switched to a micro-oxidation mixture of N2 + 0.5 vol% O2. The high-precision mass flow meter has a mixing accuracy of ±0.03 vol%, and the total flow rate is 1.5 m³ / s. 3 / h (1.5 times the furnace volume / hour), the oxygen content fluctuation in the furnace is monitored in real time by a zirconia oxygen analyzer ≤±0.04vol%, and the furnace is kept at a constant temperature of 420℃ for 30min;

[0071] Gas phase impurity removal section: Immediately switch to high-purity N2 with a purity of 99.999%, total flow rate 3m³ / h 3 / h (3 times the furnace volume / hour), adopts a front-end annular air inlet and a rear-end central outlet airflow field, 420℃ constant temperature dynamic purging for 20 minutes, continuous air inlet, micro-pressure relief dynamic balance, and condensation collection of impurity oxides at the outlet end.

[0072] Controlled reduction section: Immediately switches to a weakly reducing mixed atmosphere of N2 + 3 vol% H2, with a mixing accuracy of ±0.08 vol%, and a total flow rate of 1.5 m³ / s. 3 / h (1.5 times the furnace volume / hour), the hydrogen content fluctuation was monitored by a thermal conductivity hydrogen analyzer and was ≤±0.09 vol%. After reduction at 420℃ for 8 min, the temperature was increased to 550℃ at a rate of 250℃ / h.

[0073] Recrystallization and homogenization: After the furnace temperature is raised to 550℃, it is kept at a constant temperature for 5 hours. The furnace temperature is recorded every hour during the holding process. The temperature fluctuation is ≤±1℃ and the furnace temperature uniformity is ±3℃. The weakly reducing mixed atmosphere is continuously introduced and the pressure inside the furnace is stabilized at 0.15MPa. After the holding period, the copper rod fibrous structure is completely transformed into equiaxed recrystallized grains with an average grain size of 22μm.

[0074] S6. Programmed Cooling: After the heat preservation is completed, switch to high-purity argon gas with a purity of 99.999%. By adjusting the heating power and cooling fan speed, the temperature is linearly reduced at a cooling rate of 150℃ / h. The gauge pressure inside the furnace is maintained at 0.12MPa-0.18MPa throughout the process until the furnace temperature drops to 35℃.

[0075] S7. Charging and unloading: High-purity nitrogen is used to replace the residual hydrogen in the furnace until the hydrogen content is 0.04%. The pressure is slowly released to atmospheric pressure. After confirming that there is no safety risk, the furnace is opened and the copper rod is taken out. The surface is bright and metallic, without oxidation, discoloration, or adhesion.

[0076] Example 2

[0077] This embodiment describes a heat treatment method for preparing copper rods to suppress grain boundary impurity segregation and a plasticity enhancement process. The specific technical solutions for each step are as follows:

[0078] S1. Pretreatment for crystal control: 12mm TU1 oxygen-free copper rod billet is selected, continuously cast and rolled in hot-rolled condition, with an initial conductivity of 101.5% IACS and a total S, Pb, and Bi content of 105ppm at grain boundaries; 12-pass cold drawing deformation is employed, using polycrystalline diamond drawing dies and fully synthetic copper drawing oil for lubrication, achieving a single-pass processing rate of... The total deformation rate was 90%, resulting in a deformed copper rod with a diameter of 3.79 mm. No intermediate annealing was performed between passes. The dislocation density of the deformed copper rod was 1.5 × 10⁻⁶. 14 m -2 The average transverse grain size is 1.2 μm, the surface roughness Ra=0.5 μm, and there are no microcracks inside.

[0079] S2. Loading and Airtightness Testing: Arrange the deformed copper rods axially horizontally and in layers at equal intervals on the stainless steel rack. The loading amount is 35% of the effective volume of the furnace chamber, with a layer spacing of 20mm and a spacing of 10mm between copper rods in the same layer. Close the furnace door and lock the flange sealing structure. After pre-vacuuming, maintain pressure for 30 minutes. After the pressure rises by 2Pa, use a helium mass spectrometer leak detector to detect a leak rate of 5×10⁻⁶. -10 Pa・m 3 / s, airtightness is qualified.

