JPH03100150A - Continuous hot dip metal coating method for steel strip - Google Patents

Abstract

PURPOSE:To suppress the sticking of the powder of a metal for plating to the inside walls of a duct by providing a blow port for an atmospheric gas on the side of the duct near an annealing furnace and providing a discharge port in the terminal part which is not immersed into a pot, thereby creating the flow of the gas in the progressing direction of a steel strip. CONSTITUTION:The duct 4 to be immersed into the pot 5 of the molten metal for plating is provided with the blow port 7 for the atmospheric gas on the inlet side in the progressing direction 2 of the steel strip 1 and is provided with the discharge port 8a for the atmospheric gas near right above the liquid surface of the pot 5 on the outlet side. A part of the blown atmospheric gas flows along the progressing direction 2 of the steel strip 1. The metal vapor evaporated from the liquid surface in the duct 4 of the pot 5 is discharged together with the atmospheric gas to the outside of the duct 4 from the discharge port 8a without filling in the duct 4. The deposition of the metal vapor on the inside walls of the duct 4 is, therefore, suppressed and the appearance defect of the steel strip is prevented.

Description

[Detailed description of the invention] <Industrial application field> The present invention relates to a method for continuous hot-dip metal plating of steel strip.

<Prior Art> An example of a conventional continuous hot-dip metal plating method for steel strip will be explained with reference to FIG.

After the steel strip 1 passes through an annealing furnace 3 and is subjected to a prescribed heat treatment, it passes through a duct 4 directly connected to the outlet side of the annealing furnace 3 and enters the bath of a metal melting bot 5 for plating. When the molten metal comes out, the amount of molten metal deposited is controlled by the jet gas from the gas restricting nozzle 6, and the molten metal is sent to the next step.

Here, in order to maintain the atmosphere inside the annealing furnace 3 in a non-oxidizing or reducing state, N2 gas or N2 gas is blown in as the atmospheric gas from the atmospheric gas blowing lower.

Since the outlet of the duct 4 is "liquid sealed" in the bath of the metal melting pot 5 for plating, all the atmospheric gas flows inside the annealing furnace 3 in the opposite direction to the steel strip traveling direction 2. It is discharged from the charging port 9 to the outside of the furnace.

By the way, in the metal melting pot 5 for plating, metal vapor is constantly evaporating from the liquid surface of the molten metal. Since the lower end of the duct 4 is immersed in the molten metal liquid, metal vapor enters the duct 4, fills it, and precipitates on the inner wall of the duct where the temperature is relatively low. adheres to the surface of the steel strip 1, causing scratches and poor appearance.

Therefore, as a method for preventing such evaporation of metal vapor, for example, Japanese Patent Application Laid-Open No. 61-41754 discloses the following methods:
By controlling the atmosphere in the duct 4 to a narrow range of gas that is oxidizing to the metal vapor and non-oxidizing to the steel strip 1, the bath surface in the duct 4 of the metal melting pot 5 for plating is It has been proposed to coat the metal oxide with a metal oxide. Specifically, the water vapor partial pressure within the duct 4 is controlled by blowing in a predetermined amount of water vapor from the water vapor inlet 10 provided in the duct 4.

<Problems to be Solved by the Invention> However, this method has the following drawbacks and cannot exhibit sufficient effects.

(1) Steel strip 1. Metal melting pot for plating 5. Near the liquid surface entering the bath, the liquid surface flows due to the movement of the steel strip 1, and the metal oxide cannot completely cover the liquid surface, making it impossible to completely prevent metal vapor from evaporating. .

(2) As mentioned above, the atmospheric gas blown into the furnace from the atmospheric gas blowing lower passes through the annealing furnace 3 and flows toward the steel strip charging inlet 9, so the steam blown in from the steam inlet 10 It is actually difficult to conveniently keep the water vapor only in the duct 4, and water vapor flows into the annealing furnace 3 along with the flow of atmospheric gas, reducing the non-oxidizing or reducing nature of the atmosphere inside the annealing furnace 3. , problems such as the occurrence of pick-up scratches due to the transport rollers (not shown) and oxidation of the steel strip l occur.

The present invention provides a continuous hot-dip metal plating method for steel strips that suppresses adhesion of plating metal powder to the inner wall of a duct, prevents poor appearance of the steel strips, and significantly increases continuous operation time. (to do something)

According to the present invention, in order to achieve the above object, a steel strip is continuously passed through an annealing furnace, a duct directly connected to the annealing furnace and whose tip is immersed in a metal melting pot, and a metal melting pot. When plating, an atmospheric gas inlet is provided on the side of the duct near the annealing furnace, and an exhaust port for the ambient gas is provided at the end of the duct that is not immersed in the pot on the side farther from the annealing furnace. , there is provided a continuous hot-dip metal plating method characterized in that a flow of atmospheric gas is created in the traveling direction of the steel strip.

