What is zinc flower?
A beautiful zinc flower is a major feature of traditional hot-dip galvanized sheets. The most complete form of zinc crystals is similar to snowflakes or hexagonal stars, so the zinc crystals obtained by condensation on the surface of the strip by hot-dip process are most likely to form snowflakes or hexagonal zinc flowers.
In fact, zinc flower has only ornamental value and no practical value. On the contrary, zinc flower is not good in corrosion resistance, and it will appear convex and concave after surface coating. Therefore, the home appliance and automobile industries tend to choose small zinc flower （no zinc flower） products, while the beautiful ones have zinc flower products, and there is still a certain amount of use in the construction industry.
The high-purity zinc solution cannot be solidified to form zinc flowers upon cooling because the necessary crystal nucleus is absent from the zinc liquid. To obtain a typical zinc flower, the prerequisite is to add two kinds of alloying elements with different properties in the zinc liquid. One type of alloying element is completely compatible in the zinc liquid, and is almost completely incompatible in the solid zinc, such as lead. And titanium; another type of alloying elements have a certain solubility in liquid, solid zinc, such as aluminum, tin, antimony and the like. If only a certain type of alloying element is added, although the surface state changes after the zinc layer solidifies, it is not enough to form a typical zinc flower.
Method of forming zinc flower
The method of forming zinc flower is to try to generate a large number of crystal nuclei early, reduce the solidification temperature of the zinc liquid, and prolong the growth time of the zinc flower crystal, and facilitate the growth of the zinc flower.
（1） Add other elements to the zinc solution
When an alloying element is added to the zinc solution, the crystallization process can be prolonged, and the surface zinc solution does not solidify until it reaches its eutectic temperature.
Pure zinc solidifies at 419.5 ° C. If tin （0.5% or less） is added, the tin zinc eutectoid alloy will not solidify until it reaches 198 ° C. After the addition of cadmium, the cadmium zinc eutectoid alloy does not solidify when it is not lowered to 264 °C. If yttrium （less than 0.3%） is added, the bismuth zinc eutectoid alloy will not solidify when it is not lowered to 409 °C. After adding lead, the lead-zinc eutectoid alloy does not condense when it is not lowered to 317 °C.
Therefore, the addition of the alloy reduces the melting point of the zinc liquid, and the solidification time of the pure zinc is extended to different extents, so that the growth time of the zinc flower crystal is greatly prolonged, and the growth of the zinc flower is facilitated, so that a larger zinc flower can be obtained.
（2） Surface blowing gas method
When the surface zinc layer is to be solidified, if a vapor stream or a sulfur dioxide gas stream is passed through the surface thereof, the zinc flower crystals can be smoothly grown, and a larger pattern can be obtained. Another purpose of blowing a sulfur dioxide gas stream is to form a dense oxide film to improve the corrosion resistance of the galvanized layer.
（3） Steel wire mesh method
This method can only be used on galvanized sheets, which use magnetic rollers to bring the wire mesh into contact with the surface of the zinc layer for this purpose. Since the contact point of the steel wire mesh is first cooled, at this point, the crystal solid particles are first formed to become a crystal nucleus, and the remaining unsolidified zinc liquid, that is, the crystal nucleus starts to expand outward to form a zinc flower.
（4） Water spray method
When the pure zinc liquid on the surface of the galvanized steel is not solidified, water mist （water and steam or a mixture of water and air） is sprayed to cause a crystal nucleus. The unsolidified zinc liquid is used as a starting point to diffuse outward to form a zinc flower.
（5） Other methods
In addition to the above methods, if the surface of the plated part is smooth, the galvanized layer is relatively thin, the control temperature is uniformly lowered, and sufficient cooling time in the air is obtained, a large zinc flower can be obtained.
Zinc flower shape
The shape of the zinc flower depends on the type of alloy added in the zinc liquid. For example, lead and strontium can be selected to obtain phoenix-shaped zinc flower; if lead or tin is selected, fern-like zinc flower can be obtained.
The crystal orientation of the zinc flower on the surface of the coating differs depending on its appearance. For example, when it is simply stretched and deep-drawn, the feather-like zinc flower has the best processing property, followed by the fern-like zinc flower and the leaf-like zinc flower processing property. Worst, the leaf-like zinc flower has the best workability in bending and tension processing, followed by feather-like zinc flower and fern-like zinc flower with the worst processability.
