JPH09316620A - Device for producing hot dip galvanized steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably produce a hot dip galvanized steel strip having a beautiful plating face by feeding hot dip Zn at a high temp. largely having an Al content to the bottom part of a hot dip galvanizing bath to prevent the generation of bottom dross in the galvanizing bath. SOLUTION: In the case a steel strip 4 is continuously passed through the inside of a hot dip Zn bath 1 in a hot dip galvanizing tank and is plated with Zn, by the reaction between Fe in the steel strip 4, a sink roll 5 or the like, bottom dross composed of Fe-Zn is generated to contaminate the plating face and to disturb the operation of the sink roll 5. For preventing this, hot dip Zn relatively large in an Al content in the middle part of the plating bath tank 2 is sucked by a hot dip Zn feeding device 11, is heated by an electromagnetic induction heating device 8, is thereafter passed through a passage constituted by a tank bottom 3 and a tank bottom cover 13 and is fed from the bottom part of the plating tank 2. The Fe member is brought into reaction with Al in the Zn bath 1 to form into top dross 7, which suppresses the generation of harmful bottom dross.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for producing hot dip galvanized steel strip for stably producing a galvanized steel strip having a beautiful plated surface with little adhesion of bottom dross.

[0002]

2. Description of the Related Art Bottom dross, as the name implies,
Present at the bottom of the hot dip galvanizing bath. The formation thereof is performed by reacting Fe atoms supplied from an iron-based device in a steel strip to be plated and a plating bath with Zn atoms in molten zinc,
It is formed as an e-Zn alloy, becomes a crystal when the temperature of molten zinc in the bath during normal operation, for example, 470 ° C., and forms and deposits at the bottom of the bath due to the difference in specific gravity between the crystal and molten zinc. It is believed that.

However, a part of them may float on the flow of molten zinc generated in the bath and adhere to the steel strip to be plated together with the molten zinc to cause a serious product defect. Also,
When a large amount of bottom dross is accumulated, it may reach the height position of the sink roll in the bath and hinder the roll rotation. Therefore, removal of bottom dross is indispensable for maintaining and improving product quality and stable production of products.

Up to now, several techniques have been disclosed regarding the method of recovering or removing bottom dross, but these all require a large amount of capital investment, and the effect equivalent to the investment amount is required. In particular, it has not been obtained regarding reduction of plating surface defects due to bottom dross. The reason for this is that there are almost no examples that disclose a technique for suppressing the generation of bottom dross, regardless of the technique for recovering or removing the bottom dross. That is, since it is impossible to suppress the formation of bottom dross, it is conceivable that dross crystals, which have not been formed, float up and adhere to the steel strip to be plated.

[0005]

DISCLOSURE OF THE INVENTION The present invention relates to a technique for producing a hot-dip galvanized steel strip that solves the above-mentioned problems, and in particular to the production of bottom dross based on a new finding regarding the production and deposition of bottom dross in a hot-dip galvanizing bath. The purpose is to provide a technique for suppressing deposition. The present invention provides, in particular, an apparatus for producing hot dip galvanized steel strip, which relates to a new type hot dip galvanizing bath structure.

[0006]

When manufacturing a hot-dip galvanized steel strip, generally 0.12 wt% or more of Al is added and contained in the hot-dip galvanizing bath. The purpose is to suppress the formation of the Fe-Zn alloy that occurs when Al is not added by promoting the preferential reaction of the Fe-Al alloy at the interface between the steel strip and the molten zinc. As a result, it has been found that the adhesion between the steel strip and the plating layer increases. Further, if it is 0.12 wt% or more, it is said that the top dross is generated and conversely the bottom dross is reduced, and this is used to suppress the generation of the bottom loss. Top dross is said to be composed of Fe-Al alloy crystals, Al oxides, zinc oxides and the like, and has a relatively low specific gravity, so it floats on the plating bath surface and easily accumulates on the surface.

