JP2003277908A - Continuous galvanizing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous galvanizing method with which the development of zinc powder defect can easily and effectively be prevented. <P>SOLUTION: In the continuous galvanizing method, with which the coated thickness is controlled by blowing wiping gas 5 to a band steel 1 pulled up from a galvanizing bath 2 through a wiping nozzle 3A, a position from the upper part of the nozzle in a wiping device 3 to near supporting rolls 4 above the bath at the upper part thereof, is covered with a hood 7 from the back face side of this device. The control of the coated thickness with the gas wiping is performed while blowing purge gas 9 into the hood 7 at a flow-rate W1 exceeding the blowing flow-rate W2 of the wiping gas 5 through e.g. a gas exhausting duct 8. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous hot-dip galvanizing method and, more particularly, to the prevention of the occurrence of a product surface defect due to the adhesion of zinc powder to a plating surface in a gas wiping process. The present invention relates to a continuous hot-dip galvanizing method. 2. Description of the Related Art Conventionally, when a steel strip is continuously hot-dip galvanized, the steel strip is once drawn into a bath (bath = hot-dip zinc bath), and then vertically drawn by a sink roll in the bath. Deflected and pulled out of the bath, the excess amount of molten zinc adhering to the steel strip surface and rising was wiped off by gas blowing from a pair of wiping nozzles arranged opposite each other above the bath surface, The plating thickness is controlled to a predetermined required value. However, at this time, the zinc that has been wiped from the surface of the steel strip scatters from the steel sheet surface and the edge of the steel strip, forming a splash (splash) of zinc powder on the gas flow and re-adhering to the steel strip surface after plating. However, there is a problem that a mottled pattern is formed on the plating surface by being pressed by a roll such as a support roll on the bath. As a solution to this problem, Japanese Patent Application Laid-Open No. 57-1982
In Japanese Patent Publication No. 53 (referred to as Publication A), the bath surface and the vicinity of the wiping device are surrounded by a seal box, and a gas for sealing the inside of the box with an inert gas is discharged from above the wiping nozzle into the box, or further,
A method is disclosed in which a shielding plate for preventing the sealing gas from directly hitting the steel strip is attached to the back surface of the wiping nozzle. In Japanese Unexamined Patent Publication No. 4-231148 (hereinafter referred to as "publication B"), the upper portion of the wiping nozzle is enclosed by a seal box, and gas in this box is sucked out from a suction pipe near immediately above the wiping nozzle, and the gas above the wiping nozzle is sucked. An apparatus is disclosed in which gas is press-fitted into the box from a pressurized tube. [0005] However, in the technique described in the above-mentioned publication A, the occurrence of product surface defects (zinc powder defects) due to the adhesion of zinc powder to the steel strip plating surface at the wiping point is sufficiently reduced. Had a problem that it could not be prevented.
Further, the technique described in the above publication B has a problem that it is difficult to balance the pressurization and the suction, and a problem that the zinc dust is easily clogged in the suction pipe and a maintenance load is large. An object of the present invention is to solve these problems and to provide a continuous hot-dip galvanizing method capable of easily and effectively preventing the occurrence of zinc powder defects. Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems, and as a result, have obtained the following knowledge on the mechanism of zinc powder defect generation. That is, at the wiping point, as shown in FIG. 3, the wiping gas 5 discharged from the nozzle (wiping nozzle) 3 </ b> A of the wiping device 3 hits the steel strip 1 and then moves in the same arrow direction (up, down, left and right directions). leak. With this outflow, surrounding gas flows into the wiping point. The main flow direction of the inflowing gas 6 from the surroundings is the direction indicated by the same reference numeral (the direction in which the gas advances straight toward the surface of the steel strip 1), but the flow velocity (wind force) is not constant, and the strength is irregular at times. It changes greatly. On the other hand, the zinc powder scattered upward due to the collision of the wiping gas drifts irregularly in the air, and mainly when the inflow gas 6 is in a weak wind,
It adheres and deposits on the wiping device 3. The technique described in the above publication A cannot prevent this adhesion and deposition. [0007] The zinc powder that is drifting after being scattered from the plating surface is in the form of a needle, and is a fine particle having a length of about 30 μm.
There is no actual harm even if this fine zinc adheres to the plating surface. However, when the zinc deposit 10 attached to and deposited on the wiping device 3 and grown to a certain size receives the strong wind of the inflow gas 6, the zinc deposit 10 having a particle size of about 2000 μm or more is received therefrom.
