1.How to control alloy composition?
Synergistic Effects of Core Alloying Elements
Two types of alloying elements are added to zinc melt to promote the formation of zinc spangles:
The first type of elements are completely soluble in liquid zinc but nearly insoluble in solid zinc. These elements lower the solidification temperature of the zinc melt, prolonging its solidification time and providing more time for the spangles to grow.
The second type of elements have a certain solubility in both liquid and solid zinc. For example, Sb refines the spangles, while Sn further increases their size. In actual production, the Sb content is typically controlled at around 0.025%. Combined with cooling adjustments, this allows for the production of large spangles while avoiding the risk of reduced adhesion caused by excessive Sb content.
2.How to accurately control the cooling speed?
Staged Control of the Cooling Stage
First Cooling Stage: After the strip exits the zinc pot, the cooling rate is adjusted using a variable frequency fan. For example, setting the speed of the first fan in the first cooling stage to 50% keeps the strip's first cooling temperature ≤280°C, the secondary cooling temperature ≤160°C, and the temperature after water quenching ≤45°C, effectively controlling spangle size.
High-Span Cooling: The cooling temperature is adjusted for different strip thicknesses. For example, strips thicker than 1.2mm are cooled at 270°C, while strips thicker than 1.2mm are cooled at 230°C, balancing cooling efficiency and spangle uniformity.
Quantitative Relationship between Cooling Rate and Spangle Size
Experiments show that within the cooling rate range of 0-25K/s, spangle size decreases exponentially with increasing cooling rate. Rapid cooling (such as mist cooling) significantly refines spangles, while slow cooling (such as air cooling) promotes the formation of larger spangles.
3.How to perform substrate pretreatment and equipment parameter optimization?
Influence of Substrate Surface Condition
Roughness: A substrate roughness of 0.2-0.7μm promotes uniform spreading of the zinc solution, forming uniform spangles. Insufficient roughness can easily lead to localized aggregation of the zinc solution, resulting in spangles of uneven size.
Cleanliness: Degreasing and cleaning require a bath temperature of 50-80°C, a free alkalinity of 30-80Pt in the degreasing bath, and a free alkalinity of less than 8Pt in the rinse bath to remove surface oil and impurities that affect zinc wettability.
Air Knife and Solidification Line Control
Air Knife Parameters: When the air knife is 5-12mm from the strip and at a height of 150-400mm, the position of the zinc solution's solidification line can be precisely controlled. When the solidification line is 3-7 meters above the zinc pot, the spangle size can reach 8-13mm. Coating uniformity: By adjusting the air knife pressure, the difference in coating thickness between the upper and lower surfaces is controlled within 8% of the total coating, avoiding abnormal zinc flower distribution caused by uneven coating thickness.
4.How to adapt post-plating treatment to application scenarios?
Skin Finishing and Passivation Processes
Skin Finishing: Lightly rolling the galvanized sheet with rollers eliminates small spangles, resulting in a spangle-free surface that meets the high flatness requirements of industries like home appliances and automobiles.
Passivator Selection: Hexavalent chromium passivators form a dense passivation film that inhibits spangle growth. Trivalent chromium passivators, however, have a slower reaction rate and may result in slightly larger spangles, requiring adjustment of the passivation temperature for optimal results.
Differentiated Control Guided by Application Standards
Construction Industry: A spangle size of 8-13mm is typically required, which can be achieved by increasing the Sb content and reducing the cooling rate.
Automotive and Home Appliances: A spangle-free or small spangle finish is preferred, requiring a combination of low Sb content and rapid cooling processes.
5.What are the latest technological advances and cost optimization?
Single Zinc Pot Online Adjustment Technology
As described in the patent, by converting the fan to variable frequency control and optimizing the Sb addition process, online adjustment of spangle size can be achieved in a single zinc pot, eliminating the energy consumption and downtime associated with switching between two zinc pots.
Integrated Air Mist Cooling and Skin Finishing
Shougang and other companies use air mist cooling combined with skin finishing to refine spangles while improving the coating's anti-picking properties. For example, air mist cooling can control spangle size to 2-5mm, and skin finishing can further eliminate surface defects, meeting the demands of high-end automotive sheet metal.