A method for preparing a low hydrogen embrittlement grade narrow copper bar

Abstract

The application discloses a preparation method of a low-hydrogen embrittlement grade narrow copper bar, and a process flow of the method comprises smelting, upward drawing casting, cold deformation, annealing, four-roll flattening, annealing, drawing and winding and packaging; the narrow copper bar is prepared through the rolling forming after the cold working and the heat treatment of the upward drawing casting blank, inherent defects of the existing continuous extrusion process are avoided, the existence of the layered defects is eliminated, the reduction and the spreading in the flattening process are controlled, the annealing treatment is combined, the edge cracking of the copper bar under the deformation of a large width-thickness ratio is avoided, the uniformity of the grain structure is ensured, the requirements of the friction stir welding are met, the narrow bar with the width-thickness ratio of 5-15 is prepared, and the narrow bar meets the requirements of the high reliability of new energy batteries.

Description

Technical Field

[0001] This invention belongs to the field of copper alloy technology, specifically relating to a method for preparing a narrow copper busbar with low hydrogen embrittlement level. Background Technology

[0002] With the environmental requirements of carbon peaking and carbon neutrality, the development of new energy vehicles is rapid, and the demand for copper busbars used in new energy vehicle batteries is also increasing. New energy vehicle battery copper busbars are narrow busbars, with a width ranging from 20-40mm and a width-to-thickness ratio between 5-15. They are prepared as battery negative electrode materials through friction stir welding with aluminum rods. Currently, the production of these copper busbars mainly adopts a continuous extrusion process. For example, Chinese patent CN105551688A uses the following process: electrolytic copper smelting, continuous casting of high-purity oxygen-free copper rods, continuous extrusion of copper busbar billets, drawing, inspection and testing, and packaging. This method produces high-precision oxygen-free bright copper busbars; the process is simple, the flow is short, and uninterrupted continuous production can be achieved. However, continuous extrusion has inherent defects, mainly uncontrollable overflow, which easily leads to delamination of the copper busbar. This delamination defect causes weak friction stir welding, further leading to copper-aluminum separation in the battery negative electrode, resulting in battery power failure. This problem is fatal for new energy vehicles.

[0003] For detecting delamination defects in copper busbars, a hydrogen embrittlement test can be used for verification. A hydrogen embrittlement level of less than 3 can prevent delamination defects from occurring. Narrow copper busbar materials in new energy vehicles have extremely stringent requirements for hydrogen embrittlement levels. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for preparing a narrow copper busbar with low hydrogen embrittlement level, so as to overcome the defect of high hydrogen embrittlement level of copper busbar prepared by the prior art and reduce the occurrence of delamination defects.

[0005] The technical solution adopted by this invention to solve the above-mentioned technical problem is: a method for preparing a narrow copper busbar with low hydrogen embrittlement level, characterized in that it specifically includes the following preparation steps:

[0006] 1) Smelting: Select single-layer electrolytic plates with a copper content of 99.98wt.% or higher, cover the melting furnace and intermediate compartment with carbon particles, and cover the holding furnace with graphite flakes;

[0007] 2) Upward casting: The temperature of the holding furnace is controlled between 1140-1150℃. The casting speed is different for different upward specifications. The casting speed of φ30mm upward copper rod is controlled at 30-40cm / min. The casting speed of other specifications of copper rod is calculated according to the equal area method. For example, the casting speed of φ25mm copper rod is (30-40)×(30×30 / 25 / 25)=1.44×(30-40)cm / min. If the casting speed is too fast, the rod blank may break or become hollow. If the casting speed is too slow, cold shut defects may appear on the surface of the cast billet.

[0008] 3) Cold deformation: The upper drawing rod blank undergoes multiple cold deformation passes, with a total cold deformation rate of over 60% and a single pass rate controlled at around 20-30%. The diameter of the cold-deformed rod blank is R1.

[0009] 4) Annealing: The cold-deformed bar billet is annealed offline at a temperature between 450-650℃ for more than 2 hours to ensure that the recrystallization structure of the entire bar billet is uniform.

