Coating processes – Immersion or partial immersion – Molten metal or fused salt bath
Patent
1988-10-25
1990-11-20
Silverberg, Sam
Coating processes
Immersion or partial immersion
Molten metal or fused salt bath
C23C 206, C23C 236
Patent
active
049718428
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for controlling the thickness of an intermetallic layer on a continuous steel product in a continuous hot-dip galvanizing process. The continuous steel product is generally either a strip or a wire.
A cold-rolled steel strip can be given a good formability by means of a heat treatment disclosed in my earlier U.S. Pat. No. 4,361,448. After annealing at a temperature T.sub.1 (720.degree. to 850.degree. C.) the steel strip is slowly cooled to a temperature T.sub.2 (600.degree. to 650.degree. C.), from which temperature it is rapidly quenched in a zinc bath to a temperature T.sub.3. The time interval between T.sub.2 and T.sub.3 is about 0.5 seconds.
In the arrangement of the U.S. Pat. No. 4,361,448 a zinc bath cooler and a zinc pump, with nozzles, are separate units. Molten metal having the same temperature as the zinc bath is pumped through a snout to the immersion point of the steel strip. Therefore the end temperature T.sub.3 of the rapid cooling is rather high, and the steel strip does not reach the temperature of the zinc bath during the entire immersion time (about two seconds).
A steel strip travelling through a zinc bath causes a laminar zinc flow following the surface of the steel strip. The heat from inside the steel strip raises the temperature of the laminar zinc flow (layer) to a value higher than the operating temperature of the zinc bath. Since iron and zinc react strongly in a conventional zinc bath (containing 0.15 to 0.25% aluminium) at temperature above 480.degree. C., the result is that a thick intermetallic layer is formed on the zinc coating.
In order to achieve a good formability of the zinc coating, the intermetallic layer should be as thin as possible. In the method according to the invention, the thickness of the intermetallic layer is controlled by
rapidly cooling the steel product by quenching it in a bath of molten zinc, and controlling the structure of the coating to be formed on the steel product by regulating the end temperature of the steel product in the quenching by directing a flow of molten zinc, cooled to a temperature below the operating temperature of the zinc bath, towards the steel product as it moves through the zinc bath.
Preferably a first flow of molten zinc is directed towards the steel product close to the immersion point thereof and obliquely against the movement direction of the steel product, by means of first nozzles, and a second flow of cooled molten zinc is directed at least essentially perpendicularly towards the steel product at a point after said obliquely directed flow, by means of second nozzles.
The flow of molten zinc directed towards the steel product is cooled e.g. by means of a heat exchanger cooler, preferably to a temperature 1.degree. to 15.degree. C. below the operating temperature of the zinc bath, the flow of zinc through the cooler to said nozzles being separated from the rest of the zinc bath.
The essential feature of locally cooling the zinc bath brings about the additional important advantage that the iron content of the zinc bath is lowered.
The iron content in a zinc bath, in a continuous hot-dip galvanizing process of a thin steel sheet is generally at saturation, according to the respective temperature. Even a small change in the temperature causes a precipitation of iron and zinc, i.e. either at the bottom of the bath or as a drift of precipitates onto the surface of the steel strip to be galvanized, which impairs the quality of the coating.
Thus, to maintain a good quality, variations in the temperature of the zinc bath should be avoided. Therefore, some galvanizing lines are provided with separate pots for preliminary melting of zinc so that e.g. the melting temperature of the zinc to be added would not change the temperature of the zinc bath.
The solubility of iron in molten zinc is generally a linear function of the temperature; at a normal galvanizing temperature of approximately 455.degree. C., the iron content is about 0.06%, and at a temperature of about 420.degree. C., the iron con
REFERENCES:
patent: 4752508 (1988-06-01), Sippola
Rasmet Ky
Silverberg Sam
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