News Center

Home page

What you don't know about hot-dip galvanizing zinc flower - an article that you can brag about after reading

Category: Industry News Publish Time：2018-10-10

What is zinc flower? Beautiful zinc flowers are a main characteristic of traditional hot-dip galvanized steel. The most complete form of zinc crystals is similar to snowflakes or hexagons, so the zinc crystals obtained by condensation on the surface of steel strip through hot-dipping process are most likely to form snowflake or hexagon-shaped zinc flowers. In fact, zinc flowers only have ornamental value and no practical value. On the contrary, zinc flowers are not good in corrosion resistance, and will appear uneven after surface coating. Therefore, home appliance and automobile industries tend to choose small zinc flower (no zinc flower) products, while the beautiful zinc flower products still have a certain amount of use in the construction industry. High-purity zinc liquid in

　　What is zinc spangle?

　　The presence of beautiful zinc spangles is a key characteristic of traditional hot-dip galvanized steel sheets. The most complete form of zinc crystals is similar to snowflakes or hexagrams. Therefore, the zinc crystals obtained by condensation on the surface of steel strip through hot-dipping are most likely to form snowflake or hexagram-shaped zinc spangles.

　　In fact, zinc spangles only have ornamental value and no practical value. On the contrary, zinc spangles are not good in terms of corrosion resistance, and they will appear uneven after surface coating. Therefore, the home appliance and automotive industries tend to choose products with small zinc spangles (no zinc spangles), while products with beautiful zinc spangles still have a certain amount of use in the construction industry.

　　High-purity zinc liquid cannot solidify to form zinc spangles when cooled because the zinc liquid lacks the necessary crystal nuclei. To obtain typical zinc spangles, it is a prerequisite to add an appropriate amount of two types of alloy elements with different properties to the zinc liquid. One type of alloy element is completely soluble in the zinc liquid but almost completely insoluble in solid zinc, such as lead and titanium; the other type of alloy element has a certain solubility in both liquid and solid zinc, such as aluminum, tin, and antimony. If only one type of alloy element is added, although the surface state will change after the zinc layer solidifies, it is not enough to form typical zinc spangles.

　　Methods of forming zinc spangles

　　The method of forming zinc spangles is to try to generate a large number of crystal nuclei as early as possible, lower the solidification temperature of the zinc liquid, extend the growth time of the zinc spangle crystals, and facilitate the growth of zinc spangles.

　　(1) Adding other elements to the zinc liquid

　　When a certain alloy element is added to the zinc liquid, its crystallization process can be prolonged, and the surface zinc liquid will not solidify before reaching its eutectic temperature.

　　Pure zinc solidifies at 419.5℃. If tin (less than 0.5%) is added, the tin-zinc eutectic alloy will not solidify until it drops below 198℃. After adding cadmium, the cadmium-zinc eutectic alloy will not solidify until it drops below 264℃. If antimony (less than 0.3%) is added, its antimony-zinc eutectic alloy will not solidify until it drops below 409℃. After adding lead, its lead-zinc eutectic alloy will not solidify until it drops below 317℃.

　　Therefore, adding alloy elements to lower the melting point of the zinc liquid prolongs the solidification time compared to pure zinc to varying degrees, greatly extending the growth time of the zinc spangle crystals and facilitating the growth of zinc spangles, so larger zinc spangles can be obtained.

　　(2) Surface blowing gas method

　　When the surface zinc layer is about to solidify, if a steam flow or sulfur dioxide gas flow passes through its surface, the zinc spangle crystals can grow smoothly, and larger patterns can be obtained. Another purpose of blowing sulfur dioxide gas flow is to form a dense oxide film to improve the corrosion resistance of the galvanized layer.

　　(3) Wire mesh method

　　This method can only be used on galvanized thin plates. It uses a magnetic roller to make the wire mesh contact the surface of the zinc layer to achieve this purpose. Because the knotted contact points of the wire mesh are cooled first, solid particles are generated and become crystal nuclei at these points. The remaining unsolidified zinc liquid starts to crystallize outward from this as a crystal nucleus, generating zinc spangles.

　　(4) Water mist spraying method

　　When the pure zinc liquid on the surface of the galvanized steel part is not solidified, water mist (a mixture of water and steam or water and air) is sprayed to create crystal nuclei. The unsolidified zinc liquid diffuses outward from this point to generate zinc spangles.

　　(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 temperature is controlled to decrease uniformly, and there is sufficient air cooling time, larger zinc spangles can also be obtained.

　　Shape of zinc spangles

　　The shape of zinc spangles depends on the type of alloy added to the zinc liquid. For example, choosing lead and antimony can obtain phoenix-tail-shaped zinc spangles; choosing lead and tin can obtain fern-shaped zinc spangles, and so on.

