1.How to regulate the alloy composition to control the shape of zinc flowers?
Traditional application of lead
Mechanism of action: Lead is insoluble in solid zinc, segregates at the grain boundary to reduce surface energy, promotes uniform distribution of crystal nuclei, and forms coarse zinc flowers (diameter 5~10mm).
Bidirectional regulation of aluminum:
Small zinc flower control: When the aluminum content is 0.1%~0.2%, Al₂O₃ film is formed first to inhibit the oxidation of zinc liquid, while reducing the growth rate of crystal nuclei to form 1~5mm medium zinc flowers.
Refining effect of trace elements
Adding antimony and bismuth: In the lead-free process, 0.05%~0.1% Sb or 0.01%~0.05% Bi can be used as a crystal nucleus refiner to increase the number of crystal nuclei and make the zinc flower diameter < 1mm (small zinc flower).
Synergistic effect of nickel and magnesium: A small amount of Ni (0.01%~0.03%) or Mg (0.005%~0.01%) can form a composite oxide with aluminum to further inhibit crystal growth, which is suitable for high-end zinc flower-free coating.
2.How does cooling rate affect spangle type?
Rapid cooling (small zinc flowers/no zinc flowers): Air knife plating: Excess zinc liquid is blown away by high-pressure air or nitrogen, and the cooling rate of the steel plate surface is increased to 20~50℃/s. The increase in supercooling leads to a surge in the number of crystal nuclei, but the growth time is short, forming small zinc flowers < 1mm (such as home appliance plates).
Water cooling/mist cooling: Spray water or water mist directly on the galvanized sheet, the cooling rate can reach more than 100℃/s, forcibly inhibiting crystal growth and obtaining a zinc flower-free surface (automotive plates, medical devices).
Slow cooling (large zinc flowers): Air cooling/furnace cooling: Natural cooling or cooling at a rate of 5~10℃/s in the annealing furnace, the crystal nuclei have sufficient time to grow, forming 5~10mm large zinc flowers (steel structures, pipelines).
3.What effect does cooling temperature have on the size of zinc flowers?
Undercooling control: The theoretical solidification point of zinc is 419.5℃. In actual production, the temperature of the steel plate needs to be controlled at 410~417℃ (undercooling 2~9℃):
High undercooling (such as 9℃): more crystal nuclei, fast growth, easy to form fine zinc flowers;
Low undercooling (such as 2℃): fewer crystal nuclei, slow growth, forming coarse zinc flowers.
4.How does post-processing improve the appearance and performance of zinc spangles?
Skin-pass rolling (flattening treatment)
Process parameters: After galvanizing, the steel is rolled with a reduction rate of 1%~3% through a flattening machine to flatten the zinc flower protrusions on the surface and blur the pattern (reducing the diameter by 50%~70%).
Application scenarios: home appliance panels, building decorative panels, improving surface flatness, and improving the uniformity of spraying or printing.
Alloying annealing treatment
Process parameters: After galvanizing, heat to 450~500℃ in the annealing furnace and keep warm for 10~30 minutes to make the zinc layer react with the iron substrate to form a Fe-Zn alloy layer.
Effect: The zinc spangles disappear completely, the surface is uniform silver-gray, and the corrosion resistance is improved. It is suitable for scenes with high corrosion resistance requirements such as automobile chassis and fasteners.
Surface passivation treatment
Chromate passivation: In the traditional process, a passivation film is formed on the surface of the zinc layer through a chromate solution to fill the gaps between the zinc flowers and weaken the visual pattern.
Chromium-free passivation: The environmentally friendly process uses silane, titanium zirconium salt and other treatments. Although it has no direct effect on the size of the zinc flowers, it can improve the surface uniformity and is suitable for precision fields such as electronic components.
5.What are the causes and solutions for uneven zinc flower size?
Cause: Fluctuation of zinc liquid composition (such as high lead content in some areas) and blockage of cooling nozzles lead to inconsistent local cooling rates.
Solution: Strengthen real-time monitoring of zinc liquid composition (test Pb and Al content every hour), clean air knife nozzles regularly (daily maintenance) to ensure cooling uniformity.