[0010] 5) Four-roll flattening: During flattening deformation, the roll gap between the upper and lower rolls is S1, and the roll gap between the left and right rolls is S2. The following relationship should exist between S1 and S2:

[0011] S2 = C × (1.5 × R1 - 0.5 × S1)

[0012] Where C is the limiting spread constant, and the range of C is controlled between 0.80 and 1.0;

[0013] The process involves multi-pass four-roll rolling to the minimum allowable temperature, with a single-pass deformation of 10%-40% and a total cold rolling deformation controlled at 60-80%.

[0014] 6) Annealing: The cold-rolled flat billet is annealed offline at a temperature between 300-500℃ for more than 2 hours to ensure that the recrystallization structure of the entire billet is uniform.

[0015] 7) Drawing: The annealed flat blank is cold-drawn to obtain a finished flat bar with an arc. The cold drawing rate is controlled between 15-30%.

[0016] 8) Rolling and packaging.

[0017] Preferably, in step 1), a single-layer electrolytic plate with a copper content of at least 99.98 wt.% is selected, with no obvious copper beads on the surface. Each time an electrolytic plate is added to the melting furnace, half of the electrolytic plate is first placed into the melting furnace, and after half a minute, the electrolytic plate can be added to the melting furnace to ensure that the electrolytic plate is fully dried before entering the melting furnace. The melting furnace and the intermediate compartment are covered with carbon particles with a carbon content greater than 98 wt.%, which has a lower moisture content than ordinary charcoal, and the covering thickness is more than 5 cm. The holding furnace is covered with graphite flakes with a covering thickness of more than 20 cm to ensure that the casting billet will not have pores due to air absorption.

[0018] As a preferred option, step 3) cold deformation: the upper drawing rod blank undergoes four cold deformation processes.

[0019] Preferably, in step 3), cold rolling is selected for cold deformation. Compared to cold drawing, the deformation of the surface metal is much greater than that of the center, resulting in a significant difference in microstructure between the center and the edges during subsequent annealing. In contrast, during cold rolling, the surface deformation easily penetrates to the center, resulting in a more uniform microstructure.

[0020] Preferably, in step 4), the annealing temperature is controlled between 500-600℃, and the annealing time is controlled between 3-6 hours. This process is mainly to eliminate cracking or wrinkling caused by internal porosity defects and rough surfaces during the flattening process. Conventionally, copper under 60% cold deformation recrystallizes at 300℃. To ensure sufficient recrystallization, the temperature is increased to above 500℃, but not exceeding 600℃, otherwise abnormal grain growth occurs, and energy consumption is high. Through this process, the grain size of the annealed billet is controlled between 0.05mm and 0.07mm, with the difference in grain size between the center and the edge not exceeding 0.005mm.

[0021] Preferably, in step 5), the limiting width constant C is between 0.95 and 0.99. Because the width-to-thickness ratio of the finished flat busbar is relatively large, the deformation of the narrow busbar along the thickness direction is much greater than that along the width direction. Therefore, the edge of the narrow busbar bears a large tensile stress. When the tensile stress exceeds the yield strength of the copper busbar, wrinkles or even cracks begin to appear at the edge. Therefore, a clamping force must be applied to the edge of the copper busbar to prevent cracking. However, if the limiting width is too large, it will cause bulging at the edge. Furthermore, the greater the limitation, the larger the required size of the upper drawing rod blank. The larger the size, the more difficult the upper drawing becomes. Therefore, the limiting width constant C is preferably between 0.95 and 0.99.

[0022] As a preferred option, step 5) of the preparation process involves three to four passes of multi-pass four-roll rolling.

[0023] Preferably, in step 6), the annealing temperature is controlled between 350-450℃, and the annealing time is 2-3 hours. Because narrow bars undergo work hardening after flattening, the subsequent temperature rise during friction stir welding can cause a significant difference in grain size between the welded area and other areas, posing a risk of detachment. Therefore, annealing at 350-450℃ for 2-3 hours controls the grain size to be between 0.03-0.04 mm to ensure weld quality.