[0080] S3. Vacuum-Argon Atmosphere Replacement: Start the three-stage vacuum system to evacuate the furnace chamber at a pumping rate of 60L / s until the gauge pressure is -0.1MPa and the ultimate vacuum is 4Pa. The pre-vacuuming time is 35min. Then, high-purity argon gas with a purity of 99.999% is introduced until the gauge pressure is 0.1MPa. Repeat the vacuum-argon replacement process 3 times. Finally, the residual oxygen content in the furnace is 4ppm and the water vapor content is 2ppm. Start the pressure closed-loop control system to maintain the gauge pressure in the furnace at 0.1MPa-0.15MPa throughout the process.

[0081] S4. Segmented Temperature Control and Atmosphere Heat Treatment: The furnace temperature is linearly increased at a rate of 200℃ / h. After reaching 400℃, the temperature is stabilized for 30 minutes, with a furnace temperature uniformity of ±3℃. Three-stage heat treatment is then initiated, with each stage's atmosphere switching time ≤7s.

[0082] Micro-oxidation section: Argon gas supply is stopped, and the atmosphere is switched to a micro-oxidation mixture of N2 + 0.3 vol% O2. The high-precision mass flow meter has a mixing accuracy of ±0.03 vol%, and the total flow rate is 1 m³ / s. 3 / h (1 times the furnace volume / hour), the zirconia oxygen analyzer monitors the oxygen content fluctuation in the furnace in real time ≤±0.03vol%, and holds at 400℃ for 45min.

[0083] Gas phase impurity removal section: Immediately switch to high-purity N2 with a purity of 99.999%, total flow rate 2m³ / h. 3 / h (twice the volume of the furnace chamber / hour), adopts a front-end annular air inlet and a rear-end central air outlet airflow field, 400℃ constant temperature dynamic purging for 30 minutes, continuous air inlet, micro-pressure relief dynamic balance, and condensation collection of impurity oxides at the outlet end.

[0084] Controlled reduction section: Immediately switches to a weakly reducing mixed atmosphere of N2 + 2 vol% H2, with a mixing accuracy of ±0.07 vol%, and a total flow rate of 1 m³ / s. 3 / h (1 times the furnace volume / hour), the hydrogen content fluctuation was monitored by a thermal conductivity hydrogen analyzer and was ≤±0.08 vol%. After reduction at 400℃ for 10 min, the temperature was increased to 500℃ at a rate of 200℃ / h.

[0085] S5. Recrystallization and homogenization: After the furnace temperature is raised to 500℃, it is kept at a constant temperature for 6 hours. The furnace temperature is recorded every hour during the holding process. The temperature fluctuation is ≤±1℃ and the furnace temperature uniformity is ±4℃. The weakly reducing mixed atmosphere is continuously introduced, and the pressure inside the furnace is stabilized at 0.1MPa. After the holding is completed, the copper rod fibrous structure is completely transformed into equiaxed recrystallized grains with an average grain size of 15μm.

[0086] S6. Programmed Cooling: After the heat preservation is completed, switch to high-purity argon gas with a purity of 99.999%. By adjusting the heating power and cooling fan speed, the temperature is linearly reduced at a cooling rate of 100℃ / h. The gauge pressure inside the furnace is maintained at 0.1MPa-0.15MPa throughout the process until the furnace temperature drops to 38℃.

[0087] S7. Charging and unloading: High-purity nitrogen is used to replace the residual hydrogen in the furnace until the hydrogen content is 0.03%. The pressure is slowly released to atmospheric pressure. After confirming that there is no safety risk, the furnace is opened and the copper rod is taken out. The surface is bright and metallic, without oxidation, discoloration, or adhesion.

