Process flow of hot-dip galvanized channel steel: raw material inspection → pickling → cleaning → zinc assistance → drying → galvanizing → cooling → passivation → cleaning → finished product inspection → inspection and packaging, etc.

According to tradition, hot-dip galvanizing processes are often divided into two categories based on different pre-treatment methods: out of line annealing and in line annealing.

1. Off line annealing

Off line annealing refers to the process of recrystallization annealing of hot-rolled or cold-rolled steel before entering the hot-dip galvanizing line in a bottom type annealing furnace or a hood type annealing furnace, so that 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 contaminants. This method first removes the annealed surface oxide scale by acid washing, and then applies a layer of solvent composed of zinc chloride or a mixture of ammonium chloride and zinc chloride for protection, thereby preventing the steel from being oxidized again.

(1) Wet hot-dip galvanizing

The solvent on the surface of the steel enters the zinc solution covered with molten solvent without drying (i.e. the surface is still wet) for hot-dip galvanizing. The drawbacks of this method are:

a. Galvanizing can only be done in a lead-free state, with a thick alloy layer and poor adhesion of the coating;

b. The generated zinc slag accumulates at the interface between zinc liquid and lead liquid and cannot be deposited 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), which pollutes the surface of the steel due to passing through the zinc layer.

Therefore, wet hot-dip galvanizing has been basically eliminated.

(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 galvanized iron sheet on the surface of the steel is removed with sulfuric acid or hydrochloric acid. After pickling, the steel is immediately immersed in a water tank and waiting for galvanizing to prevent further oxidation of the steel. Afterwards, it undergoes acid washing, water cleaning, extrusion drying, drying, and enters a zinc pot (maintained at 445-465 ℃) for hot-dip galvanizing, followed by oil coating and chroming treatment. The quality of hot-dip galvanized sheet produced by this method is significantly improved compared to wet galvanizing finished products, and it only has certain value for small-scale production.

(3) Huilin Fa Re

The continuous galvanizing production line includes a series of pre-treatment processes such as alkali degreasing, hydrochloric acid pickling, water washing, solvent coating, drying, etc. Moreover, the original sheet needs to undergo hood furnace annealing before entering the galvanizing line for galvanizing. This method has a complex production process, high production costs, and most importantly, the products produced by this method often have solvent defects, which affect the corrosion resistance of the coating. And the AL in the zinc pot often reacts with the solvent on the surface of the steel to produce 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. In line annealing

In line annealing is a process in which a strip is directly provided by a cold rolling or hot rolling workshop as the raw material for hot-dip galvanizing, and undergoes gas shielded recrystallization annealing in the hot-dip galvanizing operation line. The hot-dip galvanizing methods belonging to this industry include: Sengimir method, improved Sengimir method, American Steel Union method (similar to Japan's Kawasaki method), Silas method, and Sharon method.

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