English
High‑temperature hot‑dip galvanizing is a technical solution to address galvanizing defects of high‑silicon reactive steel (wSi≈0.3%).
1. Principle
When the zinc bath temperature exceeds 530 °C, the ζ phase disappears. Liquid zinc is in equilibrium with the δ phase within 530‑665 °C. No ζ phase forms at 550 °C; the δ phase is stable and grows slowly. The influence of silicon on the iron‑zinc reaction is greatly weakened, so a uniform and normal coating can be obtained.
At 550 °C, the coating thickness is far less sensitive to the silicon content in steel compared with the conventional 460 °C process.
2. Zinc Bath Composition and Process Parameters
(1) After exiting the zinc bath, the δ phase continues to grow, forming a mixed δ+η microstructure on the surface, and the coating appears light‑grey. Adding aluminium (wAl: 0.03%‑0.05%) yields bright coatings; lead addition improves coating adhesion.
(2) The solubility of iron in molten zinc rises sharply at high temperatures, reaching 0.3% at 550 °C. Elevated iron content thickens the coating, degrades mechanical properties, generates large volumes of zinc dross and consumes zinc. Therefore, the dipping time shall be shorter than that of the conventional process.
(3) Ideal operating conditions: temperature 560 °C; iron content wFe controlled at 0.1%‑0.2%; lead content wPb approx. 1%; aluminium content wAl approx. 0.05%; cooling rate must be strictly controlled.
3. Coating Performance
At 560 °C: the coating exhibits better hardness, wear resistance and corrosion resistance than conventional hot‑dip galvanized coatings.
Upon cooling to 520 °C: a small amount of ζ phase precipitates again, which improves the ductility of the coating.
4. Drawbacks and Application Status
Molten zinc at 550 °C causes severe corrosion to steel crucibles, so ceramic/refractory‑material crucibles must be adopted, bringing complex supporting heating equipment. This process is rarely applied in large‑scale mass‑production lines.