Quality Defects of Galvanized Steel Sheet

Surface contaminationAfter hot-dip galvanizing the substrate, some of the coating has a small surface defects, rough look like embedded in the coating of dross particles, under the scanning electron microscope with secondary electrons or backscattered electron observation, found as a patch or die oxide film. These spots and oxide films cause local surface contamination of the coating to discolor.

The solution is: enhanced cleaning, including lye scrubbing and electrolytic degreasing, can remove the cold-rolled strip surface iron and oil and other impurities, the production of high surface quality and coating adhesion requirements of high zinc content is essential. The new hot-dip galvanizing unit cleaning section includes alkali liquid immersion, lye scrubbing, electrolytic cleaning, hot water scrubbing, hot water rinsing, cleaning and drying and drying, the strip surface can be residual oil and residual carbon control In the 20mg / m2 (single) below, to achieve the quality of cleaning requirements; the other with a SiO2 abrasive brush roll, cleaning the strip surface grinding, but also improve the GL product coating adhesion.

Leakage pointLeakage point refers to the hot dip plating is not completely infiltrated caused by the unplated area. In the case of hot-dip galvanizing, only the complete rolling of the rolled metal oxide, rust, oxide, oil and cleaning solution, and the exposed substrate surface in order to ensure excellent wetting. Rolling mill is usually in the A1 content of more than 0. 2% of the zinc liquid in the production of automotive exterior cover with hot-dip galvanized steel. The galvanized steel sheet produced in this zinc pot is relatively bright, but when the zinc liquid reaches such a high A1 content, the substrate surface cleanliness is more stringent in order to obtain excellent wettability. High strength steel plate contains C, P, Mn and Si alloy elements, these alloying elements in the heat treatment may be segregated to the steel surface, into oxide. Therefore, the substrate containing Si will cause a lot of leakage and melting point, poor wetting, poor adhesion of the coating and poor surface smoothness and other defects.

The solution is:(1) At the furnace temperature is 1180?, the control of excess air coefficient of 0.9 or so, you can make the oxidation-free section to the weak atmosphere, reduce the surface of the oxide layer depth, reducing the furnace to further reduce the reduction, to reduce the reduction Furnace H2 inlet.(2) To reduce the strip in the oxygen-free section of the outlet temperature can also reduce the surface oxidation of the depth of strip. According to domestic and foreign production experience, strip of the export temperature of 650? or so more appropriate. It is best not to exceed 680?.(3) For all the flange interface, the focus is the cooling fan and the furnace shell interface, the cooler and the furnace shell interface using high temperature elastic sealing gasket, can ensure that the flange after thermal expansion and contraction does not leak. Ensure that the dew point of the furnace meets the process requirements.

Embedded in the coating of dross particlesIn the hot galvanized plate edge, the surface of the coating is often a massive dark spots. The samples with dark spots were observed by scanning electron microscopy (EDX) with EDX. It was found that there was a lot of oxygen in the defect area. The dark spot is the area covered by the oxide film. These oxide film is obviously out of the steel out of the pot. When the air knife sprayed the coating to break the oxide film, in the dark spot will form a bright crack. After observation with an optical microscope, it was found that the defective coating contained a large amount of dross particles. These dross particles are associated with the oxide film on the surface of the zinc liquid and are taken out of the zinc pot with the oxide film. Strip out of the zinc pot, the nearby area of??zinc liquid surface scum.The solution is:(1) Zinc liquid temperature control: In the galvanized production, due to the chemical properties of aluminum and zinc liquid temperature, it is decided that the production of dross is inevitable, the main component of the bottom slag is FeZn7, the main scum The composition is Fe2Al5. In the modern continuous hot-dip galvanizing production, the general removal of the bottom slag does not use the way of fishing residue, and the use of chemical methods; scum generally take the way of slag, we must pay attention to the furnace in the strip at the exit of scum fishing, Because the scum here is easy to adhere to the strip surface.(2) Slag: in the galvanized production, due to the chemical properties of aluminum and zinc liquid temperature, determines the production of dross is inevitable, the main component of the bottom slag FeZn7, scum the main component of Fe2Al5 The In the modern continuous hot-dip galvanizing production, the general removal of the bottom slag does not use the way of fishing residue, and the use of chemical methods; scum generally take the way of slag, we must pay attention to the furnace in the strip at the exit of scum fishing, Because the scum here is easy to adhere to the strip surface.

Hot-dip galvanized after the surface defects are more complex, the reasons are more, the solution is multifaceted, need to do a lot of difficult and detailed work, so hot step to add a step.

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