[0024] As a preferred option, in step 7), the cold drawing amount is about 15-30% to finish the rolled flat bar, ensure dimensional accuracy, and facilitate clamping for subsequent automatic friction stir welding.

[0025] As an improvement, hydrogen embrittlement level detection is added between preparation steps 7 and 8).

[0026] Compared with the prior art, the advantages of this invention are as follows: by rolling the upper-cast billet after cold working and heat treatment, the inherent defects of the existing continuous extrusion process are avoided, the existence of delamination defects is eliminated, and the hydrogen embrittlement level of the finished product is guaranteed to be level three or below. At the same time, the compression and widening during the flattening process are controlled, and combined with annealing treatment, the edge cracking of the copper busbar under large width-to-thickness ratio deformation is avoided, the uniformity of the grain structure is guaranteed, and the requirements of friction stir welding are met. The width-to-thickness ratio of the prepared copper busbar is between 5 and 15, and this narrow busbar can meet the high reliability requirements of new energy batteries. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the embodiments.

[0028] This invention provides three embodiments and one comparative example.

[0029] The specifications of the narrow busbar in Example 1 are: 3mm thick and 21mm wide. The process flow for this low-hydrogen-embrittlement grade narrow copper busbar includes smelting → upward casting → cold deformation → annealing → four-roll flattening → annealing → drawing → coiling and packaging; the specific preparation steps are as follows:

[0030] 1) Smelting: Single-layer electrolytic plates with a copper content of 99.98% are selected, with no obvious copper granules on the surface. Each time an electrolytic plate is added to the melting furnace, half of the electrolytic plate is first placed into the molten copper in the furnace. After half a minute, the electrolytic plate can be put into the melting furnace to ensure that the moisture in the electrolytic plate is fully dried before entering the melting furnace, reducing the disadvantages caused by moisture. The melting furnace and the intermediate compartment are covered with carbon particles with a carbon content of more than 98% and a coverage thickness of more than 5cm, which has a lower moisture content than ordinary charcoal. The holding furnace is covered with graphite flakes with a coverage thickness of more than 20cm. Through the combined effect of high carbon particles and graphite flakes, it is ensured that the interior of the cast billet will not have pores due to air absorption.

[0031] 2) Upward Casting: The holding furnace temperature is controlled between 1140-1150℃. The casting speed varies depending on the upward casting specifications. In this upward casting, the diameter of the copper rod is φ26mm. The casting speed calculation method is as follows: for φ30mm upward copper rods, the casting speed is controlled at 30cm / min, calculated using the equal area method. For copper rods with a diameter of φ26mm, the casting speed is 40cm / min. Too high a casting speed can easily lead to rod breakage or hollowness, while too low a casting speed can easily cause cold shut defects on the surface of the cast billet.

[0032] 3) Cold deformation: The φ26mm upper drawing rod blank undergoes 4 passes of cold rolling, with a total cold deformation rate of over 60% and a single pass processing rate controlled at around 20-30%. The diameter R1 of the cold-deformed rod blank is φ16mm.

[0033] 4) Annealing: The cold-deformed billet was annealed offline at a temperature of 600℃ for 2 hours. After annealing, the grain size in the center and edge of the microstructure was measured to be 0.06 mm.

[0034] 5) Four-roll flattening: During flattening deformation, the roll gap between the upper and lower rolls is S1, and the roll gap between the left and right rolls is S2. The following relationship should exist between S1 and S2:

[0035] S2 = C × (1.5 × R1 - 0.5 × S1)

[0036] C is a limiting spread constant, and the range of C is controlled between 0.80 and 1.0;

[0037] The rolling process involves multiple passes of four-roll rolling to the minimum allowable thickness, with a single pass deformation ranging from 10% to 40%, and the total cold rolling deformation controlled at 60% to 80%. In this rolling process, four passes were selected: the first pass rolled to 8mm × 20mm, the second pass rolled to 6mm × 20.5mm, the third pass rolled to 4mm × 21.5mm, and the fourth pass rolled to 3.5mm × 22mm.

