JPH03115554A - Method and device for gas wiping in continuous hot dip coating - Google Patents

Abstract

PURPOSE:To easily produce a beautifully plated steel sheet at a low cost with a uniform plating thickness by controlling the molten metal on a strip surface to a prescribed plating thickness by gas wiping, then immediately and rapidly cooling and solidifying the molten metal. CONSTITUTION:The strip 2 immersed in the molten metal is pulled up and the wiping jets of the gases from slit gaps 3-3 of a wiping nozzle 3 are blown thereto. The molten metal on the strip 2 surface is controlled by the above- mentioned wiping jets to the prescribed plating thickness. The nozzle body 3-1 in the above-mentioned gas wiping method for continuous hot dip coating is provided with through-holes 3-5 which communicate a cooling medium tank 3-2 contg. cooling water 8 and fine holes 3-6 provided in an upper lip 3-4 of a slit gap 3-3. The cooling water 8 is discharged from the fine holes 3-6 in this way, is atomized by the wiping jets and is blown to the strip 2. The molten metal 1 on the surface of the strip 2 is rapidly cooled and solidified right after the gas wiping, by which the generation of an edge overcoat, sag, etc., is prevented.

Description

[Detailed description of the invention] Industrial applications This invention uses continuous hot-dip galvanizing or continuous galvanizing.

The present invention relates to a gas wiping method and apparatus for producing a plated steel plate with a uniform plating thickness in the width direction of the strip and a beautiful surface in molten metal plating such as molten aluminum plating.

Conventional technology Continuous molten metal plating generally involves immersing a steel plate whose surface has been surface-activated using a flux method or hydrogen reduction method, etc., into molten metal, and then squeezing out the excess molten metal to coat the surface of the steel plate with iron. A metal film of a predetermined thickness is formed through the alloy layer.

As a method for wiping away excess molten metal, a roll wiping method was used in the past, in which a steel plate is sandwiched between a pair of grooved rolls placed directly above a molten metal plating bath to wipe away excess molten metal. However, this method has drawbacks such as the possibility of sheet breakage when the sheet threading speed becomes high, the tendency of surface defects to occur on the plated steel sheet, and the inability to achieve thin coatings.

On the other hand, the gas wiping method is superior to roll wiping in terms of uniformity of area weight, thinner area weight, high speed, controllability, etc., and is currently used in most continuous hot-dip plating equipment.

In recent years, with the increase in demand for hot-dip galvanized steel sheets as rust-proof steel sheets for automobiles and home appliances, there has been a demand for uniform and beautiful surface quality, especially the thickness of galvanization.

The surface quality of hot-dip galvanized steel sheets using the gas wiping method is
Although it is much improved compared to products made using the roll wiping method, it is significantly inferior to electroplated steel sheets in terms of surface beauty and uniformity of plating thickness.

For example, regarding the uniformity of the plating thickness in the width direction of the strip, in hot-dip plating using the gas wiping method, an edge overcoat occurs in which the plating thickness at the edge of the strip is thicker than that at the center of the strip. This edge overcoat is applied to the plating thickness at the center of the strip.
1 to 1.7 times.

Particularly in galvanealing (alloying hot-dip galvanizing), where heating and alloying treatment is performed immediately after plating, thick plating can cause untreated edges that delay alloying at the edges, and if the plating thickness is uneven, then This results in a decrease in surface quality.

Furthermore, even in the case of ordinary hot-dip plated products, it not only causes the load to collapse during coil winding, but also causes edge elongation during unwinding, resulting in poor flatness of the steel plate.

Conventionally, various preventive measures have been taken against this edge overcoat.

In the first example, as shown in Fig. 7, in the slit gap of the wiping nozzle, a slope (33) is applied toward both sides of the nozzle from a position slightly narrower than the width of the strip (2), and the wiping collides with the edge of the strip. This method increases the squeezing force of molten zinc at the edge by increasing the dynamic pressure of the jet.

The second example is an auxiliary nozzle system as typified by Japanese Patent Publication No. 55-41295 or Japanese Patent Application Laid-open No. 57-158363.

As shown in Fig. 8, this method is achieved by providing auxiliary nozzles (34) movable in the nozzle width direction on both sides of the edge of the strip (2) at the top of the wiping nozzle, thereby increasing the squeezing force at the edge of the strip. This is a way to strengthen the

The third example is the edge plate method. This method is
As shown in Figure 9, at the strip edge, a dummy plate (3
5) is a method of preventing edge overcoat.