　　The crystal orientation of zinc spangles on the surface of the coating is different in appearance and also differs in processing. For example, in simple stretching and deep drawing, feather-shaped zinc spangles have the best processability, followed by fern-shaped zinc spangles, and leaf-shaped zinc spangles have the worst processability. In bending and tension processing, leaf-shaped zinc spangles have the best processability, followed by feather-shaped zinc spangles, and fern-shaped zinc spangles have the worst processability.

　　Zinc spangle size

　　According to the provisions of European standard EN 10346:2015, the size of hot-dip galvanized zinc spangles can be divided into normal spangle and minimized spangle.

　　Normal spangle refers to the size and morphology of zinc crystals formed after the normal solidification of zinc liquid, while no spangle (also called minimized spangle) is the size and morphology of zinc crystals obtained by special control of the solidification process of zinc. If the user needs to specify the size of the zinc spangles, it needs to be negotiated during ordering.

　　Many factors affect the size of zinc spangles. When the zinc liquid composition meets the conditions for the formation of zinc spangles, the following factors affect the size of zinc spangles:

　　(1) Raw materials: The thicker the raw steel plate and the smaller the surface roughness, the larger the zinc spangles.

　　(2) Cooling speed: The faster the cooling speed, the shorter the crystal growth time, and the smaller the zinc spangles.

　　(3) Number of crystal nuclei: The size of zinc spangles is closely related to the number of zinc spangle nuclei. When the surface of the steel substrate is rough or the surface cleanliness is poor, the zinc spangles are relatively small. Zinc crystals are formed on the steel substrate. The more nuclei, the smaller the zinc spangles. The water mist spraying method in the minimized spangle production process utilizes this principle.

　　(4) Other factors (production environment, etc.).

　　In order to obtain minimized spangles, during galvanizing, when the zinc liquid is close to the solidification temperature, water droplets with a diameter of less than 0.1 mm are immediately sprayed onto the surface of the galvanized plate to form fine and uniform crystal nuclei; in order to obtain smooth zinc spangles, leveling rolling is performed with a reduction of less than 1%. Minimized spangles are suitable for coating, and smooth zinc spangles are suitable for deep drawing.

　　Adhesion of zinc layer

　　When the heated steel strip passes through the molten zinc (zinc bath), an iron-zinc alloy layer is first formed on the substrate, and a pure zinc layer is covered on the outside of the alloy layer. The iron-zinc alloy layer has uneven thickness, is relatively loose, brittle, has poor ductility, and is prone to cracking.

　　If a certain amount of iron-based salt remains on the surface of the steel plate before galvanizing, the resulting iron-zinc alloy will have 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, large zinc flowers are formed, or the continuity of the zinc layer is destroyed, and small cracks in the pure zinc layer are easily generated.

　　When the galvanized layer is too thick, its bendability also deteriorates. When aluminum is added to the zinc liquid, the aluminum in the zinc liquid reacts with the substrate, first generating Fe 2 Al 5 or Fe 2 Al 3 , reducing the thickness of the iron-zinc alloy coating and improving the adhesion of the zinc layer.

　　Uneven zinc flowers and influencing factors

　　Uneven zinc flowers on the surface of the coating are a surface defect in hot-dip galvanized products. Although it does not have a significant impact on the use of most products, it affects the surface appearance and limits the application of the product in certain market areas. Some products with uneven zinc flowers also have poor zinc layer adhesion, which directly affects the use of the product.

　　The uneven state of zinc flowers on the surface of the hot-dip galvanized plate coating can be summarized as follows:

　　(1) Zinc flowers on one side, no zinc flowers on the other side. The diameter of the zinc flowers on the side with zinc flowers is about 6mm.

　　(2) Along the longitudinal direction of the steel strip, the size of the zinc flowers shows a regular fluctuation.

　　(3) Along the transverse direction of the steel strip, the zinc flowers are large on one side and small on the other side.

　　(4) The zinc flowers are small at the edges of the steel strip and large in the middle.

　　The main factors affecting the unevenness of zinc flowers include: the ratio of the two types of alloying elements in the zinc liquid composition, the air knife control process, the steel strip temperature, the impurity content in the gas and combustion air, the surface cleanliness of the steel strip and the reduction state in the furnace, the formation of nodules on the furnace rolls, scratches on the steel strip and iron oxide scale on the raw material surface, the temperature of the steel strip entering the zinc pot, the temperature of the zinc liquid, and the post-plating cooling temperature, etc.

Keywords: What you don't know about hot-dip galvanizing zinc flower - an article that you can brag about after reading

Previous: Research progress of self-healing coating materials

Next: What are the alloy strengthening mechanisms?