Difficulties and countermeasures of hot galvanizing production

- Feb 12, 2019-

The difficulty of hot galvanizing production is: steel leakage defect, zinc layer falling off, thick edge.

  1. Steel leakage defect

In order to reduce the cost and increase the strength of steel, higher silicon element is often added to the steel base.Excessive addition of silicon will increase the surface tension, reduce the wetting ability of steel plate to zinc solution, and increase the possibility of plating leakage.In addition, silicon is easy to be oxidized and generate SiO2, which is not easy to be removed when it is acidic, thus forming steel leakage defects in SiO2 aggregation area.


In order to solve this problem, the continuous hot galvanizing process of hot rolled steel strip without pickling was adopted.Without pickling directly into the reduction furnace of hot rolling steel strip, and to increase the strip in the reducing furnace temperature to 650-750 ℃, holding time increased to 120-400 - s.The volume fraction of H2 was increased to more than 20%.At this point, cracks and voids are generated in hot rolled strip steel due to the volume expansion of oxide scale, and H2 can enter through these cracks and voids, so that the oxide scale becomes a porous layer composed of pure Fe and FeO.Fe-al-zn alloy layer is formed on the surface during hot dip galvanizing, which can improve the adhesion of the coating and eliminate steel leakage defects.


2. Zinc layer falls off


In the no-oxidation furnace, the oxidizing atmosphere appears due to the low heating value of the gas and the fluctuation of the pressure, which causes the zinc layer to fall off.


The measures taken are as follows: 1) choose a gas burner with excellent combustion performance;To maintain the pressure of air and gas within the prescribed range;2) the fluctuation range of heating value of gas is less 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, thick edge


The specification of hot rolled sheet is thicker, and the running speed of the strip is low.This defect is particularly harmful to strip coiling. The side part is thick, and the diameter of the side part of the coil will be larger than the middle diameter after coiling. When the diameter of the side part reaches, the tensile stress will be generated, so it is easy to stretch the side part, thus forming the wavy edge defect.


The defects formed at high speed and low speed are high speed thick edge.1) the high speed thick edge is mainly caused by the bad Angle adjustment of air knife, which leads to convection and turbulence.Proper adjustment of the air knife Angle can eliminate this defect.2) low-speed thick edge is mainly due to the heat loss in the low-speed edge, and part of the airflow at both ends of the nozzle dissipates outward, which reduces the impulse of the airflow at the edge, resulting in a smaller amount of zinc scraping at the edge than in the middle, so the low-speed thick edge is formed.In order to eliminate the defect, an additional nozzle can be installed at the edge of the strip to increase the air injection pressure at the edge of the strip.In addition, the use of movable baffles on both sides of the strip can eliminate the defect of low speed and thick edge.


Cangzhou Steel Pipe Group (CSPG) Co., Ltd

Cangzhou Steel Pipe Group (CSPG) Co., Ltd. is a large-sized, key metallurgic enterprise of Hebei Province in North China, whose history dates back to 1994. CSPG currently em/paces six member companies with products varying from SSAW, LSAW, ERW, seamless steel pipes to 3PE pipes, galvanized pipes, casing pipes, etc. A Joint-stock corporation, CSPG occupies an area of 600,000 square meters with a total asset of $530 million.



Specialized in the production of straight welded pipe spiral pipe galvanized pipe 3PE/3PP/FBE/TPEP internal and external epoxy powder internal and external epoxy resin cement mortar two cloth three oil buried pipeline IPN8710 non-toxic drinking water internal and external plastic coated lining plastic and other anti-corrosion pipe fittings for oil and gas pipeline water conservancy projects



Executive standard :DIN30670 DIN30678 CSA Z245 AFNOR nf49-710/711 NACE rp0394/0490 AWWAC 210/C213GB/T9711 API 5L ISO 3183Material: Q235B/Q355BGR A GR B x42-x80 l245-l555

 

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