Galvanized pipe coating process

A continuous galvanized pipe coating production technology and equipment, and the process includes the following steps: passivation, cleaning; paint; curing, with drying preheating before coating, and the drying preheating temperature is about ±20 ℃ of the curing temperature. The device is based on the known device, before the dip-coating slot with drying preheating furnace. The process and equipment can not only in bulk for continuous galvanized pipe paint production, but also ensure the quality of paint, the surface no bubbles, no orange peel, efficiency can be improved twenty or thirty times.

Hot dip galvanizing production difficulty is steel leakage defect, zinc layer falls off and edge thick.

1. Steel leakage defect

In order to reduce cost and increase the strength of steel, much more silicon are often be added to the steel base. The exceeded silicon can increase the surface tension, reduce the steel plate to the zinc liquid wetting ability and increase the plating leakage possibility. In addition, the silicon is easy oxidation and generate SiO2. It is hard to removed in the acid condition, so that the steel leakage defects often happens in SiO2 zones.

To solve this problem, the no-pickling hot-rolled steel belt continuous hot galvanizing process can be adopted. Hot-rolled steel belt directly into the reduction furnace of hot rolling steel strip without pickling, and increase the strip in the reducing furnace temperature to 650-750 ℃, holding time increased to 120-400s. Protective gas H2 volume fraction increased to more than 20%. At this time, because of the volume expansion of scale of hot rolling steel strip produced cracks and voids, H2 can go through the cracks and gaps, making scale composed of pure Fe and FeO porous layer. On the surface of hot dip galvanized, to form Fe – Al – zinc alloy layer, can improve the coating adhesion, eliminate the leakage defects of steel.

2. Zinc layer falls off

Due to the low calorific value of gas of no-oxidation furnace, pressure fluctuations makes oxidizing atmosphere, resulting in the zinc layer falls off.

Measures to take: 1) choose combustion gas burner with good performance to keep the air and gas pressure within the prescribed scope; 2) the variation of gas calorific value is lower than ±5%; 3) firing period of furnace temperature higher than 1000 ℃; 4) no burner preheating section temperature higher than 800 ℃; 5) strip out without oxidation furnace temperature below 650 ℃.

3. Edge thick

Thicker plate specification, low hot strip speed, when the hot dip galvanized zinc layer is thicker than the middle of zinc layer, the thick edge defect happens.This kind of defect particularly harmful to strip coiling. After rolling, the edge will be greatly thicker than the diameter of central, the diameter of edge will generate tensile stress, which is so easy to put the side spin and the wave edge defects.

Under the high-speed, the strip steel forms high-speed thick edge defect, and under the low speed, it forms the low-speed thick edge defect. 1) the high-speed thick edge is mainly due to the bad air blade, and by adjusting the air blade, the angle can eliminate the defect. 2) low-speed thick edge is mainly because of the large heat dissipating In order to eliminate the defects, additional jet can be installed to increase the strip edge. In addition, the movable baffle can be used to eliminate the thick edge defects at low speed.