June 2015 Archives

(3) Cooling

Lift up the product from a zinc bath and allow excess molten zinc to drip off. Then, cool the product.There are two cooling methods available: water-cooling and air-cooling. In general, air-cooling is more commonly used.To prevent distortion caused by rapid cooling, use hot water at 70 °C or higher instead of cold water.Use air-cooling for thick plates to minimize distortion. However, air-cooling is known to degrade the appearance compared to water-cooling.

(4) Finishing

Finish the plated product by removing zinc drop on the product end, zinc residue and non-plating agent on the non-plating area, zinc accumulated in the pore, excessive roughness, etc.
Residual stress that occurs during the production process of the steel products is released by plating heat, which causes distortion to appear on the product.From the commercial point of view, it is necessary to consider whether the degree of distortion is within the permissible limit and corrections are needed or feasible.The figure below is a specific example of distortion caused by welding methods.

(5) Partial non-plating

Some steel frame products require welding after the plating process. With such types of products, there are areas not to be plated.In this case, non-plating measures are necessary.As a non-plating measure, you can remove the plated area using a grinder. However, applying this method increases the cost.Another non-plating measure is to protect the non-plating area by masking.
Masking methods include the followings:

1.Masking using caustic lime or heat-resistant materials.
2.Acid pickling on the non-plating area after applying chemical-proof coating so that rust and scale remain on the material.
3.Perform Step 1 after Step 2.

* The next section explains the continuous manufacturing of surface-treated steel sheets in a mass production line.

(2) Hot dipping process

After the pretreatment process, dip the product into a bath of molten zinc around 460°C. Wait for an appropriate length of time, and then lift the product up from the molten zinc bath after removing zinc oxide particles floating on the surface.After allowing excess molten zinc to drip off by applying light impact, cool the product.

As shown in the cross-section image above, the plating film consists of a zinc layer on the top and an alloy layer formed by the reaction of molten zinc with steel.The thickness and composition of an alloy layer are determined by the chemical composition of steel and operating conditions. The thickness of the topmost zinc layer is determined by factors such as the bath temperature and speed of withdrawing the product from the plating bath.

(1) Plating temperature

In most cases, the temperature is set between 440 and 470°C. Special items such as bolts may be plated at a higher temperature.The plating temperature should be kept as low as possible so that it will not compromise the drip-off performance when withdrawing the product from the zinc-plating bath.
If the bath temperature is too high, it increases the zinc reactivity. As a result, it generates more zinc oxide and shortens the life of the plating bath.Although a higher temperature improves the fluidity of zinc, it roughens the alloy layer structure and its surface. On the other hand, if the bath temperature is too low, the plating film tends to be extremely think and uneven.

(2) Immersion time

In general, the longer the immersion time, the greater the amount of zinc deposition is.[Fig.1] shows the deposition conditions for general steel and high-tensile steel materials.