Hot galvanized sheet surface zinc flower knowledge

◆ What is zinc flower?

Beautiful zinc flowers are one of the main features of traditional hot galvanized sheets. The most complete form of zinc crystals is approximately snowflake or hexagonal star shape, so the zinc crystals obtained by condensation on the surface of the hot plating process are most likely to form snowflake or hexagonal star zinc flowers.

In fact, zinc flowers only have ornamental value, no practical value, on the contrary, zinc flowers in corrosion resistance is not good, the surface will appear convex and concave level after painting. Therefore, the home appliances and automotive industries tend to choose small zinc flowers (non-zinc flowers) products, and zinc flowers that look beautiful, and there is a certain amount of use in the construction industry.

High purity zinc liquid can not solidify to form zinc flowers when it is cooled, because the zinc liquid lacks the necessary crystallization nuclei. To obtain the typical zinc flower, the prerequisite is to add two kinds of alloying elements with different properties in the zinc liquid, one kind of alloying elements are completely soluble in the zinc liquid, and almost completely insoluble in the solid zinc, such as lead and titanium; Another class of alloying elements have certain solubility in liquid and solid zinc, such as aluminum, tin, antimony and so on. If only a certain class of alloying elements are added, although the surface state of the zinc layer will change after solidification, it is not enough to form a typical zinc flower.

◆ Method of forming zinc flower

The method of forming zinc flowers is to try to generate a large number of crystallization nuclei early, reduce the solidification temperature of zinc liquid, so as to prolong the growth time of zinc flower crystals and facilitate the growth of zinc flowers.

(1) Add other elements to the zinc solution

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, and if tin (below 0.5%) is added, the tin-zinc eutectoid alloy will not solidify until it is reduced to 198 ° C. After adding cadmium, the cadmium-zinc eutectoid alloy will not solidify until it is lowered to 264 ° C. If antimony is added (below 0.3%), the antimony zinc eutectoid alloy will not solidify until it is reduced to 409℃. After adding lead, its lead-zinc eutectoid alloy will not coagulate until it is reduced to 317℃.

Therefore, the addition of alloy to reduce the melting point of zinc liquid has different degrees of extension than the solidification time of pure zinc, so that the growth time of zinc flower crystals is greatly extended, which is convenient for the growth of zinc flowers, so that larger zinc flowers can be obtained.

(2) Surface gas blowing method

When the zinc layer on the surface is about to solidify, if the steam flow or sulfur dioxide flow passes through the surface, the zinc flower crystal can grow smoothly, and a larger pattern can be obtained. Another purpose of blowing sulfur dioxide gas flow 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 sheet, which is achieved by using a magnetic roller to make the wire mesh contact with the surface of the zinc layer. Because the joint contact of the wire mesh is first cooled, crystallized solid particles are first formed there and become crystallization nuclei, and the rest of the unsolidified zinc liquid begins to crystallize and expand outwards to form zinc flowers as crystallization nuclei.

(4) Spray fog method

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

(5) Other methods

In addition to the above methods, if the surface of the plated parts can be smooth, the galvanized layer is relatively thin, the temperature is controlled to drop evenly and the cooling time in the air is sufficient, the larger zinc flowers can also be obtained.

▲ Zinc flower shape

The shape of the zinc flower depends on the type of alloy added in the zinc liquid, such as lead, antimony can obtain a phoenix-shaped zinc flower; Choose lead and tin to get fernlike zinc flowers and so on.

The crystal orientation of zinc flowers on the surface of the coating is different because of its appearance, and there are also differences in processing, such as simple drawing and deep drawing, the processing of feathery zinc flowers is the best, followed by the ferrat-shaped zinc flowers and leaf zinc flowers are the worst, and in bending and tension processing, the processing of leafy zinc flowers is the best, followed by the feather-shaped zinc flowers and ferrat-shaped zinc flowers are the worst.

▲ Zinc flower size

In accordance with the provisions of European standard EN 10346:2015, the dimensions of hot-dip galvanized zinc flowers can be divided into normal spangle and minimized spangle.

The conventional zinc flower refers to the zinc grain size and morphology formed after the normal solidification of zinc liquid, while the zinc-free flower (also known as the small zinc flower) is the zinc grain size and morphology obtained by special control of the zinc solidification process. If the user needs to specify the size of the zinc flower, it needs to be negotiated at the time of ordering.

There are many factors affecting the size of zinc flowers. When the composition of zinc liquid has the conditions for the formation of zinc flowers, the following factors affect the size of zinc flowers:

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

(2) Cooling rate: the faster the cooling rate, the shorter the crystal growth time, the smaller the zinc flower.

(3) Number of crystal nuclei: The size of zinc flowers is closely related to the number of crystal nuclei of zinc flowers. When the surface of the steel base is rough or the surface cleanliness is poor, the zinc flower is relatively small. Zinc grains are formed on the steel base, and the more the crystal nuclei, the smaller the zinc flowers. This principle is used in the production process of small zinc flowers.

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

In order to obtain small zinc flowers, when galvanized, when the zinc liquid is near the solidification temperature, water droplets with a diameter of less than 0.1mm are immediately sprayed onto the surface of the galvanized sheet to form a small and uniform crystallization nucleus; In order to get the finished zinc flower, then use less than 1% reduction to smooth rolling. Small zinc flowers are suitable for painting, and smooth zinc flowers are suitable for deep stamping.

★ Zinc layer adhesion

When the heated steel strip passes through the molten zinc liquid (zinc tank), the iron-zinc alloy layer is formed on the substrate first, and the pure zinc layer is covered on the alloy layer. Iron zinc alloy layer thickness is not uniform, relatively loose, easy to brittleness, poor ductility, easy to crack.

If there is a certain amount of iron base salt remaining on the surface of the steel plate before galvanized, the generated iron zinc alloy has poor adhesion, poor ductility, and is easy to flake 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 of the pure zinc layer are easy to occur.

When the galvanized layer is too thick, its bendability is also poor. When aluminum is added to the zinc solution, the aluminum in the zinc solution reacts with the substrate to form Fe2Al5 or Fe2Al3, which thinens the iron-zinc alloy coating and improves the adhesion of the zinc layer.

★ Uneven zinc flower and influencing factors

The uneven zinc flower on the surface of the coating is a surface defect of hot dip galvanized products, although it will not have a great impact on the use of most products, but affect the appearance of the surface, resulting in the application of products in some areas of the market is limited. Some products with uneven zinc flowers also have poor adhesion of zinc layer, which directly affects the use of products.

★ The uneven state of zinc flowers on the surface of hot galvanized sheet coating can be summarized as follows:

(1) There are zinc flowers on one side and no zinc flowers on the other side. The diameter of the zinc flower on the zinc flower side is about 6mm.

(2) The size of zinc flowers fluctuates regularly along the longitudinal strip.

(3) Transverse along the strip steel, one side of the zinc flower is large, the other side of the zinc flower is small.

(4) The strip edge zinc flower is small, and the middle zinc flower is large.

The main factors affecting the uneven zinc bloom include: ratio of two kinds of alloying elements in the composition of zinc liquid, gas knife control technology, strip temperature, impurity content in gas and combustion air, strip surface cleanliness and furnace reduction state, roll nodding, strip scratch and surface oxidation of raw materials, strip into the zinc pot temperature, zinc liquid temperature, cooling temperature after plating, etc.