JPH0941112A - Method for suppressing formation of dross in galvanizing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the formation of dross so as to improve product quality and to stabilize plating operation by supplying molten zinc adjusted in temp. and concn. of Al to the bottom of a plating bath in which a galvanizing bath added with Al is housed. SOLUTION: A steel strip 9 is continuously introduced via a sink roll 8 into the galvanizing bath added with the Al housed in the plating bath 1 and is galvanized. A molten zinc introducing wall 4 is arranged at the bottom of the plating bath 1 to form an introducing path between the side wall 2 of the plating bath and the bath bottom 5. Next, the molten zinc adjusted the temp. and Al concn. equal to or higher than those of the molten zinc existing in the upper part in the plating bath, for example, temp. higher by 0 to 20 deg.C and the Al concn. higher by 0 to 0.020wt.%, is supplied via an auxiliary bath 3 for sucking and discharging the plating bath from the aperture 6 of the bottom cover 7 through the introducing path described above. As a result, the formation of the dross and more particularly the bottom dross is effectively suppressed.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for effectively suppressing the formation of dross, particularly bottom dross, in hot dip galvanizing of steel strip.

[0002]

2. Description of the Related Art In general, hot-dip galvanizing of steel strip is performed by continuously annealing a steel strip whose surface has been cleaned and whose material has been improved.
It is manufactured by introducing it into a hot dip galvanizing bath containing Al, plating, and then withdrawing and adjusting the coating amount.
In such a manufacturing process, in particular, the surface appearance of the plated steel strip is deteriorated, and a problem that causes a hindrance factor of the process is the generation of dross in the hot dip galvanizing bath,
Especially, it is called "bottom dross" that is generated and deposited at the bottom of the plating bath.

This bottom dross is a Fe-Zn alloy in which Fe atoms supplied from an apparatus in a plating bath such as a sink roll made of a steel strip to be plated and an iron-based material react with Zn in molten zinc. Of the molten zinc in the normal operation, for example at 470 ℃, becomes crystalline,
Since its specific gravity is larger than that of molten zinc, it is considered to have been deposited at the bottom of the plating bath. A part of the dross thus generated floats on the flow of molten zinc in the plating bath and adheres to the steel strip to be plated together with the molten zinc, impairing the surface quality and becoming a serious product defect. Sometimes. Further, when a large amount of bottom dross accumulates on the bottom of the plating bath, it may reach the height position of the sink roll in the plating bath and hinder the rotation of the roll. Therefore, it is essential to suppress the generation of bottom dross or remove the generated bottom dross to the outside of the plating bath in order to maintain and improve the quality of the product and to stably manufacture the product.

By the way, in the case of producing a hot-dip galvanized steel strip, generally, Al is added to the hot-dip zinc in the plating bath,
The dissolved Al concentration in the plating bath (hereinafter simply referred to as "Al concentration") is kept at 0.12 wt% or more. The purpose is
While suppressing the formation reaction of the Fe-Zn alloy that occurs at the interface between the molten zinc and the steel strip, it accelerates the formation reaction of the Fe-Al alloy (generally called the Al-rich layer), and the steel strip and the plated layer Improves the adhesion to the steel strip, and top dross that is harmful when attached to steel strip (Fe-Al alloy crystals are the main component, and because the specific gravity is relatively small, it floats on the plating bath surface, metal zinc, zinc oxide, etc. And dross that floats on the surface of the plating bath) are formed. However, if this top dross is formed, the formation of bottom dross tends to be suppressed. It is to suppress the generation. However, in actual operation, even if the Al concentration was 0.12 wt% or more, bottom dross still remained, causing the above-mentioned problems.

Therefore, some studies have been conducted so far on the method of removing the generated bottom dross. For example, the method disclosed in Japanese Patent Laid-Open No. 1-147074 guides the generated bottom dross to a sub pot separately provided, and dissolves the Al-containing zinc ingot in the sub pot to increase the Al concentration, thereby reducing the bottom dross. After reacting with Al to reduce the specific gravity and floating and separating the dross, it is returned to the plating bath. However, this method requires a large amount of capital investment, and the investment has not achieved a sufficient effect, in particular, elimination of product defects due to bottom dross. Further, if the generation of bottom dross itself is suppressed, it is most desirable because it does not require the removal of the generated dross, but there are few reports on this suppression technique, and it is only slightly disclosed in JP-A-53-88633. is there. In this technique, an auxiliary pot that is in circulation communication with the plating bath is provided, and when the molten zinc flows into the auxiliary pot, it is cooled so that the iron-zinc crystals are allowed to sink and heated when flowing into the plating bath from the auxiliary pot. is there.