[0038] 6) Annealing: Cold-rolled 3.5 mm × 22 mm flat billets were annealed offline at a temperature of 350℃ for more than 3 hours. The grain size of the middle and edge parts was measured to be 0.03 mm.

[0039] 7) Drawing: Annealed 3.5×22mm flat blanks are cold-drawn to 3.0 mm×21mm to obtain finished flat bars with an arc.

[0040] 8) Oxygen content and hydrogen embrittlement level testing;

[0041] 8) Roll up and pack.

[0042] The specifications of the narrow busbar in Example 2 are: 3mm thick and 25mm wide. The process flow for this low-hydrogen-embrittlement-grade narrow copper busbar includes smelting → upward casting → cold deformation → annealing → four-roll flattening → annealing → drawing → coiling and packaging; the specific preparation steps are as follows:

[0043] 1) Smelting: Single-layer electrolytic plates with a copper content of 99.98% are selected, with no obvious copper inclusions on the surface. When adding electrolytic plates to the melting furnace each time, half of the electrolytic plates are first placed into the molten copper in the furnace. After half a minute, the electrolytic plates can be put into the melting furnace to ensure that the moisture in the electrolytic plates is fully dried before entering the melting furnace, reducing the disadvantages caused by moisture. The melting furnace and the intermediate compartment are covered with carbon particles with a carbon content of more than 98% and a coverage thickness of more than 5cm, which has a lower moisture content than ordinary charcoal. The holding furnace is covered with graphite flakes with a coverage thickness of more than 20cm. Through the combined effect of high carbon particles and graphite flakes, it is ensured that the interior of the cast billet will not have pores due to air absorption.

[0044] 2) Upward casting: The temperature of the holding furnace is controlled between 1140-1150℃. In this case, a φ30mm copper rod is selected and the casting speed is 30cm / min.

[0045] 3) Cold deformation: Select an upward drawing rod blank with a diameter of φ30mm and cold roll it through 4 passes. The total cold deformation processing rate is over 60%, and the single pass processing rate is controlled at around 20-30%. The diameter of the cold-deformed rod blank is 19mm.

[0046] 4) Annealing: The cold-deformed billet was annealed offline at a temperature of 500℃ for 2 hours. After annealing, the grain size in the center and edge of the microstructure was measured to be 0.065 mm.

[0047] 5) Four-roll flattening: During flattening deformation, the roll gap between the upper and lower rolls is S1, and the roll gap between the left and right rolls is S2. The following relationship should exist between S1 and S2:

[0048] S2 = C × (1.5 × R1 - 0.5 × S1)

[0049] C is a limiting spread constant, and the range of C is controlled between 0.80 and 1.0;

[0050] The rolling process involves multiple passes of four-roll rolling to the minimum allowable thickness, with a single pass deformation ranging from 10% to 40%, and the total cold rolling deformation controlled at 60% to 80%. In this rolling process, three passes were selected: the first pass rolled to 10mm × 23.5mm, the second pass rolled to 6mm × 25.5mm, and the third pass rolled to 4mm × 26mm.

[0051] 6) Annealing: The cold-rolled 4×26mm flat billet was annealed offline at a temperature of 400℃ for more than 3 hours. The grain size of the middle and edge parts was measured to be 0.035mm.

[0052] 7) Drawing: Annealed 4×26mm flat blanks are cold-drawn to 3×25mm to obtain finished flat bars with an arc shape;

[0053] 8) Oxygen content and hydrogen embrittlement level testing;

[0054] 9) Roll up and pack.