Furthermore, it is preferable that a valve is provided at the outlet of the exhaust port to prevent backflow of atmospheric gas and to adjust the flow rate.

Furthermore, it is preferable to provide an inlet for water vapor or a water vapor-containing gas between the inlet and the outlet for the atmospheric gas.

The present invention will be explained in more detail below based on one embodiment.

FIG. 1 is a partially sectional side view showing an example of a hot-dip metal plating apparatus for implementing the present invention.

After passing through the annealing furnace 3, the steel strip 1 passes through a duct 4 directly connected to the outlet side thereof, enters the bath of the metal melting pot 5 for plating, is continuously pulled up, and is further placed facing each other. The amount of molten metal deposited is controlled to a desired level by the injected gas from the two gas restricting nozzles 6, and the molten metal is sent to the next step.

An atmospheric gas blowing lower is provided on the inlet side of the duct 4 in the direction of movement 2 of the steel strip 1, and an atmospheric gas exhaust port 8a is provided on the exit side, right above the liquid level of the metal melting bot 5. An atmospheric gas exhaust pipe 8b is connected to the outside of the atmospheric gas exhaust port 8a. Furthermore, it is preferable to provide a flow control valve 11 in the discharge pipe 8b.

Furthermore, it is preferable that a predetermined amount of water vapor be blown from the steam inlet 10 between the atmospheric gas inlet and the outlet, preferably in the vicinity directly above the liquid level of the duct 4.

In the annealing furnace 3 and duct 4 configured in this way, when N2 or H2 gas or a mixture thereof is blown from the atmospheric gas blowing lower, the pressure distribution inside the furnace is such that the highest pressure is at the position of the atmospheric gas blowing lower, This becomes a pressure watershed, and the pressure decreases toward the exhaust port 8a or the steel strip charging port 9, and the atmospheric gas blown from the atmospheric gas blowing lower flows toward the exhaust port 8a or the steel strip charging port 9.

Therefore, inside the duct 4, a part of the blown atmospheric gas flows along the traveling direction 2 of the steel strip 1. Therefore, the metal vapor evaporated from the liquid level in the duct 4 of the metal melting bot 5 for plating is discharged to the outside of the duct 4 from the exhaust port 8a along with the atmospheric gas without filling the duct 4. The precipitation of metal vapor on the inner wall of the duct 4 can be completely prevented. Note that it is desirable to provide the exhaust port 8a as close to the liquid level in the duct 4 as possible, since this can enhance the above effect.

In order to prevent metal vapor from being deposited in the atmospheric gas exhaust pipe 8b, the outside of the exhaust pipe may be covered with a heat insulating material (not shown) or may be heated from the outside by an appropriate heating means.

The metal for plating in the present invention is not particularly limited, and Zn%
It can be applied to all molten metal plating such as Aj2, Sn, and Pb.

The atmospheric gas is for maintaining the inside of the annealing furnace 3 in a non-oxidizing or reducing state, and is N2 or H2 gas or a mixed gas thereof.

According to the present invention, even if water vapor or a water vapor-containing gas is blown into the duct 4 from the inlet 10 in order to suppress the evaporation of metal vapor, the atmospheric gas does not flow in the duct 4 in the traveling direction of the steel strip 1. Since the water vapor or water vapor-containing gas is formed, it remains in the duct 4 and does not flow into the annealing furnace 3.

Further, in the present invention, it is necessary to increase the amount of atmospheric gas blown from the blowing lower by the amount of atmospheric gas exhausted from the exhaust port 8a, but the flow rate provided in the atmospheric gas exhaust pipe 8b following the exhaust port 8a is The control valve 11 is used to prevent atmospheric gas from flowing into the duct 4 side more than necessary, or to prevent water vapor or water vapor-containing gas from flowing into the annealing furnace 3.
That is, by adjusting the flow rate of gas discharged from the exhaust port 8a to prevent backflow, it is possible to minimize the increase in the amount of atmospheric gas blown into the atmosphere.

<Example> The present invention will be specifically explained below based on Examples.

