Knowledge of hot-dip galvanized sheet

　　1. Main Purpose

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

　　2. Classification of galvanizing in steel mills

　　There are two types of galvanized products produced by steel mills: electrogalvanizing and hot-dip galvanizing, which are further subdivided. Electrogalvanizing can be divided into three types based on the thickness of the zinc layer: single sided, double sided, and differential thickness coating. Hot dip galvanizing does not have single sided and different thickness coatings, but it is divided into galvanizing and galvanized iron alloy, and electroplating does not have zinc iron alloy. Electroplated coatings have been extended to include zinc plating, nickel plating, tin plating, chromium plating, and hot dip galvanizing in terms of coating types, including hot dip galvanizing, zinc iron, aluminum zinc zinc, zinc aluminum aluminum, and zinc aluminum magnesium.

　　3. Annealing method

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

　　4. Common types of steel

　　Product category: General Commodity Roll (CQ), Deep Drawn Hot Dip Galvanized Sheet (DQ), Extra Deep Drawn Hot Dip Galvanized Sheet (DDQ-EDQ/SEDDQ), Structural Galvanized Sheet (HSLA), Bake Hardened Hot Dip Galvanized Sheet (BH), Dual Phase Steel (DP), TRIP Steel (Phase Change Induced Plastic Steel), Composite Phase Steel (CP), etc.

　　5. Defects in hot-dip galvanizing

　　Zinc layer peeling, scratches, passivation spots, zinc particles, thick edges, air knife stripes, air knife scratches, exposed steel bars, inclusions, mechanical damage, poor steel base performance, wavy edges, warping, inappropriate size, pressure printing, zinc layers of different thicknesses, roller printing, etc.

　　6. Main causes of zinc layer detachment

　　The main reason for the peeling of the zinc layer is the mismatch between raw material production and processing. Due to surface oxidation, silicon compounds, high oxidation atmosphere and protective gas dew point in the NOF section, unreasonable air fuel ratio, low hydrogen flow rate, aerobic infiltration in the furnace, low temperature of strip steel entering the furnace, and low furnace temperature in the NOF section. The oil evaporates endlessly, the aluminum content in the zinc pot is low, the unit speed is too fast, the reduction is insufficient, the residence time in the zinc solution is too short, and the coating is too thick. Processing mismatches include inconsistent bending radii, mold wear, scratches, mold gaps that are too large or too small, insufficient stamping lubricant, long mold working hours without the need for grinding and maintenance, etc

　　7. Reasons for white rust and black spots

　　The main reasons for 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 steel;

　　(4) Passivation not completely dried;

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

　　(6) Long storage time of finished products;

　　(7) Galvanized sheet is stored in contact or together with other corrosive media such as acid and alkali.

　　White rust may evolve into black spots, but black spots may not necessarily be caused by white rust, such as friction black

　　8. Maximum allowed storage time

　　If oiled, packaged and stored in place. Some products can be stored for more than a year, but it is best to use them for three months. If not oiled, the time is shorter to avoid oxidation caused by long-term storage. The specific storage time should be executed according to the corresponding execution standards for the specific product.