[0088] Example 3

[0089] This embodiment describes a heat treatment method for preparing copper rods to suppress grain boundary impurity segregation and a plasticity enhancement process. The specific technical solutions for each step are as follows:

[0090] S1. Pretreatment and Crystal Control: φ6mm T1 low-oxygen copper rod billet is selected, continuously cast and rolled in hot-rolled condition, with an initial conductivity of 101.2% IACS and a total S, Pb, and Bi content of 142ppm at grain boundaries; 8-pass cold drawing deformation is employed, using polycrystalline diamond drawing dies and fully synthetic copper drawing oil for lubrication, achieving a single-pass processing rate of... The total deformation was 70%, resulting in a deformed copper rod with a diameter of 3.29 mm. No intermediate annealing was performed between passes. The dislocation density of the deformed copper rod was 1.0 × 10⁻⁶. 14 m -2 The average transverse grain size is 2.0 μm, the surface roughness Ra=0.7 μm, and there are no microcracks inside.

[0091] S2. Loading and Airtightness Testing: Arrange the deformed copper rods axially horizontally and in layers at equal intervals on the stainless steel rack. The loading amount is 45% of the effective volume of the furnace chamber, with a layer spacing of 30mm and a spacing of 20mm between copper rods in the same layer. Close the furnace door and lock the flange sealing structure. After pre-vacuuming, maintain pressure for 30 minutes. After the pressure rises to 4Pa, use a helium mass spectrometer leak detector to detect a leak rate of 9×10⁻⁶. -10 Pa・m 3 / s, airtightness is qualified.

[0092] S3. Vacuum-Argon Atmosphere Replacement: Start the three-stage vacuum system to evacuate the furnace chamber at a pumping rate of 60 L / s until the gauge pressure is -0.2 MPa and the ultimate vacuum is 2 Pa. The pre-vacuuming time is 45 min. Then, high-purity argon gas with a purity of 99.999% is introduced until the gauge pressure is 0.2 MPa. Repeat the vacuum-argon replacement process twice. The final residual oxygen content in the furnace is 5 ppm and the water vapor content is 2.5 ppm. Start the pressure closed-loop control system to maintain the gauge pressure in the furnace at 0.15 MPa-0.2 MPa throughout the process.

[0093] S4. Segmented Temperature Control and Atmosphere Heat Treatment: The furnace temperature is linearly increased at a rate of 300℃ / h. After reaching 450℃, the temperature is stabilized for 30 minutes, with a furnace temperature uniformity of ±2℃. Three-stage heat treatment is then initiated, with each stage's atmosphere switching time ≤9s.

[0094] Micro-oxidation section: Argon gas supply is stopped, and the atmosphere is switched to a micro-oxidation mixture of N2 + 0.8 vol% O2. The high-precision mass flow meter has a mixing accuracy of ±0.04 vol%, and the total flow rate is 2 m³ / s. 3 / h (twice the furnace volume / hour), the zirconia oxygen analyzer monitors the oxygen content fluctuation in the furnace in real time ≤±0.05vol%, and holds at a constant temperature of 450℃ for 15min;

[0095] Gas phase impurity removal section: Immediately switch to high-purity N2 with a purity of 99.999%, total flow rate 4m³ / h. 3 / h (4 times the furnace volume / hour), adopts a front-end annular air inlet and a rear-end central outlet airflow field, 450℃ constant temperature dynamic purging for 10 minutes, continuous air inlet, micro-pressure relief dynamic balance, and condensation collection of impurity oxides at the outlet end.

[0096] Controlled reduction section: Immediately switches to a weakly reducing mixed atmosphere of N2 + 4 vol% H2, with a mixing accuracy of ±0.09 vol% and a total flow rate of 2 m³ / s. 3 / h (twice the furnace volume / hour), the hydrogen content fluctuation was monitored by a thermal conductivity hydrogen analyzer and was ≤±0.1 vol%. After reduction at 450℃ for 5 min, the temperature was increased to 600℃ at a rate of 300℃ / h.

[0097] S5. Recrystallization and homogenization: After the furnace temperature is raised to 600℃, it is kept at a constant temperature for 4 hours. The furnace temperature is recorded every hour during the holding process. The temperature fluctuation is ≤±2℃ and the furnace temperature uniformity is ±3℃. The weakly reducing mixed atmosphere is continuously introduced, and the pressure inside the furnace is stabilized at 0.2MPa. After the holding is completed, the copper rod fibrous structure is completely transformed into equiaxed recrystallized grains with an average grain size of 28μm.

