Introduction to hot-dip galvanized coils

1. Main Application

Hot-dip galvanized sheets are mainly used in industries such as automobiles, home appliances, construction, machinery, electronics, and light industry.

2. Classification of galvanized sheets

There are two types of galvanized products: electro-galvanizing and hot-dip galvanizing. These two types are further subdivided. Electro-galvanizing is divided into three types based on the thickness of the zinc layer: single-sided, double-sided and differential thickness coating, while hot-dip galvanizing does not have single-sided and differential thickness coating. However, hot-dip galvanizing is divided into galvanizing and galvanized iron alloy coating, while electroplating does not have zinc-iron alloy. In terms of coating types, electroplating coatings are extended to electro-galvanizing, electro-galvanizing zinc-nickel, electro-tin, and electro-chromium. Hot-dip galvanizing has more extensions, including hot-dip galvanizing, zinc-iron, aluminum-zinc, zinc-aluminum, and zinc-aluminum-magnesium.

3. Annealing method

There are two types of annealing methods for conventional cold rolled coated products: continuous annealing and cover annealing.

4. Commonly used steel grades

Product categories: general commodity coils (CQ), deep drawing hot-dip galvanized sheets (DQ), ultra deep drawing hot-dip galvanized sheets (DDQ-EDDQ/SEDDQ), structural galvanized sheets (HSLA), bake hardening hot-dip galvanized sheets (BH), duplex steel (DP), TRIP steel (transformation induced plasticity steel), complex phase steel (CP), etc.

5. Defects of hot-dip galvanizing

Zinc layer peeling, scratches, passivation spots, zinc particles, thick edges, air knife streaks, air knife scratches, exposed steel, inclusions, mechanical damage, poor steel base performance, wavy edges, warping, inconsistent dimensions, embossing, inconsistent zinc layer thickness, roller marks, etc.

6. The main reason for zinc layer shedding

The main reasons for the zinc layer falling off include raw material production reasons and processing mismatch. The raw materials have surface oxidation, silicon compounds, the oxidation atmosphere in the NOF section and the protective gas dew point is too high, the air-fuel ratio is unreasonable, the hydrogen flow rate is low, the furnace has oxygen infiltration, the strip entering the pot temperature is low, the NOF section furnace temperature is low, the grease is not completely evaporated, the aluminum content of the zinc pot is low, the unit speed is too fast, the reduction is not sufficient, the residence time in the zinc liquid is too short, and the coating is too thick. Processing mismatch includes bending radius mismatch, mold wear, scratches, mold gap is too large or too small, less stamping lubricant, and mold working time is too long without grinding and maintenance.

7. Causes of white rust and black spots

The main causes of white rust are:

(1) Poor passivation, insufficient or uneven passivation film thickness;

(2) The surface is not oiled;

(3) Residual moisture on the surface of the strip;

(4) The passivation was not completely dried;

(5) Affected by moisture or rain during transportation or storage;

(6) Finished products are stored for too long;

(7) Galvanized sheets are in contact with or stored together with other corrosive media such as acids and alkalis.

White rust may evolve into black spots, but black spots are not necessarily caused by white rust. For example, black spots caused by friction

8. Maximum allowed storage time

If oiled, and the packaging and storage are in place, some products can be stored for more than 1 year, but it is best to use them up within three months. If not oiled, the storage time should be even shorter to avoid oxidation caused by excessive storage. The specific storage time should be based on the implementation standards corresponding to the specific product.

9. Tolerance

Negative values are not allowed for length tolerance, and the maximum value is not allowed to exceed +6mm. Regarding tolerances, whether it is length, width, or thickness tolerance, you can check it in the tolerance checker in our mini program.

10. Classification of zinc flowers

Zinc flowers are generally divided into large zinc flowers, small zinc flowers, and no zinc flowers.

11. Protection principle of zinc layer

In a corrosive environment, zinc will corrode before steel, thus protecting the steel base. The zinc layer will form a certain protective film to prevent excessive oxidation and slow down the corrosion rate. In addition, zinc powder paint can be applied during maintenance to prevent corrosion of the steel and ensure the mechanical properties and other safety properties of the material.

12. Passivation principle

Chromate passivation treatment can form a passivation film on galvanized sheets. The trivalent chromium in the solution passivation group is difficult to dissolve in water, chemically inactive, and acts as a skeleton, while the hexavalent chromium is easily soluble in water and can play a role of re-passivation when the passivation film is scratched, and has a healing effect on the passivation film. Therefore, within a certain limit, the passivation film can prevent steam or humid air from directly corroding the galvanized sheet and play a protective role.

13. Methods of corrosion resistance

There are three methods to test the corrosion resistance of hot-dip galvanized sheets:

(1) Salt spray test; (2) Humidity test; (3) Corrosion test.