(Example 1) Cold-rolled vA with a thickness of 0.7 mm and a width of 1400 mm was prepared using a hot-dip metal plating apparatus configured as shown in Fig. 1 under the following conditions.
1! : Galvanized.

Steel strip line speed: 60mpm.

Production volume: 30 t/h Atmospheric gas composition: 8220%, N280% Temperature of molten zinc in melting bot: 480℃ Inner diameter 100mm
The exhaust port 8a was installed at a position 50 mm from the bath surface of the zinc melting bot 5, and atmospheric gas was blown at 1508 m'/h from the blowing lower, of which 100 Nm'/h was directed toward the steel strip charging port 9. When the remaining 50 Nm'/h was allowed to flow through the duct 4 in the traveling direction 2 of the steel strip and exhausted from the exhaust port 8a, the zinc vapor did not fill the duct 4 and was released from the exhaust port 8a. As a result, the continuous operation time was extended from 40 hours to 150 hours, almost four times as long as before.

Furthermore, when the amount of atmospheric gas blown was 200 N1M3/h and 100 N+a'/h was exhausted from the exhaust port 8a, the continuous operation time was about 180 hours.

Next, under conditions of exhausting atmospheric gas at 50 Nm'/h from the exhaust port 8a,
0,5 from the steam inlet 10 provided at the 00mm position
When the operation was carried out under the same conditions as for steam except that steam was injected at a rate of Nm''/h, most of the steam was confined between the steam inlet and the zinc bath surface, and flowed upward into the duct 4 and toward the annealing furnace 3. The evaporation of zinc vapor was suppressed under the above conditions without flowing, and the continuous operation time was significantly increased to 30 days.

(Comparative Example 1) The same steel strip as in Example 1 was galvanized using a hot-dip metal plating apparatus configured as shown in FIG. 2 under the following conditions.

Steel strip line speed: 60mpm.

Production volume: 30 t/h Atmospheric gas composition: N220%, N280% Atmospheric gas injection amount: 100 No+3/h Molten zinc temperature in melting pot: 480°C Immediately after the start of operation, zinc powder adheres to the inner wall of duct 4. Initially, after about 36 hours, the zinc layer on the powder grew to a thickness of about 1 mm.

At this stage, the chances of this powdered zinc peeling off from the inner wall of the duct 4 increased, and the powdered zinc adhered to the surface of the steel strip 1, resulting in a sharp increase in the incidence of appearance defects.

Therefore, the operation was stopped approximately 40 hours after the start of operation.
The duct 4 was opened and the interior was cleaned to remove powdered zinc.

This work is manual work in a harsh environment in the vicinity of the zinc melting pot 5, which poses a major safety problem and at the same time greatly impedes the productivity of the line.

<Effects of the Invention> Since the present invention is configured as described above, the method of the present invention can significantly suppress the evaporation of plating metal and the adhesion of condensed powder to the inner wall of the duct, and improve the appearance of the steel strip. The operating time for continuous hot-dip metal plating without defects has been significantly extended.

Explanation of symbols 1... steel strip, 2... traveling direction of steel strip l, 3... annealing furnace, 4...Duct, 5...metal melting pot for plating, 6... Gas throttle nozzle, 7... Atmosphere gas inlet, 8a...atmosphere gas exhaust port, 8b...atmosphere gas exhaust pipe, 9... Steel strip charging inlet of annealing furnace, lO...steam inlet, 11...Flow rate control valve

[Brief explanation of drawings]

FIG. 1 is a partially sectional side view showing an example of a hot-dip metal plating apparatus for implementing the present invention. FIG. 2 is an explanatory diagram showing an example of a conventional hot-dip metal plating apparatus. FIG, 2 FIG.

Claims (3)

[Claims] (1) When continuously plating a steel strip through an annealing furnace, a duct that is directly connected to this and whose tip is immersed in a metal melting pot, and the metal melting pot, an atmospheric gas is blown into the side of the duct near the annealing furnace. A continuous method characterized in that an exhaust port for the atmospheric gas is provided at the end of the duct which is far from the annealing furnace and is not immersed in the pot, and a flow of the atmospheric gas is created in the traveling direction of the steel strip. Hot dip metal plating method. (2) The method for continuous hot-dip metal plating of a steel strip according to claim 1, further comprising a valve at the outlet of the exhaust port to prevent backflow of atmospheric gas and to adjust the flow rate. (3) The method for continuous hot-dip metal plating of a steel strip according to claim 1 or 2, wherein an inlet for water vapor or a water vapor-containing gas is provided between the inlet and the outlet for the atmospheric gas.