The production process for high-precision brass strip requires a combination of metallurgical, press-forming, heat-treating, and precision testing technologies. The following are the core process flows and key technical highlights:
1. Raw Material Preparation: Precise Control of Alloy Composition
1. Batching and Melting
-Alloy Composition: Typical high-precision brass strip includes H65 (Cu65%, Zn35%) and H62 (Cu62%, Zn38%), with trace elements added (such as 0.1%-0.3% Pb to improve machinability and 0.2%-0.5% Sn to enhance corrosion resistance).
-Melting Process: A vacuum melting furnace or medium-frequency induction furnace is used, with the melting temperature controlled at 1100-1200°C to prevent zinc volatilization (zinc boiling point is 907°C) and lead to segregation.
A covering agent (such as charcoal) is added to isolate the strip from air and reduce oxidation. Nitrogen or argon is introduced during the final stages of melting to remove inclusions (target oxide content ≤ 0.005%, gas content ≤ 5ppm). 2. Casting Process
-Continuous Casting: Horizontal or upward continuous casting is used. Ingot sizes are typically 20-50mm thick and 100-300mm wide.
-Key Controls: Cooling rate: The mold water temperature is maintained at 20-30°C, and the casting speed is maintained at 0.5-2m/min to ensure fine ingot grains (average grain size ≤50μm) and the absence of shrinkage cavities/air holes. Ingot annealing: Homogenization annealing is performed immediately after casting (temperature 550-650°C, hold for 2-4 hours) to eliminate dendritic segregation and improve hot rolling processability.
II. Plastic Processing: Multi-pass Rolling Achieves High Precision
(I) Hot Rolling
-Purpose: Roll the ingot into a strip with a thickness of 3-10mm, break up the cast structure, and increase density. Process Parameters: Heating temperature: 650-800°C (decreases with increasing copper content, e.g., 750°C for H65 and 850°C for H80), holding time: 1-2 hours. Rolling passes: A 4-roll hot rolling mill is used, with a total reduction of 60%-80% and a single-pass reduction of ≤25% to avoid cracking. Post-rolling cooling: Air or water cooling to room temperature, with surface oxide scale thickness controlled to ≤5μm.
(II) Cold Rolling
Core Process: Multiple cold rolling passes achieve thickness reduction (from 3mm to 0.05-1.5mm) and performance control, requiring a high-precision rolling mill and tension control system.
Key Technologies:
1. Mill Type:
For medium-thick strip (≥0.3mm): A 4-roll reversing mill is used, with a roll diameter ratio (work roll/backup roll) of 1:3-1:5 and a rolling force control accuracy of ±1%. Ultra-thin strip (<0.3mm): Use a 12- or 20-roll Sendzimir mill (such as the Sendzimir mill). Minimum rolling thickness can reach 0.01mm, with a thickness tolerance of ±1%.
2. Rolling Process:
Pass Reduction: 15%-25% for initial passes, ≤10% for subsequent precision rolling passes to avoid excessive work hardening.
Rolling Oil Lubrication: Use low-viscosity mineral oil (kinematic viscosity 4-8 mm²/s) or synthetic ester oil. Control the rolling oil temperature at 30-50°C and ensure a surface roughness Ra ≤ 0.8μm.
Tension Control: Post-tension / pre-tension = 1.1-1.3 to prevent strip deviation or wave formation. Maintain thickness uniformity within ±0.5%.
III. Heat Treatment: Dual Control of Performance and Precision
1. Intermediate Annealing (Recrystallization Annealing)
Purpose: Eliminate cold rolling work hardening, restore plasticity, and prepare for subsequent rolling. - Process: Temperature: 500-650°C (580°C for H65 brass, for example), hold time: 30-120 minutes, increasing with thickness.
- Equipment: Use a continuous annealing furnace (such as a roller-hearth furnace) with a nitrogen atmosphere (oxygen content ≤ 10 ppm) to prevent dezincification due to oxidation.
- Goal: Grain size controlled to ASTM grade 6-8 (average grain size 20-40 μm), hardness reduced to HV80-120.
2. Finish Annealing (Stress Relief Annealing)
- Purpose: Eliminate residual stresses, stabilize dimensions, and adjust mechanical properties (such as hardness and tensile strength).
- Process: Temperature: 200-300°C, hold time: 1-2 hours. Suitable for products requiring a certain degree of strength (such as elastic components). Alternatively, low-temperature annealing (150-200°C) can be used to relieve stress without changing the work-hardening state. Suitable for applications requiring high hardness (such as cutting strip). IV. Surface Treatment and Precision Machining
1. Pickling and Descaling
- Use a mixture of sulfuric acid and hydrochloric acid (5%-10% concentration, 50-70°C) or electrolytic pickling to remove scale produced during hot/cold rolling and ensure surface cleanliness (iron content ≤ 50 ppm).
- After pickling, neutralize with a 5% sodium carbonate solution and ultrasonically clean the strip to prevent acid residue from corroding the strip.
2. Surface Finishing
- Pass-milling: Use a low reduction of 0.5%-2% to improve surface smoothness (Ra ≤ 0.4 μm) and dimensional accuracy.
- Plating: Nickel plating, tin plating, or anti-rust oil coating are used as required. For example, brass strip for electronic connectors is often nickel-plated (thickness 1-3 μm) to enhance conductivity and oxidation resistance. 3. Precision Slitting and Straightening
-Slitting: Circular shears or laser slitting machines are used, with width accuracy controlled within ±0.05mm (narrow strips) or ±0.1mm (wide strips), and edge burrs ≤0.01mm.
-Straightening: A multi-roll straightening machine (15-21 rollers) eliminates wavy and camber, maintaining a straightness of ≤1mm/m, ensuring the strip flatness meets the requirements of automated stamping equipment.
V. Quality Inspection and Control
1. Dimensional Accuracy Inspection
-Thickness: β-ray thickness gauge or laser thickness gauge, accuracy of ±0.5μm.
-Width: Visual inspection system (CCD camera), accuracy of ±0.02mm.
-Straightness: Tension-type straightness tester, resolution 0.1I unit (I unit = 10^-5 relative elongation within 1m length).
2. Mechanical Properties Testing
-Hardness: Vickers hardness tester (load 1-5kg), accuracy ±5HV. Tensile Strength/Elongation: Micro-tensile testing machine (specimen width 5-10mm), test accuracy ±2%.
3. Surface Quality Inspection
Visual Inspection: Light intensity ≥ 1000 lux, inspect for defects such as scratches, pits, and color shift (maximum defect size ≤ 0.05mm).
Scanning Electron Microscope (SEM): Analyze the surface microstructure to ensure there are no rolling cracks or residual oxide film.
The company has a cluster of leading copper processing production lines in China, including:
German imported precision copper tube production line (annual output of 30,000 tons)
Japanese technology copper foil rolling line (thinnest up to 6μm)
Fully automatic copper bar continuous extrusion line
Intelligent copper sheet and strip finishing mill unit
Digitalized control and management of the whole production process is realized through MES system, and the dimensional accuracy of the products can reach ±0.01mm.
