Galvanized Channel Steel Process Flow

The process flow of hot-dip galvanized channel steel: raw material inspection → acid washing → cleaning → zinc assistance → drying → galvanizing → cooling → passivation → cleaning → finished product inspection → inspection and packaging, etc.
According to customs, hot-dip galvanizing processes are often divided into two categories based on different pre-treatment methods: offline annealing and online annealing.
1. Off line annealing
Off line annealing refers to the recrystallization annealing of hot-rolled or cold-rolled steel in a bottomed annealing furnace or bell type annealing furnace before entering the hot-dip galvanizing line. In this way, there is no annealing process for the galvanizing line. Before hot-dip galvanizing, steel must maintain a clean, pure iron active surface free of oxides and other dirt. This method involves first removing the annealed surface iron oxide scale by acid washing, and then applying a layer of zinc chloride or a solvent composed of ammonium chloride and zinc chloride for protection to prevent the steel from being oxidized again.
(1) Wet hot dip galvanizing
The solvent on the surface of the steel does not need to be dried (i.e. the surface is still wet) before entering the zinc liquid covered with molten solvent on the surface for hot dip galvanizing. The drawbacks of this method are:
a. Zinc plating can only be carried out in a lead-free state, with a thick alloy layer and poor adhesion;
b. The generated zinc slag accumulates at the interface between zinc liquid and lead liquid, and cannot deposit at the bottom of the pot (because the specific gravity of zinc slag is greater than that of zinc liquid and less than that of lead liquid), resulting in surface contamination of the steel due to passing through the zinc layer.
Therefore, wet hot dip galvanizing has been basically phased out.
(2) Single steel
This method generally uses hot-rolled laminated plates as raw materials. Firstly, the annealed steel is sent to the pickling workshop, and the oxygen hot dip galvanizing method is used to remove the iron scale on the steel surface with sulfuric acid or hydrochloric acid. After pickling, the steel immediately enters the water tank to soak and wait for galvanizing, which can prevent further oxidation of the steel. Afterwards, it undergoes acid washing, water cleaning, squeezing drying, drying, and hot dip galvanizing in a zinc pot (maintained at a temperature of 445-465 ℃), followed by oil coating and chrome treatment. The quality of hot-dip galvanized sheet produced by this method is significantly improved compared to that of wet galvanizing, and it is only of certain value for small-scale production.
(3) Huilinfa fever
The continuous galvanizing production line includes a series of pre-treatment processes such as alkaline degreasing, hydrochloric acid pickling, water flushing, solvent coating, drying, etc., and the raw plate also needs to be annealed in a hood furnace before entering the galvanizing line for galvanizing. This method has a complex production process and high production cost. More importantly, the products produced by this method often have solvent defects that affect the corrosion resistance of the coating. And the AL in the zinc pot often reacts with the solvent on the steel surface to generate aluminum trichloride, which is consumed and the adhesion of the coating deteriorates. Therefore, although this method has been around for nearly thirty years, it has not been developed in the global hot-dip galvanizing industry.
2. Inline annealing
In line annealing refers to the gas protected recrystallization annealing of hot-dip galvanized raw plates directly provided by the cold rolling or hot rolling workshop as coils. The hot-dip galvanizing methods belonging to this industry include: Senjimil method, improved Senjimil method, American Steel Union method (same as Kawasaki method in Japan), Silas method, and Sharon method.