Zinc flower size
According to the European standard EN 10346:2015, the size of hot-dip galvanized flowers can be divided into ordinary spangles and minimized spangles.
Conventional zinc flower refers to the size and morphology of zinc crystal formed after the normal solidification of zinc solution, and there is no zinc flower （also known as small zinc flower）. This is the zinc grain size and shape obtained by special control of the solidification process of zinc. appearance. If the user needs to specify the size of the zinc flower, it is necessary to negotiate when ordering.
There are many factors affecting the size of zinc flower. When the zinc component has the condition of zinc flower formation, the following factors have an effect on the size of the zinc flower:
（1） Raw materials: The thicker the raw steel sheet, the smaller the surface roughness, and the larger the zinc flower.
（2） Cooling rate: The faster the cooling rate, the shorter the crystal growth time and the smaller the zinc flower.
（3） Number of crystal nuclei: The size of zinc flower is closely related to the number of zinc nuclei. When the steel base surface is rough or the surface cleanliness is poor, the zinc flower is relatively small. Zinc grains are formed on the steel base, the more crystal nuclei, the smaller the zinc flower, and the water spray method of the small zinc flower production process utilizes this principle.
（4） Other factors （production environment, etc.）.
In order to obtain a small zinc flower, in the case of galvanizing, when the zinc liquid is near the solidification temperature, water droplets having a diameter of 0.1 mm or less are immediately sprayed onto the surface of the galvanized sheet to form a fine and uniform crystal nucleus; The flower is further subjected to temper rolling with a reduction of less than 1%. Small zinc flowers are suitable for painting, and smooth zinc flowers are suitable for deep drawing.
Zinc layer adhesion
When the heated steel strip passes through the molten zinc liquid （zinc bath）, an iron-zinc alloy layer is first formed on the substrate, and a pure zinc layer is covered on the outer layer of the alloy layer. The thickness of the iron-zinc alloy layer is uneven, loose, brittle, poor in ductility, and easy to crack.
If a certain amount of iron-based salt remains on the surface of the steel sheet before galvanizing, the resulting iron-zinc alloy has poor adhesion, poor ductility, and is easily peeled off. When the pure zinc coating contains harmful impurities such as zinc oxide, zinc slag, cadmium and lead, a large zinc flower is formed, or the continuity of the zinc layer is broken, and small cracking of the pure zinc layer is liable to occur.
When the galvanized layer is too thick, its bendability is also deteriorated. When aluminum is added to the zinc solution, the aluminum in the zinc solution reacts with the substrate, firstly forming Fe2Al5 or Fe2Al3, and thinning the iron-zinc alloy plating layer, thereby improving the adhesion of the zinc layer.
Zinc flower unevenness and influencing factors
The uneven zinc coating on the surface of the coating is a surface defect in the hot-dip galvanizing product. Although it does not have a great influence on the use of most products, it affects the appearance of the surface and causes the application of the product in some fields of the market to be affected. limit. Some products with uneven zinc flower have the same adhesion of zinc layer, which directly affects the use of the product.
The uneven zinc flower surface on the surface of the hot-dip galvanized sheet can be summarized as follows:
（1） There are zinc flowers on one side and no zinc flowers on the other side. The zinc flower with one side of the zinc flower is about 6 mm in diameter.
（2） along the longitudinal direction of the strip, the size of the zinc flower shows a regular fluctuation.
（3） along the strip transverse direction, one side of the zinc flower is large, and the other side of the zinc flower is small.
（4） The zinc flower on the edge of the strip is small, and the zinc flower in the middle is large.
The main factors affecting the unevenness of zinc flower include: the proportion of two kinds of alloying elements in the composition of zinc liquid, the air knife control process, the temperature of the strip steel, the impurity content in the gas, the combustion air, the cleanliness of the strip surface and the reduction state in the furnace. Roll nodulation, strip scratching and iron oxide scale on the surface of the material, the temperature of the steel into the zinc pot, the temperature of the zinc bath, the cooling temperature after plating.