The Al content is 0.12 in actual operation.
In most cases, the range is 0.15 wt%. The reason for this is that it is known that the Al concentration is necessary and sufficient for obtaining the effect of the Fe-Al alloy described above, and thus high A
The adverse effect of top dross due to the l concentration is further reduced. However, on the other hand, A in molten zinc
It has been known that bottom dross is generated and deposited even when the l concentration is 0.12 to 0.15 wt% and the above-mentioned trouble occurs.

Regarding the reason for this, according to the findings of the inventors, the following has been found. The temperature of the molten zinc at the bottom of the plating bath is 10 to 20 ° C. lower than the temperature at the top of the plating bath, and the temperature of the bottom wall of the bath made of refractory is even lower. Therefore, the molten zinc reaching the bottom is Fe
And the solubility of Al decreases, and while Fe-Al alloy (a component of top dross) is precipitated,
With the decrease of the l concentration, the Fe-Zn alloy, that is, the main component of the bottom dross is finally generated.

The reason why the temperature of the bottom of the plating bath lowers, which causes the generation of bottom dross, is to maintain the temperature of the molten zinc in the plating bath by heating it by a heat source outside the bath, for example, usually by an electromagnetic induction heating device, and by plating. Although it is established on the balance of heat carried in by the steel strip and heat taken out, in a normal plating bath, the heating device such as the electromagnetic induction heating device is not provided at the bottom of the plating bath, and most of them are It is installed on the side wall away from the bottom of the tank. as a result,
Only heat is dissipated from the bottom of the plating bath, and the temperature is lower than other parts. In particular, it was found that the molten zinc was less likely to stay in the corners from the side wall of the bottom, so that the molten zinc was likely to stay, and the temperature was particularly low.

Based on the above findings, molten zinc having a relatively high Al concentration and a relatively high temperature is continuously induced and supplied to the bottom of the bath to make the bottom of the bath have a relatively high temperature and a high Al concentration. , Has developed a technology to suppress the formation and accumulation of bottom dross. The present invention further provides a new type of hot dip galvanizing bath in order to achieve this technique easily, efficiently and effectively without requiring a large capital investment. It is a thing.

That is, the constitution is, in addition to the conventional basic constitution, (a) means for supplying molten zinc having a relatively high temperature and a high Al concentration to the bottom of the bath, and (b) supplied molten zinc. And (c) bottom coating means for separating the bath bottom from other parts in the bath so that the bath bottom has a relatively high temperature and a high Al concentration. The hot dip galvanizing bath characterized by the above is intended to achieve the above object. In the present invention, “relatively high temperature” means 20 to 3 higher than the melting temperature of zinc.
It means a temperature as high as 0 ° C. The relatively high Al content means a concentration close to 0.15 wt% of 0.12 to 0.15 wt%.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, for example, molten zinc in the middle of a plating bath having a relatively high temperature and a high Al concentration is sucked on the suction side of an electromagnetic induction heating device and heated in the electromagnetic induction heating device. A supply pipe for directing the molten zinc after heating toward the bottom of the bath is provided, and for example, a guide plate for guiding the molten zinc after heating is provided at the bottom of the bath, and the induced molten zinc after heating is further provided. After the series of heating, a flow of molten zinc was induced to induce a gap between the coating at the bottom of the bath and the bottom of the bath. This can be done relatively easily,
And the equipment cost can be reduced. In addition, since it is possible to generate a series of molten zinc flow after heating, the molten zinc at the bottom of the bath is always kept at a relatively high temperature.
The high Al concentration can be maintained. In addition, for a series of flows, the flow rate, flow velocity direction, adjustment of the guide plate, etc., the size of the opening area connected to the molten zinc in the bath,
It can be changed according to its shape, the cross-sectional area of the flow path, etc., and the temperature of the molten zinc after heating can be changed easily.
It can be 1 concentration. As a result, it is easy to prevent the generation and accumulation of bottom dross.

[0013]

【Example】

(Embodiment 1) FIG. 1 shows one embodiment of the present invention.
Sink roll 5 in plating tank 2 holding molten zinc bath 1
The steel strip 4 is wrapped around and rises. Top dross 7 floats on the surface 6 of the molten zinc bath 1.