However, these methods have the following problems.

First, in the first example, when the strip width changes, the nozzle slit gap corresponding to the strip edge differs, so the degree of edge overcoat differs, and in order to sufficiently prevent edge overcoat, it is necessary to replace the nozzle. The disadvantage is that productivity decreases. Furthermore, since the slit gap at the edge portion is wide and the amount of gas ejected is large, splash and noise are significantly generated. Especially a lot of splashes.

There is a problem in that the air contaminates the nozzle and the strip, leading to decreased productivity and quality due to nozzle cleaning.

In the second example, only the auxiliary nozzle needs to be moved each time the strip width changes, and although the problem of decreased productivity due to nozzle replacement as in the first example can be avoided, it is difficult to avoid the generation of splash and noise. In this case, as in the first example, frequent cleaning of the nozzle is inevitable.

In the third example, if the gap between the edge plate and the strip becomes too large, the effect of preventing edge overcoat will be reduced, so the edge plate must be kept at a constant interval to follow the vibration of the strip due to passing. This follow-up mechanism has the disadvantage that high responsiveness is required for high-speed sheet threading, leading to a rise in equipment costs.

On the other hand, regarding the surface quality, a "dripping" pattern of the plating solution occurs, especially during thick plating. This "sag" is a phenomenon in which the wiped molten metal in an unsolidified state sags due to gravity, and it not only looks bad but also causes uneven plating thickness, and countermeasures are required to prevent it.

Based on the experience of on-site workers, drooping can be prevented by lowering the bath temperature or material temperature and increasing the viscosity of the plating solution.

However, a decrease in the bath temperature or material temperature causes a decrease in the adhesion of the plating or the mechanical properties of the plated steel sheet, leaving problems in terms of product quality.

Problems to be Solved by the Invention In view of the above-mentioned circumstances, this invention investigates the causes of edge overcoat and "sag", and after understanding the mechanism of occurrence, prevents them and improves plating thickness. The purpose of this invention is to propose a gas wiping method and apparatus that make it possible to manufacture plated steel sheets with uniform and beautiful coatings.

Means for Solving the Problems The inventor conducted various experimental studies regarding the flow of a liquid film on a strip surface, and as a result, obtained the following knowledge regarding the occurrence of edge overcoat.

That is, as shown in FIG. 4, (1) Surface tension acts on the adhered liquid film that has been reduced to a predetermined thickness by gas wiping but is still unsolidified.

■ Due to the surface tension of the adhered liquid film, the liquid adhering to the strip edge wraps around to the strip surface side.

■ Due to the liquid that has flowed around to the strip surface side, the thickness of the deposited film at the edges is thicker than that at the center of the strip, and the velocity distribution within the liquid film is also different. That is, the flow velocity VL on the surface of the liquid film, which increases along with the strip, is slower at the edge portions where the film is thicker than at the center portion where the film is thinner.

■ Because the surface flow velocity of the liquid film is different, the frictional stress caused by the difference in speed between the surface flow and the wiping jet impinging on the strip is different between the center and edge of the strip. That is,
The frictional stress acting upward on the liquid film surface is larger at the edge of the strip than at the center of the strip.

■ Due to the difference in frictional stress between the center and edge of the strip, a component of frictional stress is generated in the width direction of the strip, and this component of frictional stress and surface tension are balanced, and the edge overcoat is generated at a fixed position from the edge of the strip. do.

As explained above, it has been found that the edge overcoat develops after wiping due to the surface tension of the liquid adhering to the strip edge.

FIG. 5 shows the results of measuring the occurrence of edge overcoat in a cold model test using an aqueous glycerin solution.

Immediately after wiping, i.e. 15 minutes from the wiping nozzle position
At 215 mm above the wiping position, the edge overcoat is not generated, but at 215 mm above the wiping position, the edge overcoat is 1.4 times that at the center of the strip, indicating that the edge overcoat is caused by the surface tension of the liquid. This supports the above-mentioned mechanism of development that occurs after being wiped.