[0006]

However, in this method, the molten zinc is once cooled and then heated again, so that the thermal energy loss is large, and in order to reduce the bottom dross that has already formed and accumulated at the bottom of the plating bath. There were problems such as almost no effect and zinc loss occurring at the same time as bottom dross formation. An object of the present invention is to solve the problems of the above-mentioned conventional techniques in hot-dip galvanizing of steel strips, and to propose a method for effectively suppressing the formation of bottom dross.

[0007]

[Means for Solving the Problems] In order to achieve the above object, as a result of intensive research on a mechanism for suppressing bottom dross and a method for suppressing the generation, the present invention has been completed. The summary structure is as follows.

(1) In a hot dip galvanizing bath containing Al,
When introducing steel strip continuously for hot dip galvanizing,
At the bottom of the galvanizing bath containing the hot dip galvanizing bath, equal to or more than the hot dip zinc in / up the galvanizing bath,
A method for suppressing dross generation in hot dip galvanizing, which comprises supplying hot dip zinc adjusted to temperature and Al concentration.

(2) The temperature is 0 to 20 ° C. higher than that of molten zinc in the plating bath and in the upper portion, and 0 to 0.020 wt%
The method for suppressing dross production according to (1) above, in which molten zinc adjusted to a high Al concentration is supplied.

(3) On the side surface of the plating bath, an auxiliary bath which communicates with the plating bath at the upper and lower sides is provided, and the molten zinc in the plating bath is taken into the auxiliary bath through the upper communication port, and the temperature of the taken molten zinc is taken. And the Al concentration is adjusted in an auxiliary bath, and the adjusted molten zinc is supplied to the bottom of the plating bath through the lower communication port, and the method for suppressing dross production according to (1) or (2) above.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The inventors of the present invention first of all, in the actual operation,
The reason why bottom dross is generated even when the Al concentration is 0.12 wt% or more was investigated and examined. The outline of the result is as follows.

The temperature of the molten zinc at the bottom of the plating bath is lower by about 10 to 20 ° C. than that at the top / bottom of the bath, and the temperature of the bath wall surface at the bottom is even lower. Therefore, the solubilities of Fe and Al in the molten zinc at the bottom are lowered, and Fe-Al alloy (main component of top dross) is deposited. Therefore, it is considered that the Al concentration gradually decreases, and finally Fe-Zn alloy, that is, bottom dross, is formed. On the other hand, the reason why the bottom of the plating bath becomes relatively low is because the heating of the plating bath is generally performed by a heating device provided on the side wall, for example, an induction heating device, and the bottom is hardly heated. It is considered that the bottom of the bathtub becomes relatively cold. Also, in the bottom corner, there is almost no flow of molten zinc,
Since hot molten zinc rarely reaches the corners,
It is thought that the temperature becomes even lower.

Therefore, in order to cope with these phenomena described above, the present invention has a higher temperature and a higher temperature than the inside / top of the plating bath.
By continuously supplying molten zinc having an Al concentration to the bottom of the plating bath, the temperature and Al concentration of the molten zinc at the bottom of the plating bath are made equal to or higher than those of the molten zinc at the top and bottom of the plating bath. It was completed based on the finding that it is possible to suppress the generation of bottom dross by taking the following measures.

In the present invention, specifically, the production of bottom dross may be suppressed by the following method. The temperature of molten zinc in the plating bath depends on the Fe-Al
In order to suppress the precipitation of alloys and Fe-Zn alloys (caused by the decrease in the solubility of Fe and Al in molten zinc), 460
Within the range of ~ 480 ° C, control as constant a temperature range as possible, for example, 470 ± 5 ° C. Therefore, as mentioned above, the temperature of molten zinc at the bottom of the plating bath is about 10 to 20 ° C lower than that of the middle and upper parts, so if this temperature is made equal to or higher than that of the middle and upper parts, the formation of bottom dross will occur. Can be suppressed. Therefore, in the present invention, it is necessary to make the temperature of the molten zinc at the bottom of the plating bath equal to or higher than that of the molten zinc at the middle and upper portions. As a means for this, it is extremely effective to supply molten zinc satisfying the above conditions to the bottom of the plating bath. However, if the temperature of the molten zinc for this supply is too high, the temperature of the entire plating bath becomes high, and a harmful Fe-Zn alloy layer is generated at the interface between the plating layer and the steel strip, so it is supplied to the bottom of the plating bath. The temperature of the molten zinc is preferably in the range of 0 to 20 ° C. higher than the temperatures of the molten zinc in the middle and upper portions. In actual operation, it is desirable to keep the temperature of the entire plating bath constant, so the temperature should be set in consideration of the heat balance.

