The Influence Of The Temperature And Time On The Galvanized Coating

Hot-dip galvanizing is the most widely used technology to protect the steels from corrosion and obtains the wide-spreading application in the production. Galvanizing temperature and galvanizing time are the key technological parameters in hot-dip galvanizing. So it is significant to study the effects of galvanizing temperature and galvanizing time on the coating, and can provide the theory guidance in making the reasonable processing parameter. Si is always added into the steels as the element to strengthen the steels and deoxidate in the continuous casting of the steel. When there is some Si in the steels, it can results in the decrease of coating quality because of the Si reactivity. So it is also significant to study the effect of galvanizing temperature on the coating of the silicon steel. A series of investigation on the changes of the coating morphology were done by optical microscopy and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS).When the pure iron was galvanized, the experimental results confirmed that there is the parabolic relationship between the growth rate of the coating and the galvanizing temperature. When the temperature increases from 450℃to 480℃, the formation velocity of the coating quickens, and reaches the maximum at 480℃. When the galvanizing temperature is above 480℃, the formation velocity of the coating becomes slowly. The depth of the coating increases a little after the galvanizing temperature exceeding 500℃. The investigations on the changes of the coating morphology confirm that the main reason of the formation velocity of the coatings changing with the temperature is the change of the morphology of theζphase with the galvanizing temperature. In the temperature range of 450～470℃,the morphology ofζphase is coherent and columnar. When galvanized at 480℃, the morphology ofζphase changes from columnar to granular, and there are some gaps between the grains ofζphase obviously. The liquid-Zn can contact withδphase, accelerating the diffusion of Fe-Zn and the formation of the coating. When the temperature is above 480℃,ζphase becomes thinner. When galvanized above 500℃,ζphase disappears in the coating that makes the formation of the coating slowly. When galvanized at different temperature, the thickness of the coating increases with the immersing time.When the Sandelin (Si≈0.035~0.120%) steel was galvanized, there is also the parabolic relationship between the growth of the coating and the galvanizing temperature. The phase in the coating changed most obviously with the galvanized temperature isζphase. But the coating grows fastest at 470℃. There are some similarity in the character between the coating structure of Sandelin steel and that of the pure iron galvanized at 480℃. Si has the restraining effect on the growth ofδphase.