Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Patent
1998-06-19
2000-03-28
Parker, Fred J.
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
427461, 427482, 427185, 427321, 4273768, 118309, 118634, 118DIG5, B05D 106, B05D 124, B05C 1902
Patent
active
060428929
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and a method for coating zinc on steel sheets for use on automobiles and electronic apparatuses. More specifically, the present invention relates to an apparatus and a method for coating zinc on steel sheets by using zinc powders.
Zinc performs a sacrificing action for steel to extend the life expectancy of steel, and therefore, conventionally zinc has been used in zinc-coating a steel sheet. There are many kinds of zinc coating methods such as hot dip galvanization, electroplating, and zinc powder-using zinc coating. The zinc coated steel sheets are mostly used as automobile body sheets and outer and inner sheets of electronic apparatuses. They are manufactured by electroplating or hot dip galvanization. The reason is as follows. That is, when a steel strip is continuously coated, not only the product quality but also the productivity and the workability have to be considered. In this respect, electroplating and hot dip galvanization are advantageous.
Electroplating is carried out in the following manner. That is, cold rolled steel sheets are made to undergo a batch annealing or a continuous annealing so as to improve mechanical properties. Then, electroplating is carried out within an electrolyte containing zinc ions, thereby obtaining a zinc deposition layer of the target thickness. In this method, the mechanical properties which have been obtained through heat treatments are not degraded during the plating process. Further, the coated amount (deposition thickness) is varied in accordance with the applied electric power, and therefore, the coated amount can be accurately controlled.
However, it has the following disadvantages. That is, as the coated amount is increased, much more electric power is required. Consequently, the productivity is aggravated in the case of a thick plating. Therefore, electroplating is adopted in the case where the coated amount is 40 g/m.sup.2 or less for one face of the steel sheets. Meanwhile, the plating speed is limited by the current density limit, and therefore, if the productivity is to be improved, the zinc coating chamber has to be long. This becomes a facility burden. Because of such limiting factors, electroplating should be preferably carried out at a strip velocity of about 200 m/min and at a coated amount of 40 g/m.sup.2 or less for one face of the steel sheet.
Meanwhile, hot dip galvanization is carried out in the same facility as that of the annealing. Therefore the manufacturing cost becomes lower, and a thick zinc coating is possible. However, it has the following disadvantages. That is, a sink roll and a guide roll which are immersed in a hot dipping pot are corroded, and therefore, they have to be replaced periodically. Further, as the line speed becomes fast, the resistivity of the melted zinc is increased. Therefore, the sink roll cannot move synchronously with the steel sheet to produce slips, and therefore, the surface of the steel sheet may be scratched so much as to lead to a product defect. Further, as the line speed is increased, or as the coated thickness is decreased, splashes are increased during an air wiping, with the result that the generation of dross is increased. Besides, if the zinc adhered on the surface is to be solidified, some cooling period is required, and therefore, the velocity of the steel sheet is limited to about 200 m/min. Further, the adjustment of the coated thickness is difficult, and therefore, the manufacturing becomes difficult if the coated amount per face is less than 40 g/m.sup.2.
A method for coating zinc by using zinc powders is disclosed in Japanese Patent Application Laid-open No. Hei-5-311388.
This method is illustrated in FIG. 1. As shown in this drawing, an object to be coated (steel sheet) 1 is heated to above the melting point of the powder metal, and zinc powders loaded on a gas are spouted by means of a spouting nozzle 6 onto the object 1 within a zinc coating chamber 3 containing a reducing atmosphere. Thus the zinc powders are melt-
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patent: 5384165 (1995-01-01), Sellitto
patent: 5551981 (1996-09-01), Sellitto
Cho Noi Ha
Chung Jin Hwan
Kim Sang Hean
Parker Fred J.
Pohang Iron & Steel Co. Ltd.
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