JPS59166668A - Manufacture of hot-dip aluminum-coated steel sheet having coating layer of uniform thickness along longitudinal direction of steel strip - Google Patents

Abstract

PURPOSE:To obtain a hot-dip Al-coated steel sheet having a coating layer of uniform thickness along its longitudinal direction, by spraying unoxidizing or reducing gas having oxygen content controlled onto the surface of a steel strip having passed through an Al coating bath to wipe the coated surface. CONSTITUTION:In continuously hot-dip coating a steel sheet with Al, wiping gas is sprayed onto the steel strip having passed through a coating bath inside a sealed box. The wiping is performed using unoxidizing or reducing gas having oxygen content controlled below 1% as said wiping gas and an atmospheric gas in its vicinity. By this method, a hot-dip Al-coated steel sheet excellent in external appearance having a coating layer of uniform thickness along its longitudinal direction is obtained without the formation of a flow pattern.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a hot-dip aluminized steel sheet, which continuously runs in a molten aluminum plating bath or an aluminum alloy plating bath (hereinafter simply referred to as an aluminium-plating bath).

Conventionally, hot-dip aluminized steel sheets are manufactured by using gas wiping, which sprays high-pressure gas through a nozzle onto the surface of the IJ tube to adjust the amount of plating. There is. This gas wiping method is widely used because it can be easily controlled according to the traveling speed of the steel plate and the plated cover.

However, conventional wiping methods adjust the amount of plating deposited by spraying combustion waste gas such as heavy oil or kerosene, or high-pressure gas such as air, nitrogen, or superheated steam, resulting in flow-patterned plating surface defects. There was a simple drawback. The flow pattern of the plating is due to unevenness in the thickness of the plating layer, which causes problems such as peeling off of the plating during bending, and the product's corrosion resistance and heat resistance are not fully demonstrated.

The present inventors have confirmed that the plating flow pattern that causes this problem is related to the presence of an oxide film formed on the plating surface during wiping. The present invention aims to solve these problems in hot-dip aluminizing, and its gist is to use wiping gas, which sprays high-pressure gas onto the surface of a steel strip that has passed through a hot-dip aluminizing bath, and oxygen gas in the atmosphere near the wiping gas. A non-oxidizing gas or a reducing gas with a content of 1% or less is used.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is presented to explain the method of the present invention.
1 shows an embodiment of a conventional continuous hot-dip aluminum plating apparatus. 1 is a hot-dip aluminum plating pot, which melts and stores aluminum. A snout 3 is connected to a continuous heating furnace 4 and has its other end immersed in an aluminizing bath 5. 6 is a turn roll installed inside the aluminizing bath 5. 7
is a seal box, and 8 is a gas wiping nozzle. 9 is a steel strip.

That is, the steel strip 9 is passed from the continuous heating furnace 4 through the snout 3, through the hot-dip aluminizing bath 5, rotated around the turn roll 6, and raised, while in the sealing box 7, a non-oxidized material having an oxygen content of 1 z or less is passed through the hot-dip aluminum plating bath 5. Wipe with a reactive or reducing gas. In this case, the oxygen content contained in the wiping gas and the surrounding atmosphere gas is reduced to 1 treble or less (preferably 1 tres or less, more preferably 0.0050% or less) as shown in Figure 2. , the steel strip exhibits a good plating appearance without producing a longitudinal mesh plating pattern.

Next, examples of the present invention will be described.

Figure 3 shows the plating layer thickness (μ) in the longitudinal direction of the strip when a 1.2 rtrmO steel strip was plated in molten aluminum (Az=-9q6Si) at a running speed of 30 cm. This figure shows a comparison between the wiping gas and the oxygen content in the seal box (2 ppm) and the vJC method (superheated steam method).

In the method of the present invention, the thickness of the plating layer in the longitudinal direction of the steel strip is uniform, and the corrosion resistance (salt spray test 240 hours) and heat resistance (temperature 500 CXIO hours) are superior to the VJ'C method.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a conventional continuous hot-dip aluminizing apparatus, Figure 2 is a diagram showing the variation in the plating thickness in the longitudinal direction of a plated steel strip due to the oxygen content in the seal box, and Figure 3 is a diagram of the present invention. FIG. 3 is a diagram showing the thickness of the plating layer in the longitudinal direction of the IJ tube in comparison with the vJC method. 1... Molten aluminum plating pot 3... Snout 4...
Continuous heating furnace 5... Aluminum plating bath 6... Turn roll 7... Seal box 8... Scrap wiping nozzle 9... Steel strip Fig. 1

Claims (1)

[Claims] A non-oxidizing gas or a reducing gas with an oxygen content of 1% or less is used for the wiping gas that sprays high-pressure gas onto the surface of the steel strip passed through the hot-dip aluminizing bath and the surrounding atmosphere gas. A method for producing hot-dip aluminized steel sheets with a uniform coating layer thickness in the longitudinal direction of the steel strip.