[0098] S6. Programmed Cooling: After the heat preservation is completed, switch to high-purity argon gas with a purity of 99.999%. By adjusting the heating power and cooling fan speed, the temperature is linearly reduced at a cooling rate of 200℃ / h. The gauge pressure inside the furnace is maintained at 0.15MPa-0.2MPa throughout the process until the furnace temperature drops to 32℃.

[0099] S7. Charging and unloading: High-purity nitrogen is used to replace the residual hydrogen in the furnace until the hydrogen content is 0.04%. The pressure is slowly released to atmospheric pressure. After confirming that there is no safety risk, the furnace is opened and the copper rod is taken out. The surface is bright and metallic, without oxidation, discoloration, or adhesion.

[0100] Comparative Example 1

[0101] This comparative example uses the exact same copper rod billet, pretreatment crystal control, furnace loading, atmosphere replacement, and heating / cooling parameters as Example 1. The only difference is that there is no three-stage continuous controllable atmosphere heat treatment process; the temperature is directly raised to 550°C and held for 5 hours, with high-purity argon gas used for protection throughout the process.

[0102] Comparative Example 2

[0103] This comparative example uses the same batch of copper melt as Example 1, with 0.05wt% rare earth Ce added during smelting, and is cast into φ8mm copper rod billets. The subsequent pretreatment crystal control and conventional vacuum annealing processes are completely consistent with those of Comparative Example 1.

[0104] Comparative Example 3

[0105] This comparative example uses the exact same process parameters as Example 1, the only difference being that the total deformation degree of the pre-treatment control crystal is 60%, which is lower than the 70%-95% range specified in this invention.

[0106] Comparative Example 4

[0107] This comparative example uses the exact same copper rod blank, three-stage heat treatment, recrystallization heat preservation, and heating and cooling parameters as Example 1. The only difference is that there is no pretreatment crystal control process, and the original φ8mm copper rod blank is directly heat treated.

[0108]

[0109] In summary, this invention, through a full-chain synergistic process of high-processing-rate pretreatment for crystal control, three-stage continuous controllable atmosphere targeted impurity removal, and precise recrystallization heat treatment, solves the problem of intergranular brittle fracture caused by the segregation of harmful impurities at the grain boundaries of copper rods in existing technologies. Examples 1-3 of this invention all achieve a grain boundary S, Pb, and Bi harmful impurity removal rate of ≥60%, a grain boundary bonding strength improvement of more than 50%, a room temperature elongation of ≥42%, and a deep drawing yield of ≥98.8%, which is superior to existing processes such as conventional vacuum annealing and alloying modification. It eliminates the brittle film at the grain boundaries and solves the problem of brittle fracture.

[0110] This invention eliminates the need for alloying modification. Through precise micro-oxidation-reduction process control, the conductivity loss of the copper rod is ≤0.2% IACS, resulting in almost no loss of conductivity. This balances the dual requirements of high conductivity and high plasticity, while existing alloying processes result in conductivity losses as high as 2.8% IACS, failing to meet the requirements for high-conductivity electrical copper materials. The high-processing-rate pretreatment and three-stage heat treatment of this invention create a significant synergistic effect. Compared to other methods with no pretreatment or insufficient pretreatment deformation, the impurity removal rate is only about 30%, and the plasticity improvement effect is extremely poor. This fully verifies the integrity and necessity of the process system of this invention.