The same applies to the Al concentration of molten zinc, and if the Al concentration at the bottom of the plating bath is equal to or higher than that in the top and bottom of the plating bath, the formation of bottom dross can be suppressed. Here, the Al concentration in the molten zinc supplied to the bottom of the plating bath is 0.12 wt% or more, and in particular, 0.13 to 0 when assuming a local temperature drop in the plating bath.
16wt% is desirable. In other words, it is preferable that the Al concentration in the molten zinc supplied to the bottom of the plating bath is in the range of 0 to 0.020 wt% higher than that in the top and bottom of the plating bath. The effect varies depending on the amount of molten zinc supplied, and the Al concentration in the entire plating bath needs to be kept constant. Therefore, the Al concentration should be set in consideration of the balance of Al. In this way, by supplying high temperature, molten zinc with a high Al concentration to the bottom of the plating bath, it is possible to make the temperature and Al concentration at the bottom equal to or higher than that of the inside / top of the plating bath. Needless to say, continuous supply is essential to maintain the state.

The adjustment of the temperature and the Al concentration of the molten zinc to be supplied to the bottom of the plating bath is conducted by, for example, the equipment shown in FIG. An auxiliary bath is provided, and the molten zinc in the plating bath is taken into the auxiliary bath from the upper communication port.
The temperature and Al concentration of the taken-in molten zinc may be adjusted in an auxiliary bath (including a communication tube), and the adjusted molten zinc may be circulated and supplied to the bottom of the plating bath through the lower communication port.
To adjust the temperature and Al concentration, an induction heating device is installed in the auxiliary bath to control the temperature, a predetermined amount of zinc ingot with a high Al concentration is charged, and the Al concentration is adjusted according to the charged amount. I'll do it.

FIG. 1 shows an auxiliary bath 3 (induction heating and Al addition is possible) 3 provided with the molten zinc in the plating bath 1 in communication with both side walls 2 at upper and lower portions thereof from an inlet b. After taking in molten zinc and adjusting the temperature and Al concentration of this molten zinc to desired values, the molten zinc was taken out from the outlet a, and passed through between the molten zinc introduction wall 4 and the plating bath side wall along the side wall to the bottom of the plating bath. The molten zinc having a high temperature and a high Al concentration can be supplied from the opening of the bottom cover (guide) 7. Here, it is preferable that the bottom cover 7 surrounds almost the entire bottom of the plating bath, and the opening provided in the bottom cover is a corner of the bath considering that the temperature and Al concentration of the entire plating bath are made uniform. It is good to install it in the section.

[0018]

EXAMPLES The present invention will be described below based on examples. Inventive Example 1 In a general Zenzimer method and hot-dip galvanized steel strip production line, the hot-dip galvanizing bath in the line was modified as shown in FIG. The temperature of molten zinc discharged from outlet a is 485 ± 5 ℃, Al concentration is 0.135 ±
It was 0.005 wt%. The temperature and Al concentration of molten zinc in and above the plating bath were 470 ± 5 ° C and 0.135 ± 0, respectively.
It was 005 wt%. This state was continuously continued for 7 days to produce a hot-dip galvanized steel strip. Comparative Example 1 (conventional method) With the molten zinc introducing wall and the bottom cover completely removed,
The temperature and the Al concentration of the molten zinc at the outlet a were set to the same conditions as in Invention Example 1, and the temperature in the plating bath / upper part and the Al
With the same concentration, the hot-dip galvanized steel strip was manufactured by this method continuously for 7 days. Inventive Example 2 Using the equipment shown in FIG. 1 in which the bottom cover is provided on the bottom of the plating bath as in Inventive Example 1, a temperature of 480 ± 5 ° C. and an Al concentration of 0.145 ± in an auxiliary bath provided adjacent to the plating bath. 0.005 wt
The hot-dip galvanized steel strip was continuously produced for 7 days by the method of supplying the hot-dip zinc adjusted to 10% to the bottom of the plating bath. In addition, the temperature of the molten zinc in and above the plating bath and
The Al concentrations were 470 ± 5 ° C and 0.135 ± 0.005 wt%, respectively. Comparative Example 2 As in the case of Invention Example 2, the temperature and Al concentration of molten zinc at the outlet a were 480 ± 5 ° C., respectively, using the auxiliary bath and completely removing the molten zinc introduction wall and the bottom cover. 0.135
Adjusted to ± 0.005 wt%. In addition, the temperature and Al concentration in the plating bath / upper bottom were set to the same conditions. using this method,
Hot-dip galvanized steel strip was manufactured for 7 consecutive days.

