Coating processes – Immersion or partial immersion – Molten metal or fused salt bath
Reexamination Certificate
2000-06-20
2001-06-05
Beck, Shrive (Department: 1762)
Coating processes
Immersion or partial immersion
Molten metal or fused salt bath
C427S349000, C427S383700, C427S431000, C427S434200, C427S434400, C427S436000, C242S615200
Reexamination Certificate
active
06242048
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a hot dip coated metal strip. More particularly, the present invention relates to a method of manufacturing a hot dip coated metal strip having a coating layer of a uniform thickness by reducing the vibration of the metal strip which is lifted from a hot dip coating bath and travels vertically at an approximately constant speed.
2. Description of the Related Art
In general, hot dip galvanizing is applied to the surfaces of a steel strip using a continuous hot dip galvanizing apparatus (also referred to as a line) as described below.
First, as shown in
FIG. 2
, a steel strip
1
as a material to be coated is introduced into a hot dip galvanizing bath
2
, the direction of travel of the steel strip
1
is diverted upward by a sink roll
3
disposed in the galvanizing bath
2
, the crossbow of the steel strip
1
is corrected by a pair of upper and lower support rolls
4
disposed in the galvanizing bath
2
so as to clamp both the surfaces of the steel strip
1
, and then the steel strip
1
is lifted vertically from the galvanizing bath
2
. During that time, molten zinc is deposited on the surfaces of the steel strip
1
. A gas
6
(referred to as a wiping gas) is blown onto the surfaces of the steel strip
1
, on which the molten zinc has been deposited and which travels upward, through nozzles
5
(referred to as wiping nozzles because they wipe off the coated metal) so that the amount of the molten metal deposited on the steel strip
1
is adjusted to a desired amount (so that the molten metal can be uniformly deposited on the entire surface of the steel strip
1
). A pair of touch rolls
7
, which clamp the surfaces of the steel strip
1
similarly to the support rolls
4
, are disposed above the wiping nozzles
5
to stabilize the travel of the steel strip
1
. The steel strip
1
, which has passed through the touch rolls
7
, may be subjected to an alloying treatment by travelling through an alloying furnace
8
disposed above the touch rolls
7
so that the coating layer thereof is alloyed when necessary.
By the way, recently, it has become very important to stably manufacture at high speed a hot dip galvanized steel strip which has a low coating weight (referred to as light coating). In accordance with the reduced coating weight, there has been required a technology for manufacturing a hot dip galvanized steel strip while preventing the vibration thereof due to an increase in the pressure of the wiping gas
6
, and the like. This is because the coating weight of the molten zinc deposited on the surfaces of the steel strip is greatly varied by an increase in the vibration of the steel strip and the quality of a product is thereby deteriorated.
Ordinarily, when the hot dip coated steel strip
1
, which has a particularly low coating weight (coating weight per one side is 45 g/m
2
or less), is manufactured at a high speed, the steel strip
1
is vibrated at the position where the wiping nozzles
5
are disposed in a direction vertical to the surfaces thereof in a total amplitude of vibration of 1-2 mm at all times.
Since wiping cannot be smoothly carried out when this vibration occurs, at present, the standard deviation of the variation of the coating weights on the surfaces of a steel strip &sgr; is set to a large value of 2-4 g/m
2
(&sgr;=2-4 g/m
2
) with respect to the coating weight per one side of 45 g/m
2
. However, since it is generally required by customers to guarantee the lower limit of the coating weight, when the guarantee for the lower limit is kept, molten zinc is excessively deposited. This means that a large amount of zinc is wastefully consumed from the view point of manufacturers.
When a hot dip galvannealed steel strip is manufactured, the large variation of the coating weight directly leads to the variation of the coating weight of hot dip galvannealing. Thus, when the steel strip
1
is manufactured, the coating is often undesirably exfoliated in a powder state (referred to as powdering) from a portion of the steel strip
1
where zinc is thickly deposited; moreover, a defect such as uneven alloying, and the like is liable to occur in the manufacture of the steel strip
1
.
Technologies for preventing the vibration have been vigorously developed and many of them have been published. For example, Japanese Unexamined Patent Application Publications Nos. 5-320847 and 5-078806 disclose technologies for disposing a static pressure pad to maintain the pressure of a gas which is blown to wiping nozzles at a constant pressure. Further, Japanese Unexamined Patent Application Publication No. 6-322503 discloses a technology for separately disposing nozzles for blowing a shield gas above wiping nozzles and disposing gas shield plates between the shield gas blowing nozzles and the wiping nozzles.
However, the technologies for preventing the vibration of a steel strip by means of the static pressure pad or by blowing another gas are not in practical use because high power must be specially provided to generate a desired pressure and flow rate of gas as well as the effect of the technologies is lowered when the steel strip has a relatively large thickness.
Further, Japanese Unexamined Patent Application Publications Nos. 52-113330, 6-179956 and 6-287736 disclose technologies for preventing the vibration of a steel strip using magnetic force or electromagnetic force. However, these technologies are not yet in practical use because not only do they separately require an expensive magnetic force generator and operation is made complex but also the effect of the technologies is lowered in a steel strip having a relatively large thickness.
SUMMARY OF THE INVENTION
In view of the above circumstances, an object of the present invention is to provide a method of manufacturing a hot dip coated metal strip which can provide the metal strip with stable quality by reducing the variation of the coating weight of molten metal to be deposited on the surfaces of the metal strip even if operating conditions of hot dip coating are changed as well as which can greatly lower a coating cost by preventing the excessive deposition of the molten metal.
To achieve the above object, the inventors examined the influences of tension of a traveling metal strip, target coating weight, linear speed of the metal strip, pressure of a wiping gas, distance between a touch roll disposed above wiping nozzles and a support roll disposed in a bath, and the like on the vibration of the metal strip at a gas wiping position in many test operations. Then, the inventors have completed the present invention based on a knowledge discovered from the analysis of data obtained in the examination that the vibration of a metal strip can be greatly reduced when operation is carried out by setting the distance between the touch roll and the support roll disposed in the bath within a certain range.
That is, according to the present invention, there is provided a method of manufacturing a hot dip coated metal strip which includes the steps of depositing molten metal on the surfaces of the metal strip by continuously dipping the metal strip in a hot dip coating bath, lifting the metal strip at a constant speed while supporting it with a pair of upper and lower support rolls for clamping the surfaces of the metal strip in the coating bath, adjusting the coating weights of the molten metal deposited on the surfaces of the metal strip by wiping the molten metal with gases from gas wiping nozzles disposed above the surface of the coating bath, and advancing the metal strip while supporting it with a pair of upper and lower touch rolls disposed outside the coating bath for clamping the surfaces thereof, wherein the metal strip is advanced by setting the distance L between the upper support roll disposed in the coating bath and the lower touch roll disposed outside the coating bath within the range determined by the following formula:
L≦
80
×T×W
2
/V
in which,
L: distance between the upper
Iida Sachihiro
Sugano Takahiro
Barr Michael
Beck Shrive
Kawasaki Steel Corporation
Young & Thompson
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