Coating processes – Immersion or partial immersion – Molten metal or fused salt bath
Reexamination Certificate
2000-09-28
2002-07-30
Barr, Michael (Department: 1762)
Coating processes
Immersion or partial immersion
Molten metal or fused salt bath
C427S431000
Reexamination Certificate
active
06426122
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for hot-dip galvanizing.
BACKGROUND OF THE INVENTION
Generation of surface defects on hot dip galvanized steel strip caused by dross is one of the most serious problems on the hot-dip galvanized steel strip. Dross is an intermetallic compound such as FeZn
7
generated from the reaction between iron and zinc which are eluted from the steel strip in a plating tank holding a zinc-base molten metal, and the dross has spherical equivalent diameters of from 5 to 300 microns. In a stagnant state of the molten metal in the plating tank, the dross deposits on the bottom of the plating tank.
However, with a natural convection of the molten metal, generated from the traveling steel strip, from the rotation of immersed rolls in the tank, or from the dissolved zinc-base ingot that supplies the consumed metal brought out along with the steel strip, the molten metal in the plating tank is agitated. As a result, the dross having less difference in specific gravity from the molten metal cannot deposit on the bottom of tank, or the once-deposited dross is stirred up to adhere the plated steel strip, thus causing the surface defects of the hot-dip galvanized steel strip.
To remove the dross, many proposals have been made. They include a method to sediment the dross by discharging the hot dip zinc bath to outside the plating tank, and a method to filter the hot dip zinc bath.
Nevertheless, those conventionally proposed methods are not brought into practical use. The reason is that these proposed technologies fail in practical application because of many problems in the complex mechanism, the durability, and the operability of commercial facilities, though they are reasonable in theory.
Regarding the methods for sedimentation separation of the dross, which have been proposed, the design emphasizes not to solidify the molten zinc during the transfer to outside the tank, and the design should take into account of the leak accident of molten zinc from the transfer piping. Consequently, the facilities increase the investment cost, which makes the facilities unrealistic ones.
As for the method to filter the dross, there appears a significant difference in the size of intermetallic compounds which can be filtered between the initial period of filtration and the point of degraded filtering performance after clogging the filter unit. As a result, the intermetallic compounds that cause the quality degradation cannot be efficiently and stably removed. Furthermore, on replacing a filter of the filter unit, dismounting and mounting the filter need an additional device that functions in the molten zinc bath, which also needs extra cost as in the case of molten zinc transfer. This also makes the facilities unrealistic ones.
In recent years, there have been proposed methods of direct removal of bottom dross immediately after the generation thereof, which methods stand on different point of view from the conventional methods. Typical examples of the methods are disclosed in JP-A-4-154948, (the term “JP-A” referred herein signifies “the Japanese Patent Laid-Open No.”), (hereinafter referred to as the “Prior Art 1”), JP-A-8-3707, (hereinafter referred to as the “Prior Art 2”), and JP-A-7-268587, (hereinafter referred to as the “Prior Art 3”).
The Prior Art 1 discloses a method to remove dross in a sedimentation tank installed separately from the plating tank. The characteristics of the method are that the plating tank is designed to decrease the distance between the steel strip and the tank bottom to prevent the sedimentation of the dross, that the transfer of the molten zinc from the plating tank to the sedimentation tank is conducted through a shallow flow passage to let the top dross of the plating tank flow into the sedimentation tank, and that the transfer of the molten zinc from the sedimentation tank to the plating tank is done by a pump.
The Prior Art 2 is characterized in that a flow passage to circulate the molten metal is established by placing a separation plate near an inner wall of the plating tank, that a circulation unit is mounted in the above-described flow passage to circulate the molten metal, that a heating device is mounted at inlet of the above-described flow passage to heat the molten metal to increase the size of dross to enhance the sedimentation of the dross, and that a dross recovery unit is located adjacent to the exit of the above-described flow passage to recover the sedimented dross.
The Prior Art 3 is characterized in that a plating tank having a circular bottom to plating the metal strip and a sedimentation tank to sediment and deposit the bottom dross generated in the plating bath are installed, that a connection hole is opened at near a side wall of the plating tank to let the molten metal for plating freely enter and leave between the plating bath tank and the sedimentation tank, thus the molten metal containing the dross is discharged to the sedimentation tank using the flow accompanied with the traveling steel strip to separate and sediment the bottom dross in the sedimentation tank where the flow rate is slow, and to recycle the molten metal after removed the dross to the plating tank.
According to the Prior Art 1, the suction opening for the molten zinc in the sedimentation tank has to be located at significantly below the bath level, so that the molten zinc containing sedimenting dross is sucked to the opening and transferred to the plating tank. In addition, since the transfer of the molten zinc from the sedimentation tank to the plating tank is conducted by a pump, a large amount of dross is generated in the plating tank which has a discharge opening. That is, the effect of sedimenting and removing the dross is not sufficient, and an additional problem of generation of top dross occurs.
Since the capacity of the sedimentation tank increases and since the problem of solidification and leak of molten zinc during the transfer of molten zinc between the plating and the distant sedimentation tank has not been solved, a problem of increasing investment cost and operation cost arises.
According to the Prior Art 2, the capacity of flow passage should be small as seen in an embodiment described later, so that the effect of sedimenting and removing the large amount of dross generated in the plating tank is not sufficient. Furthermore, the dross sediments and deposits in the flow passage to reduce the capacity of the flow passage, which increases the flow speed of the molten zinc. As a result, necessary sedimenting time cannot be secured, thus degrading the removal efficiency of the dross. In addition, the dross deposited in the narrow flow passage is not easily removed.
According to the Prior Art 3, since the molten zinc is discharged from the plating tank to the sedimentation tank using the flow accompanied with the traveling steel strip, the discharge flow rate cannot be controlled. Therefore, the dross in the plating tank cannot be fully discharged to the sedimentation tank, which raises a problem of accumulation and growth of the dross in the plating tank.
The Prior Art 1 and the Prior Art 3 consider only the flow of molten zinc bath in the cross sectional plane to the direction of traveling steel strip in the plating tank. FIG.
5
and
FIG. 6
show schematic drawings of the distribution status of the dross deposited in the plating tank, which are derived from a water model and commercial plant data by the inventors of the present invention.
FIG. 5
is a drawing viewed from cross sectional plane to the direction of traveling steel strip in the plating facility.
FIG. 6
is a drawing viewed in A—A cross section of FIG.
5
. In both drawings, the reference number
2
is a sink roll, and the reference number
8
is the dross.
As seen in
FIGS. 5 and 6
, the dross
8
deposits at the edge portion of the axial direction of the sink roll
2
and at the front and rear sides of the rotational direction thereof. That is, the flow pattern of the molten zinc between the sink roll and the inner wall surfaces of the plating
Akashi Kentaro
Araki Kenji
Hadada Kozo
Hatakeyama Seishi
Ishida Nobuyuki
Barr Michael
Frishauf, Holtz Goodman, Langer & Chick, P.C.
NKK Corporation
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