JPH0776761A - Method for decreasing al concentration in hot dip metal coating bath - Google Patents

Abstract

PURPOSE:To rapidly and easily lower Al concn. to a target value by lowering bath temp., bringing the Al in a bath into reaction with the Fe in the bath and floating the Al and the Fe as a compd. on a bath surface. CONSTITUTION:The temp. of the galvanizing bath 4 for producing a galvanized steel sheet 5 is so lowered as to attain the target temp. by controlling the input capacity of a heater 7 by a thermometer 10. As a result, the reaction of the formula of the Fe and Al dissolved in the bath: 2Fe+5Al Fe2Al5 is made easier to progress and Fe2Al5 floats as drop loss 11 on the bath surface. This drop loss is discharged outside the system of the bath 4 by a discharge pump 8 and the discharge rate thereof is measured by a flow meter 12. The amt. of the discharged Al is computed by a calculator 13. A zinc ingot 14 is simultaneously charged by a charger 9 at this time. consequently, the discharge pump 8 is stopped at the point of the time when the Al concn. in the bath falls down to the target value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing the concentration of bath components, especially Al, in a hot dip galvanizing bath for producing hot dip galvanized steel sheets.

[0002]

2. Description of the Related Art Conventionally, when manufacturing a hot-dip galvanized steel sheet, the Al concentration is usually adjusted to 0.10 to 0.20%, though it varies depending on the manufacturing process, manufacturing conditions and the like. For example, in order to ensure plating adhesion, a relatively high concentration of Al (0.15 to 0.20
%), But when iron-zinc alloying treatment is performed by heating after zinc plating, the Al concentration in the bath should be as low as 0.15% or less to facilitate the alloying treatment. There is.

However, the adjustment of a low Al concentration bath to a high Al concentration bath can be made relatively easily by adding Al into the bath, but it is relatively easy, but a high Al concentration bath is changed to a low Al concentration bath. If there is no method to adjust on an industrial scale, for example, high Al
The concentration bath is pumped out of the plating bath to fill the low Al concentration bath, and the operation is stopped during that time, which lowers productivity and also involves high heat work such as bath replacement and heat loss. There are drawbacks.

In order to solve these drawbacks, JP-A-1-159361 discloses a method of reducing the Al concentration in the bath by adding a de-Al agent (chloride such as ZnCl ₂ ) to the zinc bath. However, this method has a problem that Al is generated as AlCl ₃ fumes and is caught in the steel sheet and damages the surface.

[0005]

By the way, according to the present invention,
An object of the present invention is to propose a method for solving the above problems, which can easily reduce the Al concentration industrially, and which does not cause a large decrease in the yield of the plated steel sheet. It is what

[0006]

Means for Solving the Problems In the present invention, when reducing the Al concentration in a hot dip galvanizing bath, the bath temperature in the plating bath is lowered so that the Al in the bath and the bath eluted from the steel sheet
This is a method of reducing the Al concentration in the hot dip galvanizing bath, which is characterized by reacting with Fe to float on the bath surface as Fe ₂ Al ₅ (top dross) and discharging it out of the system.

[0007]

[Operation] According to the present invention, by lowering the bath temperature in the plating tank, the reaction of the following formula of Fe and Al dissolved in the bath is facilitated. 2Fe + 5Al → Fe ₂ Al _{5 By} this reaction, Al floats on the surface of the bath as Fe ₂ Al ₅ (top dross), so by discharging it to the outside of the system, the Al concentration in the bath can be rapidly reduced. You can

The lowering of the bath temperature in the plating tank can be quickly achieved by reducing the input to the heating device 7 and further by adding a zinc ingot. Regarding the decrease in bath temperature, for example, 480
Al: 0.16%, Fe: 0.04% zinc bath at ℃, bath temperature 450
By lowering it to ℃ and discharging the top dross out of the system,
It can be reduced to Al: 0.14% and Fe: 0.03%.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view showing an example of a hot dip galvanizing apparatus which can be used for carrying out the present invention. After passing through the reducing furnace 1, the hot-dip galvanized steel sheet 5 is immersed in the hot-dip galvanizing bath 4 in the hot-dip galvanizing tank 2, is pulled up from the hot-dip galvanizing tank 2 via a sink roll 6, and is then attached by the squeezing device 3 as desired. The amount of molten zinc is adjusted so as to obtain the amount.

A heating device 7, a top dross discharge pump 8, a zinc ingot feeder 9 and a bath thermometer 10 are arranged around the plating tank 2. The input capacity of the heating device 7 is controlled by the indicated value of the bath thermometer 10 so that the bath temperature becomes the target temperature, and the top dross 11 is discharged from the system of the molten zinc bath 4 by the top dross discharge pump 8. The discharge amount is measured by the flow meter 12, and the amount of Al discharged outside the system is calculated by the computer 13. At this time, if necessary, the zinc ingot 14 is simultaneously charged by the zinc ingot charging machine 9.

As a result, the top dross discharge pump 8 is stopped when the Al concentration in the bath falls to the target value. Figure 2
The solid line shows the case where the method of the present invention was carried out, and the broken line shows the conventional method in which the Al concentration was switched only by introducing the pure zinc ingot while the bath temperature was kept constant. According to this, in the conventional method, Al
Although it took 10 hours for the concentration to drop to the target value, it was shortened to 3 hours by the method of the present invention.

[0012]

EFFECTS OF THE INVENTION According to the present invention, since Al is discharged out of the system as top dross, the Al concentration can be easily lowered to the target value in a short time, the yield of hot-dip galvanized steel sheet is improved, and the thermal energy is increased. The loss of was also minimized.

[Brief description of drawings]

FIG. 1 is a sectional view of an apparatus used for carrying out the method of the present invention.

FIG. 2 is a graph showing changes in Al concentration when the method of the present invention and the conventional method are carried out.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reduction furnace 2 Hot dip galvanizing tank 3 Throttling device 4 Hot dip galvanizing bath 5 Hot dip galvanizing steel plate 6 Sink roll 7 Heating device 8 Top dross discharge pump 9 Zinc ingot injector 10 Bath thermometer 11 Top dross 12 Flow meter 13 Calculator 14 Zinc ingot

Claims (1)

[Claims] 1. When reducing the Al concentration in the hot dip galvanizing bath, the bath temperature in the plating bath is lowered, and Al in the bath reacts with Fe in the bath to produce Fe ₂ Al ₅ (top dross). A method for reducing the Al concentration in a hot-dip galvanizing bath, characterized by floating on the bath surface and discharging it from the system.