Molten zinc in the middle part of the hot dip galvanizing bath having a relatively high temperature and a high Al concentration is sucked into the molten zinc supply device 11 of the electromagnetic induction heating device 8 and heated there to further increase the temperature. After that, it is discharged and supplied from the supply port toward the bottom of the bathtub. This molten zinc is guided downward by a guide plate (molten zinc guiding device) 12 provided along the side wall and enters between the tank bottom cover (tank bottom coating device) 13 and the tank bottom 3. The tank bottom cover (tank bottom coating device) 13 is provided with an opening at the center of the tank so that molten zinc flows upward in the bath to guide molten zinc having a relatively high temperature and a high Al concentration to the bottom of the bath. Created a series of channels. This was carried out for 7 consecutive days. At this time, the temperature of the molten zinc in the middle part of the bath is 470 ± 5 ° C., and the Al concentration is 0.145 ± 0.00.
5% by weight, the temperature of the molten zinc at the bottom of the bath is 475 ± 5 ° C., Al while the continuous flow according to the present invention is continuous.
The concentration was 0.145 ± 0.005 wt%. As a result, bottom dross formation was not observed at all in the operation for 7 days. From this, it was found that the hot-dip galvanizing bath of the present invention was extremely effective in generating and suppressing bottom dross.

(Embodiment 2) In FIG. 1, the molten zinc in the middle part of the hot dip galvanizing bath is sucked by an ordinary iron-based pumping pump without passing through the electromagnetic induction heating device 8.
As it was, molten zinc was supplied between the side wall of the guide plate 12 provided along the side wall and the side wall through the outlet side pipe, and a series of flows was created in the same manner as in Example 1. The molten zinc temperature in the middle of the bath was 475 ± 5 ° C, Al during the continuous flow.
The concentration is 0.145 ± 0.005 wt%, the molten zinc temperature at the bottom of the bath is 475 ± 5 ° C., and the Al concentration is 0.145.
It was ± 0.005 wt%. In this state, the actual operation was continued for 72 hours, but the occurrence of bottom dross was not observed as in Example 1.

(Embodiment 3) In FIG. 1, molten zinc having a relatively high temperature and a high Al concentration is supplied from a sub-pot provided adjacent to a hot dip galvanizing bath through a ceramic pump without passing through an electromagnetic induction heating device. , As in Example 1 and Example 2, and created a series of molten zinc streams. This was carried out for 7 consecutive days. At this time, the molten zinc in the middle of the bath was 475 ± 5 ° C. and the Al concentration was 0.145 ± 0.005 wt.
%, The molten zinc temperature at the bottom of the tank is 478 ± 5 ° C, Al
The concentration was controlled to 0.148 ± 0.005 wt%. As a result, the formation and accumulation of bottom loss was not observed, and it was found that the present invention is extremely effective in the generation and accumulation of bottom dross.

[0017]

According to the present invention, in addition to the conventional basic constitution, the molten zinc having a relatively high temperature and a high Al concentration is efficiently and effectively supplied to the bottom of the bath to melt the bottom of the bath. Since zinc is always used as a hot dip galvanizing bath capable of maintaining a relatively high temperature and a high Al concentration, it has become possible to suppress the formation and deposition of bottom dross.

[Brief description of drawings]

FIG. 1 is a view conceptually showing a hot-dip galvanizing bath of the present invention.

[Explanation of symbols]

 1 Hot-dip zinc 2 Hot-dip galvanizing bath 3 Bath bottom 4 Steel strip 5 Sink roll 6 Bath surface 7 Top dross 8 Electromagnetic induction heating device 11 Hot-dip zinc supply device 12 Hot-dip zinc induction device 13 Vessel bottom coating device

Claims (1)

[Claims] 1. A means for supplying molten zinc having a high Al concentration to a bath at a relatively high temperature, a means for guiding the supplied molten zinc to the bottom of the bath, and the induced molten zinc and other molten zinc in the bath. An apparatus for producing a hot-dip galvanized steel strip, which is provided with a tank bottom coating means for suppressing the mixing with.