In addition, "sagging" occurs as the thickness of the plating increases, the force of gravity increases compared to the adhesion force to the strip due to the viscosity of the molten metal, and the surface flow velocity of the liquid film adhering to the strip becomes smaller than the rising speed of the strip. It is something.

Figure 6 shows an example of the relationship between the height of the ``drip'' generation position from the wiping nozzle and the plating thickness, and it can be seen that the thicker the plating, the closer the ``sag'' generation position is to the nozzle. I understand.

In addition, the 6th w! i is a nozzle with a slit gap of 0.75 mm, which blows room temperature air at a rate of 0.1 to 1 kg/cm.
It was injected at a pressure of

Based on these experimental results, edge overcoat and “
It was found that both of the sag did not occur immediately after the wiping position, but grew after wiping. Therefore, the edge overcoat and the sag did not start to grow immediately after wiping, but the molten metal film on the strip surface If it is cooled and solidified, it will form an edge overcoat and “
This will prevent "sagging".

This invention was discovered based on the above knowledge, and uses a wiping jet to gas wipe the molten metal on the strip surface, which is controlled to a predetermined plating thickness.

The gist of this invention is a gas wiping method characterized by rapid cooling and solidification immediately after.

Further, as a means for carrying out the method of the present invention, a slit or a pore is provided along the slit gap in the upper lip of the slit gap for ejecting the wiping jet, which communicates with the cooling medium tank attached to the nozzle body. A method in which the molten metal on the surface of the strip is rapidly solidified by jetting out the cooling medium from slits or pores, or a thin tube that communicates with the cooling medium tank attached to the nozzle body is installed on the outer surface of the upper lip of the wiping nozzle along the slit gap. A device is used in which piping is provided, and each piping is provided with a cooling medium flow control valve for adjusting the cooling capacity in the strip width direction.

Immediately after wiping, a cooling medium is sprayed onto the molten metal on the strip surface, which is controlled to a predetermined plating thickness using a gas wiping jet, so that the molten metal on the strip surface is rapidly cooled and solidified, forming an edge. Overcoat and "sag" growth is inhibited.

Here, the cooling medium used for rapidly cooling the molten metal is:
Water is generally used, but if a few percent aqueous solution of ammonium phosphate or sodium phosphate is used, a beautiful plated steel sheet can be obtained without damaging the surface gloss.

For example, the cooling medium tank is installed at the top of the nozzle body,
The cooling medium can be provided so as to flow out along the upper surface of the nozzle body toward the nozzle slit gap side.

When the wiping gas is ejected from the slit gap, due to the ejector effect of the wiping gas jet, the cooling medium in the tank flows out onto the gas jet ejected from the slit gap, and at the same time is atomized by the wiping gas jet. It collides with the strip, causing the molten metal immediately after wiping to cool and solidify.

The cooling rate of the molten metal varies depending on the operating conditions, but in consideration of the glossiness and beautification of the plating surface, the position where the plating is applied is preferably 1000111 or less than the wiping position.

EXAMPLES Hereinafter, examples of the present invention will be described with reference to the drawings.

FIG. 1 shows an example of a wiping device in a continuous hot-dip plating line for carrying out the method of this invention.
1) is the molten metal, (2) is the strip, and (3) is the wiping nozzle.

That is, in the wiping device according to the present invention, a cooling medium tank (3-2) having the same length as the nozzle body is attached to the upper part of the wiping nozzle body (3-1), and a wiping gas ejection slit gap (3-1) is attached to the top of the wiping nozzle body (3-1). 3), the upper lip (3-4>) is provided with a small hole (3-6) that communicates with the tank through the through hole (3-5).

The pores (3-6) are provided at appropriate intervals just above the slit gap (3-3). Further, each through hole (3-5) may be provided in the nozzle body (3-1) itself, or may be provided by embedding a thin tube within the nozzle body.

Note that the cooling medium is supplied to the cooling medium tank (3-2) by an appropriate means.

In the wiping device with the above configuration, when water is used as the cooling medium, when the wiping gas is ejected from the slit gap, due to the ejector effect of the gas jet,
Cooling water (8) in the tank (3-2) passes through the through hole (3-5) and flows out from the pore (3-6).

Figure 3 schematically shows this situation, where the cooling water that flows out is atomized by the wiping jet (5).
Collisions with strip (2). At this time, atomized cooling water (8
-1) is sprayed only onto the upper part of the strip which has been reduced to a certain weight by wiping, and the adhered molten metal (1-1) is cooled and solidified.

