When the surface of the hot-dip coating of the steel plate is solidified, there will be a crystal pattern of the metal coating, which is especially obvious for the galvanized sheet. The appearance of beautiful zinc crystals has become an important feature of the appearance of hot-dip galvanized sheets. This crystal pattern is usually called “spangle”.
High-purity zinc liquid cannot solidify to form spangles when it is cooled, because the zinc liquid lacks the necessary crystallization nuclei. To obtain typical spangles, the precondition is to add two types of alloying elements with different properties in the zinc liquid. One type of alloying elements is completely compatible in the zinc liquid, but almost completely incompatible in the solid zinc, such as lead and titanium; another type of alloying element has certain solubility in liquid and solid zinc, such as aluminum, tin, antimony, etc. If only a certain type of alloying element is added, although the surface state of the zinc layer will change after solidification, it is not enough to form a typical spangle.
The method of forming spangles is to try to generate a large number of crystalline nuclei as early as possible and reduce the solidification temperature of zinc liquid, so as to prolong the growth time of spangle crystals and facilitate the growth of spangles.
The size of hot-dip galvanized spangle can be divided into:
There are many factors that affect the size of the spangle.
When the composition of the zinc solution has the conditions for the formation of the spangle, the following factors will affect the size of the spangle:
- Raw material: The thicker the raw steel plate, the smaller the surface roughness and the larger the spangle.
- Cooling rate: The faster the cooling rate, the shorter the crystal growth time and the smaller the spangle.
- The amount of zinc crystals: The size of the spangles is related to the amount of zinc crystals. When the surface of the steel base is rough or the surface cleanliness is poor, the spangle is relatively small. Zinc crystals are formed on the steel base, and the more zinc crystals, the smaller the spangle.
The main factors that affect spangle unevenness include:
- The ratio of two types of alloying elements in the composition of molten zinc
- Air Knife Control Process
- Steel sheet temperature
- Impurity content in gas and combustion air
- Steel surface cleanliness
- Oxidation of iron on the surface of raw materials
- The temperature of the steel sheet into the zinc pot
- Zinc liquid temperature
- Cooling temperature after galvanizing, etc.