Are easily separated and scattered, and the scattered coarse-grained zinc 11 is re-adhered to the plated surface, causing zinc powder defects. The present invention has been completed based on such findings, as a result of studying means for preventing adhesion and deposition of zinc powder on a wiping device and separation of coarse zinc from the zinc powder. In a continuous hot-dip galvanizing method in which a wiping gas is sprayed from a wiping nozzle onto a steel strip pulled up from a hot-dip zinc bath to control plating thickness, a hood covers the upper portion of the nozzle of the wiping device and the vicinity of a support roll on the bath above the nozzle. A continuous hot-dip galvanizing method characterized in that a purge gas is blown into the hood at a flow rate exceeding a wiping gas blowing flow rate. In the present invention, for example, as shown in FIG. 1, a wiping nozzle 3 is attached to a steel strip 1 pulled up from a bath 2.
In the continuous hot-dip galvanizing method in which the wiping gas 5 is sprayed from A to control the plating thickness, from the upper part of the nozzle of the wiping device 3 to the vicinity of the support roll 4 above the bath, preferably from the back side of the wiping device, Covered with a hood 7, and in the hood 7, for example, via an air supply duct 8, at a flow rate exceeding the blowing flow rate of the wiping gas 5,
The plating thickness control by gas wiping is executed while blowing the purge gas 9. The blown purge gas 9 forms a steady mainstream in the direction indicated by the same arrow. It is preferable to use a non-oxidizing gas such as a nitrogen gas or an inert gas as the purge gas. By making the flow rate W1 of the purge gas 9 into the hood 7 larger than the flow rate W2 of the wiping gas (W1> W2), the flow rate from the inside of the hood 7 to the vicinity of the strip 1 pass line in front of the opening thereof is increased. The zone can be maintained at a positive pressure (higher than outside the zone), and the inflow of gas from outside into this zone is prevented. Similarly, the upward flow of the wiping gas 5 after hitting the strip 1 is also prevented. Since the upper portion of the nozzle 3A of the wiping device 3 is in the positive pressure region, the zinc powder scattered by the wiping is less likely to enter the positive pressure region, and is less likely to adhere and deposit on the wiping device 3. Become. Also, even if the zinc deposit 10 as shown in FIG. 3 grows to some extent due to some adjustment, a strong wind such as the inflow gas 6 in FIG. 3 blows into this positive pressure region from outside. Therefore, there is no possibility that the coarse zinc 11 is separated and scattered from the zinc deposit 10 and adheres to the plated surface of the steel strip 1. Thus, according to the present invention, it is possible to effectively prevent zinc powder defects from occurring in a plated steel sheet product. Further, since gas is not sucked out from the hood, there is no problem in terms of operation and maintenance as described in the above publication. [0013] In operation in a continuous hot-dip galvanizing line in which the plating thickness is controlled by gas wiping, a hood and an air duct are attached to the wiping equipment on the bath in the form shown in FIG. Gas wiping shall be performed while blowing nitrogen gas as a purge gas.
The wiping gas blowing flow rate W2 is approximately constant (unit width of steel strip 1).
about 4000 m ³ / h) and the purge gas injection flow rate W
1 was changed variously, and the number of generated zinc powder defects in the obtained plated product was investigated. As the flow rates W1 and W2, a loss due to a pipe resistance (design value) from the discharge flow rate of the air supply drive source (pump, blower, etc.) to the nozzle tip (or the air supply duct tip opening) is subtracted. Was used. The number of zinc powder defects was evaluated based on the number of defects per 100 m of steel strip when a steel strip having a width of 1200 mm was treated. FIG. 2 shows the results of the investigation. As shown in the figure,
In the case of operating under the condition of W1> W2, which corresponds to the present invention (Example), the zinc powder defect occurrence rate was remarkably reduced as compared with the case of deviating from the present invention (Comparative Example), and the effect of the present invention became apparent. . As described above, according to the present invention, it is possible to effectively prevent coarse zinc from adhering to the galvanized surface at the wiping point, and to significantly reduce the zinc powder defect occurrence rate of the continuous galvanized product. It works.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory view (a: side view, b: front view) showing an embodiment of the present invention. FIG. 2 is a graph showing a relationship between W1 / W2 and a zinc powder defect occurrence rate. FIG. 3 is an explanatory view (a: side view, b: front view) showing a mechanism of generating zinc powder defects. [Description of Signs] 1 Strip steel 2 Bath (molten zinc bath) 3 Wiping equipment 3A Nozzle (wiping nozzle) 4 Support roll 5 Wiping gas 6 Inflow gas 7 from surroundings Food 8 Air supply duct 9 Purge gas 10 Zinc deposit 11 Coarse Grain zinc

Claims (1)

Claims: 1. A continuous hot-dip galvanizing method in which a wiping gas is blown from a wiping nozzle onto a steel strip pulled up from a hot-dip galvanizing bath to control a plating thickness. A continuous hot-dip galvanizing method comprising covering a portion near a support roll with a hood, and blowing a purge gas into the hood at a flow rate exceeding a wiping gas blowing flow rate.