1. The impact of carbon
The higher the carbon content in the steel, the stronger the iron zinc reaction, the greater the loss of iron, the stronger the reaction of steel base, that is, the thicker the iron zinc alloy layer becomes, the worse the adhesion of zinc coating. Therefore,
low carbon steel sheet with carbon content less than 0.15% is suitable for hot galvanizing.
2. Influence of silicon
The high silicon content in steel substrate will bring difficulties to hot dip galvanizing. It is generally considered that
killed steel with high silicon content can not be used for hot dip galvanizing, so boiling steel with silicon content less than 0.07% is mostly used as hot-dip galvanizing plate for a long time
It has been proved by experience that the high silicon
content in the steel base will cause the severe thickening of the ξ phase in the iron zinc alloy layer of the zinc coating, forming a gray coating, and making the adhesion of the zinc coating worse.
It has been recognized that the harmful effect
of high silicon content in steel substrate on hot-dip galvanizing has been recognized. However, the principle of its action is quite complicated. However, it is generally believed that the effect of silicon in steel plate on hot-dip galvanizing mainly
depends on silicon dioxide rather than free silicon.
The main reason is that the atomic volume of silicon is small and silicon has a large affinity for oxygen, so it is difficult to be reduced by hydrogen in annealing furnace. When steel plate is
heated or recrystallized in bell type furnace, silicon surface can be enriched and the following chemical reactions can occur:
Si + O2 = SiO2
Si +2FeO = SiO2 + 2Fe
In this way, a layer of SiO2 oxide film will be formed not only on the
surface of the steel plate, but also in the interior of the steel plate.
It is found that the brittle ξ phase in the gray coating is especially developed, which leads to the sharp growth of the Fe Zn alloy layer and the decrease of the adhesion
of the coating. It can be seen that SiO2 in steel sheet is the main factor affecting hot dip galvanizing.
3. Effect of titanium
Titanium is the main additive element in the production of low alloy high strength hot-dip galvanized sheet, which
has little effect on the galvanizing process, but has a certain impact on the mechanical properties of the steel plate. Because titanium has a large affinity with carbon and nitrogen, it can become the state of tic and tin in steel. Because carbon and
nitrogen are the main elements that cause steel aging, so the steel containing titanium will fix nitrogen and carbon, which plays an important role in the anti-aging of steel.
4. Manganese, phosphorus and sulfur
Generally, it contains less
manganese and phosphorus in low carbon steel. Practice has proved that the content of manganese and sulfur in steel has little effect on the structure of zinc coating. However, phosphorus in steel has a significant bad effect on hot dip galvanizing. When
the phosphorus content is about 0.15%, the growth of ξ phase and δ 1 phase is very fast because of the thin η phase transition. Where the η phase transformation is thin or even completely free of η phase, the coating will appear dull spots and make the
5. Hot dip galvanizing
It is necessary to control the temperature of the zinc bath, the time of galvanizing and the speed of the work piece leading out of the liquid zinc. The extraction speed is generally 1.5 m / min. If the
temperature is too low, the fluidity of liquid zinc is poor, and the coating is thick and uneven, it is easy to produce sagging and poor appearance quality; when the temperature is high, the liquid zinc is easy to separate from the workpiece, which can
reduce sagging and wrinkling, with strong adhesion, thin coating, good appearance and high production efficiency; however, when the temperature is too high, the iron loss of the workpiece and zinc pot is serious, resulting in a large number of zinc slag,
which affects the quality of zinc coating and is easy to be affected The color difference makes the surface color unsightly and the zinc consumption is high.
The thickness of zinc layer depends on the temperature of liquid zinc, the time of zinc
dipping, the material of steel and the composition of liquid zinc.
In order to prevent high temperature deformation of workpiece and reduce zinc slag caused by iron loss, 450 ～ 470 ℃ is adopted for 0.5 ～ 1.5min. Some factories adopt higher temperature
for large workpieces and iron castings, but avoid the temperature range of peak iron loss. However, we suggest that the alloy with the function of removing iron and reducing eutectic temperature should be added into the liquid zinc, and the zinc plating
temperature should be reduced to 435-445 ℃.