[0111] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A heat treatment process for preparing and improving the plasticity of copper rods, characterized in that, The process includes the following specific steps: S1. Pretreatment and crystal control: The copper rod blank is subjected to high-rate plastic deformation, which is a multi-pass cold drawing deformation. The degree of deformation is controlled at 70%-95%, and the original grain size is adjusted to obtain deformed copper rod. S2. Furnace sealing: Place the deformed copper rod inside the furnace chamber of the horizontal vacuum resistance annealing furnace and seal the furnace body; S3. Atmosphere replacement: Before heating, the furnace chamber is evacuated, and the vacuum pressure is controlled at -0.1MPa to -0.2MPa. Then, high-purity argon gas is introduced into the furnace chamber until the pressure inside the furnace is 0.1MPa-0.2MPa. The furnace chamber is kept under positive pressure throughout the heat treatment process. S4. Segmented temperature control and atmosphere heat treatment: The temperature is programmed to rise at a rate of 200℃ / h-300℃ / h. When the furnace temperature reaches 400℃-450℃, the three-stage continuous controllable atmosphere heat treatment is started, including the micro-oxidation stage, the gas phase impurity removal stage and the controllable reduction stage. After completion, the temperature continues to rise to the target heat treatment temperature of 500℃-600℃. The micro-oxidation section: the furnace temperature is stabilized at 400℃-450℃, and a micro-oxidation mixed atmosphere of N2 + 0.3-0.8 vol% O2 is introduced into the furnace and kept at the temperature for 15-45 minutes, so that low-melting-point harmful impurities of S, Pb and Bi at the grain boundaries are preferentially oxidized to generate volatile oxides. The gas phase impurity removal section: After the micro-oxidation section is completed, the system switches to a high-purity N2 atmosphere for dynamic purging. The purging time is 10-30 minutes, which quickly carries the volatilized impurity oxides out of the furnace body and removes harmful impurities at the grain boundaries. The controlled reduction section: After the gas phase impurity removal section, the atmosphere is switched to a weakly reducing mixed atmosphere of N2 + 2-4 vol% H2. The atmosphere is maintained and the temperature continues to rise to the target heat treatment temperature, simultaneously reducing the trace oxide layer on the surface of the copper substrate. S5. Heat preservation and homogenization: After the furnace temperature is raised to the target heat treatment temperature, heat preservation treatment is carried out for 4-6 hours. During the heat preservation process, a weak reducing protective atmosphere is continuously introduced to maintain positive pressure inside the furnace. S6. Programmed cooling: After the heat preservation is completed, the furnace is cooled at a rate of 100℃ / h-200℃ / h. High-purity argon gas is continuously introduced during the cooling process and the positive pressure inside the furnace is maintained until the furnace temperature drops to room temperature. S7. Unloading: Open the furnace and remove the heat-treated copper rod to obtain a high-ductility, high-conductivity copper rod.

2. The heat treatment preparation and plasticity improvement process for a copper rod according to claim 1, characterized in that, In S1, the degree of deformation is controlled at 80%-90%.

3. The heat treatment preparation and plasticity improvement process for a copper rod according to claim 1, characterized in that, In the mixed atmosphere of the micro-oxidation section, the oxygen volume fraction is 0.4-0.6 vol%, and the holding time is 20-30 min.

4. The heat treatment preparation and plasticity improvement process for a copper rod according to claim 1, characterized in that, The high-purity N2 in the gas phase impurity removal section has a purity of ≥99.999%, and the dynamic purging gas flow rate is 2-4 times the furnace volume per hour.

5. The heat treatment preparation and plasticity improvement process for a copper rod according to claim 1, characterized in that, In the weakly reducing mixed atmosphere of the controllable reduction section, the hydrogen gas integral is 2.5-3.5 vol.

6. The heat treatment preparation and plasticity improvement process for a copper rod according to claim 1, characterized in that, In step S4, the heating rate is controlled at 220℃ / h-280℃ / h, and the target heat treatment temperature is 530℃-570℃.

7. The heat treatment preparation and plasticity improvement process for a copper rod according to claim 1, characterized in that, In S5, the heat preservation time is 4.5h-5.5h, a weakly reducing mixed atmosphere is continuously introduced during the heat preservation process, and the pressure inside the furnace is stabilized at 0.1MPa-0.2MPa.

8. The heat treatment preparation and plasticity improvement process for a copper rod according to claim 1, characterized in that, In step S6, the cooling rate is controlled at 120℃ / h-180℃ / h, and high-purity argon gas with a purity of ≥99.999% is introduced throughout the cooling process. The pressure inside the furnace is kept stable at 0.1MPa-0.2MPa.

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