[0055] The specifications of the narrow busbar in Example 3 are: 3mm thick and 15mm wide. The process flow for this low-hydrogen-embrittlement-grade narrow copper busbar includes smelting → upward casting → cold deformation → annealing → four-roll flattening → annealing → drawing → coiling and packaging; the specific preparation steps are as follows:

[0056] 1) Smelting: Single-layer electrolytic plates with a copper content of 99.98% are selected, with no obvious copper inclusions on the surface. When adding electrolytic plates to the melting furnace each time, half of the electrolytic plates are first placed into the molten copper in the furnace. After half a minute, the electrolytic plates can be put into the melting furnace to ensure that the moisture in the electrolytic plates is fully dried before entering the melting furnace, reducing the disadvantages caused by moisture. The melting furnace and the intermediate compartment are covered with carbon particles with a carbon content of more than 98% and a coverage thickness of more than 5cm, which has a lower moisture content than ordinary charcoal. The holding furnace is covered with graphite flakes with a coverage thickness of more than 20cm. Through the combined effect of high carbon particles and graphite flakes, it is ensured that the interior of the cast billet will not have pores due to air absorption.

[0057] 2) Upward casting: The temperature of the holding furnace is controlled between 1140-1150℃. In this case, a φ20mm copper rod is selected and the casting speed is 70cm / min.

[0058] 3) Cold deformation: Select an upward drawing rod blank with a diameter of φ20mm and cold roll it through 4 passes. The total cold deformation processing rate is over 60%, and the single pass processing rate is controlled at around 20-30%. The diameter of the cold-deformed rod blank is 12mm.

[0059] 4) Annealing: The cold-deformed rod blank is annealed offline at a temperature of 550℃ for 2 hours. After annealing, the grain size in the center and edge of the microstructure is measured to be 0.070 mm.

[0060] 5) Four-roll flattening: During flattening deformation, the roll gap between the upper and lower rolls is S1, and the roll gap between the left and right rolls is S2. The following relationship should exist between S1 and S2:

[0061] S2 = C × (1.5 × R1 - 0.5 × S1)

[0062] C is a limiting spread constant, and the range of C is controlled between 0.80 and 1.0;

[0063] The rolling process involves three four-roll mills to the minimum allowable thickness, with a single-pass deformation of 10%-40% and a total cold rolling deformation controlled at 60-80%. In this rolling process, three passes were selected: the first pass rolled to 7mm×14.5mm, the second pass rolled to 5mm×15.5mm, and the third pass rolled to 4mm×16mm.

[0064] 6) Annealing: The cold-rolled 4 mm × 16 mm flat billet was annealed offline at a temperature of 450℃ for more than 3 hours. The grain size of the middle and edge parts was measured to be 0.040 mm.

[0065] 7) Drawing: Annealed 4 mm × 26 mm flat blanks are cold-drawn to 3 mm × 15 mm to obtain finished flat bars with an arc.

[0066] 8) Oxygen content and hydrogen embrittlement level testing;

[0067] 8) Roll up and pack.

[0068] Comparative Example 4 uses a continuous extrusion process.

[0069] 1) Raw material specifications: Single-layer electrolytic plate with a copper content of 99.99% or higher;

[0070] 2) Top-draw continuous casting: The electrolytic plate is melted to form molten copper, and the casting temperature is 1140-1160℃. The top-draw rod is φ20mm.

[0071] 3) Continuous extrusion: After the ingot passes through the cavity of the continuous extrusion press, it enters the extrusion die to obtain the extruded billet. The extrusion speed is 6.5 r / min, and the extruded billet size is 6.5 mm × 203 mm.

[0072] 4) Stretching: The finished product is stretched at a speed of 4m / min, and the finished product size is 6.0mm×200mm;

[0073] 5) Cut to length: Cut copper busbars into 5m lengths.

[0074] The obtained embodiments and comparative examples were subjected to the following tests:

[0075] 1) Oxygen content detection method: The oxygen content was tested according to GB / T 5121.8-2008 Chemical analysis methods for copper and copper alloys - Part 8: Determination of oxygen content.

[0076] 2) Hydrogen embrittlement level test method: The test is carried out at 3 locations every 0.5 mm along the thickness direction of the narrow row using the metallographic test method of "YS / T 335-2009 Oxygen-free copper oxygen content".

[0077] Table 1 Test results of the examples and comparative examples

[0078]

[0079] It can be seen that the hydrogen embrittlement level of the finished products in Examples 1 to 3 can be maintained at level 1 to 2, while that of the comparative examples is level 3 to 5, which can achieve the expected technical effect and reduce the occurrence of delamination defects.

