High-precision brass strip, a copper alloy strip with high dimensional accuracy, excellent surface quality and excellent mechanical properties, is widely used in electronic information, precision machinery, aerospace, automotive parts and other fields, such as connectors, shields, elastic components and so on.
The production process of high-precision brass strips requires a combination of metallurgy, pressure processing, heat treatment and precision testing technology. The following is the core process and key technical points:
I. Raw material preparation: precise control of alloy composition
1. Batching and melting
Alloy composition: typical high-precision brass strip such as H65 (Cu65%, Zn35%), H62 (Cu62%, Zn38%), or add trace elements (such as 0.1%-0.3% Pb to improve the cutability, 0.2%-0.5% Sn to improve corrosion resistance).
-Melting process: use vacuum melting furnace, or medium frequency induction furnace, control the melting temperature of 1100-1200 ℃, to avoid zinc volatilization (zinc boiling point of 907 ℃) leading to compositional segregation.
Add a covering agent (such as charcoal) to isolate the air and reduce oxidation; late in the smelting process, through the nitrogen or argon refining, to remove inclusions (target oxides ≤ 0.005%, the gas content of ≤ 5ppm).
2.Casting process
-Continuous casting: horizontal continuous casting or upward-induced continuous casting, ingot size is usually 20-50mm in thickness and 100-300mm in width.
-Key control: cooling speed, crystallizer water temperature control at 20-30 ℃, billeting speed of 0.5-2m/min, to ensure that the ingot fine grain (average grain size ≤ 50μm), no shrinkage / porosity. Ingot annealing, casting immediately after the homogenization of annealing (temperature 550-650 ℃, insulation 2-4 hours), to eliminate dendritic segregation, improve the hot rolling processability.
Second, plastic processing: multi-pass rolling to achieve high precision
(a) hot rolling open billet
-Purpose: roll the ingot into a thickness of 3-10mm strip billet, broken casting organization, improve the density.
-Process parameters: heating temperature at 650-800 ℃ (with the increase in copper content and reduce, such as H65 take 750 ℃, H80 take 850 ℃), holding time of 1-2 hours. Rolling passes, the use of 4-roll hot rolling mill, the total under pressure rate of 60% -80%, single pass under pressure rate of ≤ 25%, to avoid cracking. Cooling after rolling, air or water cooling to room temperature, control the thickness of surface oxide skin ≤ 5μm.
(II) Cold roll forming
Core process: realize thickness thinning (from 3mm to 0.05-1.5mm) and performance regulation through multi-pass cold rolling, which needs to be supported by high-precision rolling mill and tension control system.
-Key technologies:
1. Mill type:
-Medium-thick strip (≥0.3mm): 4-roll reversible mill, roll diameter ratio (work roll/support roll) 1:3-1:5, rolling force control accuracy ±1%.
Ultra-thin strip (<0.3mm): 12-roll or 20-roll Sengemeier mill (such as Sendzimir mill), the minimum rolling thickness of up to 0.01mm, thickness tolerance ± 1%.
2. Rolling process:
-Dozing rate: initial dozing take 15% -25%, later precision rolling dozing ≤ 10%, to avoid excessive work hardening.
-Rolling oil lubrication: use low viscosity mineral oil (kinematic viscosity 4-8mm²/s) or synthetic ester oil, control the rolling oil temperature 30-50 ℃, to ensure that the surface roughness Ra ≤ 0.8μm.
-Tension control: back tension / front tension = 1.1-1.3, to prevent the strip from running or wave shape, thickness uniformity control within ± 0.5%.
III. Heat treatment: dual regulation of performance and precision
1. Intermediate annealing (recrystallization annealing)
-Role: eliminate cold rolling work hardening, restore plasticity, and prepare for subsequent rolling.
-Process: temperature, 500-650 ℃ (such as H65 brass to take 580 ℃), holding time 30-120 minutes, with the increase in thickness to extend.
-Equipment: continuous annealing furnace (such as roller-hearth furnace), nitrogen protective atmosphere (oxygen content ≤ 10ppm), to avoid oxidative dezincification.
-Objective: Grain size control at ASTM 6-8 grade (average grain 20-40μm), hardness reduced to HV80-120.
2. Finished product annealing (stress relief annealing)
-Role: eliminate residual stress, stabilize the size, adjust mechanical properties (such as hardness, tensile strength).
-Process: temperature, 200-300 ℃, insulation 1-2 hours, applicable to the need to retain a certain strength of the product (such as elastic components). Or use low-temperature annealing (150-200 ℃), only the elimination of stress without changing the state of work hardening, applicable to high hardness scenes (such as cutting with the strip).
IV. Surface treatment and precision machining
1. Acid washing and descaling
-Apply sulfuric acid + hydrochloric acid mixed acid (concentration 5%-10%, temperature 50-70 ℃) or electrolytic pickling to remove the oxidized skin generated in the process of hot/cold rolling to ensure the surface cleanliness (iron content ≤ 50ppm).
-Neutralization (5% sodium carbonate solution) and ultrasonic cleaning are carried out after pickling to prevent acid residues from corroding the strip.
2.Surface finishing
-Bright rolling treatment: Improve the surface finish (Ra≤0.4μm) and dimensional accuracy by rolling at a small underpressure rate of 0.5%-2%.
Plating treatment: Nickel-plated, tin-plated or coated with rust-resistant oil according to requirements, for example, brass strips for electronic connectors are often nickel-plated (thickness 1-3μm) to enhance electrical conductivity and oxidation resistance.
3.Precision slitting and straightening
-Slitting: use disc shear or laser slitting machine, width precision control in ± 0.05mm (narrow band) or ± 0.1mm (wide band), edge burr ≤ 0.01mm.
Straightening: Eliminate wavy and sickle bends by multi-roll straightening machine (15-21 rolls), with straightness ≤1mm/m, to ensure that the strip flatness meets the requirements of automatic stamping equipment.
V. Quality inspection and control
1. Dimensional accuracy inspection
-Thickness: adopt β-ray thickness gauge or laser thickness gauge, accuracy ±0.5μm.
-Width: Visual inspection system (CCD camera), accuracy ±0.02mm.
-Straightness: Tension type straightness detector, resolution 0.1I unit (I unit = 10^-5 relative elongation in 1m length).
2. Mechanical properties test
-Hardness: Vickers hardness tester (load 1-5kg), accuracy ±5HV.
-Tensile Strength/Elongation: Micro Tensile Tester (specimen width 5-10mm), testing accuracy ±2%.
3.Surface quality inspection
-Visual inspection: light intensity ≥1000lux, detect scratches, pits, color difference and other defects (maximum allowable defect size ≤0.05mm).
-SEM (Scanning Electron Microscope): analyze the surface microstructure to ensure that there is no rolling crack or oxide film residue.
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