The amount of bottom dross deposited on the bottom of the plating bath was measured and compared after each of the above-mentioned methods after continuous operation for 7 days. In order to strictly compare the production amount of bottom dross, the initial amount of bottom dross was quantified in advance before starting the operation by each method. As the quantitative method, 20 fixed points were provided and the deposition heights at 20 points were measured and the average value was used. Before each method
The average of 20 points was 2.0 cm. The number of dross attached to the steel strip was also measured. The results are shown in Table 1 and Table 2, respectively.

[0020]

[Table 1]

[0021]

[Table 2]

From Table 1, according to the method of the present invention, the average deposition height before the start of bottom dross was 2.0 cm, but it became 0 cm. On the other hand, in Comparative Example 1, the average deposition height before the start was increased by 2.5 cm to 4.5 cm.
From this, it is understood that the method of the present invention can extremely effectively suppress the production of bottom dross, and further can reduce the accumulated bottom dross as in the case of the present invention. Further, from Table 2, according to the method of the present invention, the number of bottom dross attached to the steel strip was smaller than that in the conventional method, and the effect became remarkable two days after the start. Therefore, it is also apparent that the method of the present invention can stably produce a beautiful hot-dip galvanized steel strip.

[0023]

As described above, according to the method of the present invention,
Bottom dross, which could not be suppressed by the conventional technology, was generated at the bottom of the plating bath at high temperature and high Al concentration (Al0.12 wt.
% Or more) by continuously supplying molten zinc,
Since the temperature at the bottom of the plating bath and the Al concentration are kept equal to or higher than those in the top and bottom of the plating bath, it is possible to effectively suppress the formation of bottom dross. Further, since the generation of bottom dross can be suppressed, the number of dross attached to the steel strip can be reduced, and the steel strip having a beautiful plated surface with less dross attachment can be stably manufactured.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of an apparatus for supplying molten zinc whose temperature and Al concentration are adjusted to the bottom of a hot dip galvanizing bath used in the method of the present invention.

[Explanation of symbols] 1 plating bath 2 side wall of plating bath 3 auxiliary bath 4 molten zinc introducing wall 5 plating bath bottom 6 bottom cover opening 7 bottom cover 8 sink roll 9 steel strip a auxiliary bath discharge port b auxiliary bath suction port

Claims (3)

[Claims] 1. When hot-dip galvanizing by continuously introducing a steel strip into a hot-dip galvanizing bath containing Al, the bottom of the hot-dip galvanizing bath containing the hot-dip galvanizing bath is located in or above the hot-dip galvanizing bath. Equal to or better than molten zinc,
A method for suppressing dross generation in hot dip galvanizing, which comprises supplying hot dip zinc adjusted to temperature and Al concentration. 2. The dross generation according to claim 1, wherein the molten zinc adjusted to have an Al concentration adjusted to 0 to 0.020 wt% higher at a temperature higher by 0 to 20 ° C. than the molten zinc in the upper part of the plating bath. Suppression method. 3. An auxiliary bath which communicates with the plating bath at an upper portion and a lower portion is provided on a side surface of the plating bath, and molten zinc in the plating bath is introduced into the auxiliary bath through a communication port at the upper portion, and the temperature of the molten zinc taken in and The method for suppressing dross generation according to claim 1 or 2, wherein the Al concentration is adjusted in an auxiliary bath, and the adjusted molten zinc is supplied to the bottom of the plating bath through a communication port at the bottom.