Claims

1. A method for preparing a narrow copper busbar with low hydrogen embrittlement level, characterized in that: The preparation steps include the following: 1) Smelting: Select single-layer electrolytic plates with a copper content of 99.98wt.% or higher, cover the melting furnace and intermediate compartment with carbon particles, and cover the holding furnace with graphite flakes. 2) Upward casting: The temperature of the holding furnace is controlled between 1140-1150℃. The casting speed is different for different upward specifications. The casting speed of φ30mm upward copper rod is controlled at 30-40cm / min. The casting speed of other specifications of copper rod is calculated according to the equal area method. 3) Cold deformation: Select an upper drawing rod blank with a diameter of R0 and perform multiple cold deformation passes. The total cold deformation rate is over 60%, and the single-pass processing rate is controlled at 20-30%. The diameter of the cold-deformed rod blank is R1. 4) Annealing: The cold-deformed bar blank is annealed offline at a temperature between 450-650℃ for more than 2 hours. 5) Four-roll flattening: During flattening deformation, the roll gap between the upper and lower rolls is S1, and the roll gap between the left and right rolls is S2. The following relationship should exist between S1 and S2: S2 = C × (1.5 × R1 - 0.5 × S1) Where C is the limiting spread constant, and the range of C is 0.80-1; The process involves multi-pass four-roll rolling to the minimum allowable temperature, with a single-pass deformation of 10%-40% and a total cold rolling deformation controlled at 60-80%. 6) Annealing: The cold-rolled flat billet is annealed offline at a temperature between 300-500℃ for a time of more than 2 hours. 7) Drawing: The annealed flat blank is cold-drawn to obtain a finished flat bar with an arc. The cold drawing rate is controlled between 15-30%. 8) Rolling and packaging.

2. The method for preparing the low-hydrogen-embrittlement-grade narrow copper busbar according to claim 1, characterized in that: Preparation step 1) Smelting: The carbon particles contain more than 98 wt.% carbon and have a covering thickness of more than 5 cm; the graphite flakes have a covering thickness of more than 20 cm; the surface of the single-layer electrolytic plate has no obvious copper beads. When adding electrolytic plates to the melting furnace each time, half of the electrolytic plates are first put into the copper in the furnace, and the electrolytic plates can be put into the melting furnace after half a minute.

3. The method for preparing the low-hydrogen-embrittlement-grade narrow copper busbar according to claim 1, characterized in that: Preparation step 3) Cold deformation: The upper drawing rod blank undergoes four cold deformation processes.

4. The method for preparing the low-hydrogen-embrittlement-grade narrow copper busbar according to claim 1, characterized in that: Preparation steps 3) Cold deformation is achieved by selecting cold rolling process.

5. The method for preparing a narrow copper busbar with low hydrogen embrittlement level according to claim 1, characterized in that: Preparation steps 4) The annealing temperature is controlled between 500-600℃, the annealing time is controlled between 3-6h, and the grain size of the annealed billet is controlled between 0.05mm-0.07mm, with the difference between the grain size in the middle and the edge not exceeding 0.005mm.

6. The method for preparing a narrow copper busbar with low hydrogen embrittlement level according to claim 1, characterized in that: Preparation step 5) restricts the breadth constant C to between 0.95 and 0.

99.

7. The method for preparing a narrow copper busbar with low hydrogen embrittlement level according to claim 1, characterized in that: Preparation step 5) Multi-pass four-roll rolling is carried out in three to four passes.

8. The method for preparing a narrow copper busbar with low hydrogen embrittlement level according to claim 1, characterized in that: Preparation step 6) The annealing temperature is controlled between 350-450℃, and the annealing time is 2-3h to ensure that the grain size is 0.03-0.04mm.

9. The method for preparing a narrow copper busbar with low hydrogen embrittlement level according to claim 1, characterized in that: Hydrogen embrittlement level detection is performed between preparation steps 7 and 8).

Images