Therefore, the cooling liquid does not flow below the nozzle where wiping is performed, and there is no reduction in squeezing force due to increased viscosity due to temperature drop, and the molten metal (14
) can be rapidly solidified.

FIG. 2 shows another embodiment of the present invention, illustrating a wiping device equipped with a coolant flow rate adjustment function.

That is, as a cooling liquid supply means, thin tubes (3-7) are arranged on the outer surface of the wiping nozzle body (3-1), and a cooling liquid flow rate control valve (9) is installed in each thin tube.

The cooling liquid amount adjustment valve (9) is provided to prevent uneven cooling in the width direction of the strip, and is effective in adjusting the amount of cooling liquid from each thin tube to obtain a product that is uniform in the width direction. .

Note that in FIGS. 1 and 2, the cooling medium tank (
3-2> is provided at the upper part of the wiping nozzle body (3-1), but the tank position may be installed at a position lower than the upper lip (3-4) since the ejector effect of the wiping jet can be utilized. It is also possible to use a pressurized tank and force injection.

As described in detail, the present invention provides the following effects.

Wiping according to the method of claim 1 in which edge overcoat and "sag" do not begin to grow.

Immediately afterwards, the molten metal film on the strip surface can be cooled and solidified, which prevents edge overcoat and "sag", resulting in a hot-dip plated steel sheet with uniform plating thickness in the width direction of the strip and a beautiful surface. It becomes possible to manufacture

According to the apparatus according to the second aspect, the cooling medium can be injected in a sliding manner onto the wiping gas jet jetted out from the upper lip of the slit gap of the wiping nozzle, so that the cooling medium has a constant basis weight by wiping. Since the molten metal is injected only onto the upper part of the narrowed strip, it is possible to cool and solidify the adhered molten metal directly above the wiping position without reducing the squeezing force due to the wiping gas.

According to the apparatus according to claim 3, in addition to the effects of the apparatus according to claim 2, the flow rate of the cooling medium injected from the upper lip of the slit gap can be adjusted, so that uneven cooling in the strip width direction can be reduced. This makes it possible to make the plating thickness more uniform in the width direction of the strip, making it possible to manufacture high-quality hot-dip plated steel sheets.

In addition to these effects, this invention can be easily applied without requiring major modifications to existing wiping nozzles, and is relatively simple in structure, so it does not require high costs. It is extremely useful.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing an apparatus according to an embodiment of the present invention, Fig. 2 is a perspective view showing an apparatus according to another embodiment of the invention, and Fig. 3 is a schematic diagram showing the principle of cooling and solidifying deposited molten metal in this invention. Figure 4 is an explanatory diagram of the occurrence of edge overcoat, Figures 5 and 6 are diagrams showing measurement examples of edge overcoat and "sag" occurrence positions, respectively, and Figures 7 to 9 are respectively FIG. 2 is an explanatory diagram showing a conventional edge overcoat prevention measure. 1... Molten metal 1-1... Adhering molten metal 2... Strip 3... Wiping nozzle 3-1... Nozzle body 3-2... Cooling medium tank 3-3... Slit gap 3-4... Upper lip 3-5... Through hole 3-6
... Pore 3-7 ... Thin tube 9 ... Coolant flow rate control valve

Claims (1)

[Claims] 1. A gas wiping method for continuous hot-dip plating, characterized in that the molten metal on the strip surface, which is controlled to a predetermined plating thickness using a wiping jet, is rapidly cooled and solidified immediately after being gas-wiped. Gas wiping method in plating. 2. A gas wiping device for continuous hot-dip plating, characterized in that the upper lip of the slit gap for ejecting a wiping jet is provided with a slit or pore that communicates with a cooling medium tank attached to the nozzle body along the slit gap. Gas wiping equipment for continuous hot-dip plating. 3. In a gas wiping device for continuous hot-dip plating, a thin tube communicating with the cooling medium tank attached to the nozzle body is installed on the outer surface of the upper lip of the wiping jet ejection slit gap along the slit gap, and each thin tube has a strip width. 1. A gas wiping device for continuous hot-dip plating, characterized in that it is equipped with a cooling medium flow rate control valve for adjusting cooling capacity in one direction.