Off-line annealing is the recrystallization annealing of the hot-rolled or cold-rolled steel plate in the draw-bottom annealing furnace or cover annealing furnace before entering the hot-plated wire line, so that there is no annealing process for the galvanized line. The steel plate must maintain a clean pure iron active surface free of oxides and other impurities before hot galvanizing. This method first removes the annealed surface oxide sheet by pickling, and then coats it with a layer of zinc chloride or a solvent composed of ammonium chloride and zinc chloride to protect it, thereby preventing the steel sheet from being oxidized again.
The solvent on the surface of the steel plate is not dried, that is, the surface is still wet, and the zinc liquid covered with molten solvent is hot plated on the surface. The disadvantage of this method is that it can only be galvanized in a lead-free state, and the alloy layer of the coating is very thick and the adhesion is very bad. The generated zinc slag is deposited at the interface of 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 the zinc liquid and less than the lead liquid, so the steel plate pollutes the surface by passing through the zinc layer. Therefore, this method has been basically eliminated.
This method generally uses hot-rolled laminated plates as raw materials, and first sends the annealed steel plate to the pickling workshop, and uses sulfuric acid or hydrochloric acid to remove the surface of the steel plate by oxygen hot-dip galvanizing. After pickling, the steel plate immediately enters the water tank to soak and wait for galvanized, which can prevent the steel plate from being reoxidized. After pickling, water cleaning, squeezing dry, drying, into the zinc pot, the temperature has been maintained at 445-465 ℃.
Hot dip galvanizing followed by oiling and chroming. The quality of the hot-galvanized sheet produced by this method is significantly improved than that of the finished product of the wet galvanized method, and it is only valuable for small-scale production. Continuous galvanizing production line includes lye degreasing, hydrochloric acid pickling, water washing, solvent coating, drying and a series of pre-treatment processes, and the original plate into the galvanizing line before galvanizing also need to be annealed under the hood furnace.
The production process of this method is complicated, the production cost is high, and the main thing is that the product produced by this method often has solvent defects, which affects the corrosion resistance of the coating. And the AL in the zinc pan often reacts with the solvent on the surface of the steel plate to produce aluminum trichloride and is consumed, and the adhesion of the coating is deteriorated. Therefore, although this method has been available for nearly 30 years, it has not been developed in the world hot dip galvanizing industry.
Post time: 14-07-23