Common problems of galvanized steel coil

Common problems of galvanized steel coil

1. How many types of galvanizing methods are currently available in the world?

Answer: There are three types of methods: electro-galvanizing, hot-dip galvanizing and galvanizing.
2. What are the commonly used steel grades for hot-dip galvanized sheets?

Answer: Product categories: general merchandise coil (CQ), structural galvanized sheet (HSLA), deep drawing hot-dip galvanized sheet (DDQ), bake hardening hot-dip galvanized sheet (BH), dual-phase steel (DP), TRIP Steel (transformation induced plasticity steel), etc.

3. What is the main purpose of hot-dip galvanized sheet?

Answer: Hot-dip galvanized sheets are mainly used in construction, home appliances, automobiles, machinery, electronics, light industry and other industries

4. Which two types of hot-dip galvanizing can be divided into different annealing methods?

Answer: It can be divided into two types: in-line annealing and out-of-line annealing, which are also called shielding gas method and flux method respectively.

5. What are the types of galvanizing annealing furnace?

Answer: Three types: vertical annealing furnace, horizontal annealing furnace and vertical and horizontal annealing furnace.

6. How many cooling methods are usually used for cooling towers?

Answer: There are two types: air-cooled and water-cooled.

7. What are the main defects of hot-dip galvanizing?

Answer: Mainly include: shedding, scratches, passivation spots, zinc particles, thick edges, air knife streaks, air knife scratches, exposed steel, inclusions, mechanical damage, poor performance of the steel base, wave edges, buckling, size Incompatibility, embossing, inconsistent zinc layer thickness, roll printing, etc.

8. Known: The produced specification is 0.75×1050mm, and the coil weight is 5 tons. What is the length of the coil? (The specific gravity of galvanized sheet is 7.85g/cm3)

Solution: L=G/(h×b×p)=(5×1000)/(0.785×1.050×7.5)=808.816m

Answer: The strip length of the coil is 808.816m.

9. What are the main reasons for the zinc layer to fall off?

Answer: The main reasons for the zinc layer to fall off are: surface oxidation, silicon compounds, too dirty cold rolled emulsion, too high oxidizing atmosphere and protective gas dew point in the NOF section, unreasonable air-fuel ratio, low hydrogen flow, oxygen infiltration in the furnace, The temperature of the strip steel entering the pot is low, the furnace pressure in the RWP section is low and the furnace door suction, the furnace temperature in the NOF section is low, the grease does not evaporate, the aluminum content of the zinc pot is low, the unit speed is too fast, the reduction is insufficient, and the zinc liquid The medium residence time is too short and the coating is too thick.

10. What are the causes of white rust and dark spots?

Answer: Black spots are formed by further oxidation of white rust. The main causes of white rust are:

Poor passivation, insufficient or uneven thickness of the passivation film; no oil on the surface or residual moisture on the surface of the strip; moisture on the surface of the strip during coiling; incomplete passivation; moisture or rain during transportation or storage ; The finished product storage time is too long; the galvanized sheet is in contact with other corrosive media such as acid and alkali or stored together.
11. How long is the maximum allowable storage time for finished products in the warehouse? Why?

Answer: It is allowed to store for three months to avoid oxidation caused by too long storage.

12. What is the length tolerance of the galvanized steel sheet?

Answer: The length tolerance is not allowed to have negative values, and the maximum is not allowed to exceed +6mm.

13. What is a big spangle? What is a small spangle?

Answer: Large spangles are normal spangles. Large spangles can be produced according to the normal production process. The crystal nucleus diameter is not less than 0.2mm; the crystal nucleus diameter is less than 0.2mm, which is called small spangles. This kind of spangles is generally unusable. Identify with the naked eye.

14. The protection principle of hot-dip galvanized layer?

Answer: Because zinc can form a film with good corrosion resistance on the surface in a corrosive environment. It not only protects the zinc layer itself, but also protects the steel base.

15. What is the principle of passivation of hot-dip galvanized sheet?

Answer: Chromate passivation treatment on the galvanized sheet can form a passivation film. The chemical reaction formula is as follows: Zn+H2GrO4-ZnGrO2=H2

The trivalent chromium in the solution passivation group is difficult to dissolve in water, and its chemical properties are inactive, which acts as a skeleton. The hexavalent chromium is easily soluble in water and can play a re-passivation effect when the passivation film is scratched. Has the healing effect of passivation film. Therefore, within a certain limit, the passivation film can prevent steam or humid air from directly corroding the galvanized sheet, and the Dun zinc layer plays a protective role.

16. What are the methods for testing the corrosion resistance of hot-dip galvanized sheets?

Answer: There are three methods for testing the corrosion resistance of hot-dip galvanized sheets: salt spray test; humidity test; corrosion test.

17. Why should anti-rust treatment be applied to hot-dip galvanized products?

Answer: When the hot-dip galvanized sheet is in humid air, especially when the air contains acidic substances such as SiO2, CO2, NO2, and NO, the surface of the galvanized layer will quickly form loose white rust. The main components of white rust are ZnO and Zn(OH)2. This white rust not only affects the appearance, but also brings great difficulties to future use.

18. What are the causes of white rust?

Answer: The causes of white rust are: hot-dip galvanizing and other corrosive media such as acid, alkali, salt, etc.; the passivation film or anti-film is destroyed; the passivation or oiling effect is not good; the storage warehouse is not well ventilated. Moisture; galvanized sheet is sprayed with water during transportation; transported at low temperature and stored at high temperature to form condensate.

19. How to calculate the thickness of the coating?

Answer: The coating thickness formula is: Dz = Gz (Sz*dz) where: Dz-thickness of the coating mm, Gz-weight per square meter of the coating g, Sz-coating area mm2, dz-specific gravity of zinc , The above formula can be simplified as: Dz=0.000139Gz

20. What are the reasons for the production of zinc particles?

Answer: The problem of the original board is mainly the cleanliness of the original board and the iron powder content of the original board; the temperature of the zinc liquid is too high, which causes the bottom dross to float; the content of Al is too high, which affects the solubility of iron.

21. What is the meaning of hot-dip galvanized sheet finishing?

Answer: With the continuous expansion of the use of hot-dip galvanized sheet, in the modern continuous hot-dip galvanizing unit of strip steel, the following purposes can be achieved by smoothing the strip:

①Improve the straightness and flatness of the thin plate, and at the same time, the zinc particles on the surface can be flattened to make the surface of the strip smooth, which is particularly advantageous for future deep drawing and other occasions with high precision;
②The smoothing roller used has undergone pre-shot peening treatment, so the surface of the smoothed plated plate has a certain degree of roughness. It can improve the adhesion of the coating and can store a certain amount of grease, which is beneficial to the lubrication of the die during deep drawing processing;
③For the plated plates to be painted later, although the surface is controlled to be small spangles, the small spangles can still be exposed through the paint layer. Therefore, for some high-demand galvanized sheets, the surface of the small spangles needs to be smoothed. In this way, the galvanized sheet can obtain a more uniform silver-white appearance;
④Through smoothing, the lower yield point can be lowered, and the yield platform disappears or is less obvious, which can prevent slip lines from appearing in the subsequent drawing or deep drawing processing, and improve the possibility of deep drawing.

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Reference source: Internet
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