What is spangle?
With beautiful spangles is a main feature of traditional hot-dip galvanized sheets. The most complete shape of zinc crystals is similar to snowflakes or hexagonal stars, so the zinc crystals obtained by condensation on the surface of the hot-dip steel strip are most likely to form snowflakes or hexagonal spangles.
In fact, spangles are only of ornamental value and have no practical value. On the contrary, spangles are not good in corrosion resistance, and the surface will appear convex and concave after coating. Therefore, the home appliance and automobile industries tend to choose small spangles (no spangles) products, and spangled products that look beautiful, and still have a certain amount of use in the construction industry.
High-purity zinc liquid cannot solidify to form spangles when cooled, because the zinc liquid lacks the necessary crystal nuclei. To obtain a typical spangle, the prerequisite is to add two types of alloying elements with different properties in the zinc liquid in an appropriate amount. One type of alloying element 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 a certain degree of solubility in both liquid and solid zinc, such as aluminum, tin, antimony and so on. 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.
Hot-dip galvanized sheet surface spangle knowledge
Method of forming spangles
The method of forming spangles is to try to generate a large number of crystal nuclei as early as possible and reduce the solidification temperature of the zinc solution, so as to extend the growth time of spangle crystals and facilitate the growth of spangles.
(1) Add other elements to the zinc liquid
When a certain alloying element is added to the zinc liquid, the crystallization process can be prolonged, and the surface zinc liquid will not solidify before reaching its eutectic temperature.
Pure zinc solidifies at 419.5°C. If tin (less than 0.5%) is added, the tin-zinc eutectoid alloy will not solidify before it drops to 198°C. After cadmium is added, the cadmium-zinc eutectoid alloy will not solidify before it drops to 264°C. If antimony (less than 0.3%) is added, the antimony-zinc eutectoid alloy will not solidify before it drops to 409°C. After adding lead, its lead-zinc eutectoid alloy will not condense before it drops to 317°C.
Therefore, adding an alloy to lower the melting point of the zinc liquid can extend the solidification time of pure zinc to different degrees, which greatly prolongs the growth time of spangle crystals and facilitates the growth of spangles, so larger spangles can be obtained.
(2) Surface blowing method
When the zinc layer on the surface is about to solidify, if steam or sulfur dioxide is passed through the surface, the spangle crystals can grow smoothly and larger patterns can be obtained. Another purpose of blowing the sulfur dioxide gas stream is to form a dense oxide film and improve the corrosion resistance of the galvanized layer.
(3) Wire mesh method
This method can only be used on galvanized sheets. It uses a magnetic roller to make the steel wire mesh contact the surface of the zinc layer to achieve this purpose. Because the contact point of the steel mesh knots is first cooled, crystalline solid particles are first formed there to become crystalline nuclei, and the rest of the unsolidified zinc liquid uses this as the crystalline nucleus to begin to crystallize and expand outward to form spangles.
(4) Water spray method
When the pure zinc liquid on the surface of galvanized steel parts is not solidified, spray water mist (a mixture of water and steam or water and air) to cause crystallization nuclei. The unsolidified zinc liquid spreads out from this as a starting point to form spangles.
(5) Other methods
In addition to the above methods, if the surface of the plated parts is smooth, the galvanized layer is relatively thin, the control temperature is uniformly lowered, and the cooling time in the air is sufficient, larger spangles can also be obtained.
Spangle shape
The shape of the spangles depends on the type of alloy added in the zinc solution. For example, choosing lead and antimony can get a phoenix-like spangles; choosing lead and tin can get a fern-like spangles and so on.
The crystal orientation of spangles on the surface of the coating is different due to its different appearance, and there are also differences in processing. For example, when simple drawing and deep drawing, the processing of feather-shaped spangles is the best, followed by fern-shaped spangles and leaf-shaped spangles. The worst, and in bending and tension processing, the processing of leaf-shaped spangles is the best, followed by feather-shaped spangles and fern-shaped spangles with the worst processing properties.
Spangle size
According to the European standard EN 10346:2015, the size of hot-dip galvanized spangles can be divided into normal spangle and minimized spangle.
Conventional spangle refers to the size and morphology of zinc grains formed after the normal solidification of zinc liquid, while no spangle (also called small spangle) is a 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 spangle, it needs to be negotiated when ordering.
There are many factors that affect the size of spangles. When the composition of the zinc solution meets the conditions for the formation of spangles, the following factors will affect the size of the spangles:
(1) Raw material: The thicker the raw steel plate, the smaller the surface roughness, and the larger the spangles.
(2) Cooling rate: The faster the cooling rate, the shorter the crystal growth time and the smaller the spangles.
(3) Number of crystal nuclei: The size of spangles is closely related to the number of spangles. When the surface of the steel base is rough or the surface cleanliness is poor, the spangles are relatively small. Zinc crystal grains are formed on the steel base. The more crystal nuclei, the smaller the spangles. This principle is used by the water spray method in the production process of small spangles.
(4) Other factors (production environment, etc.).
In order to obtain small spangles, during galvanizing, when the zinc solution is close to the solidification temperature, immediately spray water droplets with a diameter of less than 0.1mm on the surface of the galvanized sheet to form a small and uniform crystal nucleus; in order to obtain smooth zinc Flower, and then use a reduction of less than 1% for leveling rolling. Small spangles are suitable for painting, and smooth spangles are suitable for deep drawing.
Zinc layer adhesion
When the heated steel strip passes through the molten zinc bath (zinc tank), an iron-zinc alloy layer is first formed on the substrate, and a pure zinc layer is covered on the alloy layer. The thickness of the iron-zinc alloy layer is uneven, looser, brittle, poorly ductile, 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 easy to peel off. When the pure zinc coating contains harmful impurities, such as zinc oxide, zinc dross, cadmium and lead, large spangles are formed, or the continuity of the zinc layer is destroyed, and small cracks in the pure zinc layer are prone to occur.
When the galvanized layer is too thick, its flexibility also deteriorates. When aluminum is added to the zinc solution, the aluminum in the zinc solution reacts with the substrate to first produce Fe2Al5 or Fe2Al3, which thins the iron-zinc alloy coating and improves the adhesion of the zinc layer.
Uneven spangles and influencing factors
Uneven spangles on the coating surface are a kind of surface defect existing in hot-dip galvanized products. Although it will not have a great impact on the use of most products, it affects the appearance of the surface and causes the application of products in certain areas of the market to be affected. limit. Some products with uneven spangles also have poor zinc layer adhesion, which directly affects the use of the product.
The uneven state of spangles on the surface of the hot-dip galvanized sheet can be summarized as follows:
(1) There are spangles on one side and no spangles on the other side. The diameter of the spangle on the spangle side is about 6mm.
(2) Along the longitudinal direction of the strip, the spangle size presents regular fluctuations.
(3) Along the horizontal direction of the strip, the spangles are large on one side and small on the other side.
(4) The spangles at the edges of the strip are small, and the center spangles are large.
The main factors that affect the unevenness of spangles include: the ratio of the two types of alloying elements in the composition of the zinc liquid, the air knife control process, the temperature of the strip steel, the impurity content in the gas and the combustion air, the cleanliness of the strip surface and the reduction state in the furnace, Roll nodules, strip steel scratches, and raw material surface oxide scale, strip steel entering the zinc pot temperature, zinc liquid temperature, cooling temperature after plating, etc.
