Alloying process

Alloying means a process which is after zinc pot the strip with pure zinc layer into the annealing furnace with a temperature  450～550℃, in order to get zinc coat contain 7%～ 15% Zn-Fe alloy. Effectiveness factor mainly contain of effective Al, zinc pot temperature, strip temperature while entering zinc pot, alloying temperature, strip speed, strip chemical component, and the zinc coating thickness etc.

High quality alloying product don`t expect the zinc coating have teeth shape Г appearance(FeZn10), or have plenty cylinder shape ζ appearance (FeZn13). That needs to choose proper alloying coat annealing process, which require precise control to heating rate, soaking time, cooling rate.
Surfaceness process

Target is taking off strip yield point elongation in order to acquire good straightness and the proper surfaceness for galvanized strip, require to smooth the galvanized strip. Smooth process parameter mainly include Roll force, bending roll force, flat entrance tension and flat exit tension. For improve the galvanized strip straightness and surfaceness, the four leveling roller mill better than the two leveling roller mill.

At present there had new developed four leveling roller mill which could change working roller by different diameter roller, in order to control the strip’s straightness and surface-ness adopt different strip particular different strip material used in the hot drip galvanized line. Surfaceness is one of the key technology for high quality galvanized product, now the research mainly focus on the surface texturing treatment with leveling roller, there had some more factors for the surfaceness like diameter of leveling roller,roughness of roller surface etc.

In the texturing treatment, shot-blasted roller and electro-discharge texturing widely used, laser roughed roller due to the long period, high cost not used much. Recently the study mostly on chrome plated roughen and electron beam texturing, developing fast but still not popularized. Another side, the leveling roller sticking zinc has been a difficult problem in the production of high surface quality galvanizing products, require necessary upgrade leveling mill, could put cleaning detergent in leveling roller desalination water.

Post-processing

Post-processing including passivation,prephosphating,coating oil. Through passivation treatment could improve zinc coat structure and luster, could improve corrosion resistance and service life, could improve the bonding force between zinc coat and strip. Previously, passivation treatment mainly used chromate passivation.

Some activators, such as fluoride, phosphoric acid or sulfuric acid, are added to the passivation liquid to get the thicker chromate film after passivation. When fluoride in the passi-vation liquid can reduce the surface tension of the steel strip,accelerate the film reaction, can increase the chemical polishing effect, make the passivation film fine and bright. Hot galvanizing steel plate for building, in the market have special chromating coat with excellent corrosion resistance and black rust resistance.

Over the years,  researchers have conducted a great of research on inorganic corrosion inhibitors of non-toxic or low-toxic substances as passivation agents. U.S. Hicharm development of titanate is one of them, it is less toxic than chromate, corrosion resistance is equivalent to low Cr passivation. The results indicate that. The hot dip galvanizing layer is passivated with the non-toxic passivation liquid, can delay the time of white rust in the zinc coating, its corrosion resistance is close to the level of chromate, without chromium passivation instead of chromate has been the trend.