KR20010028283A - Preventing method of metallic dust formation from molten metal in snout for a hot dip coating - Google Patents

Abstract

PURPOSE: A method for restraining formation of metal dusts in a snout of a hot dipped metal plating line is provided to restrain generation of metallic dusts in the snout, and restrain occurrence of flaking, bare spot or defects of product surface by artificially controlling gas environment and infusing the gas into the snout so that a ratio of hydrogen to vapor is less than 4:1, and a vapor content has a dew point of -40 to -5 deg.C. CONSTITUTION: The method for restraining formation of metal dusts in a snout of a hot dipped metal plating line comprises the process of infusing an environment gas into the snout so that a ratio of hydrogen to vapor is less than 4:1, and a vapor content has a dew point of -40 to -5 deg.C so as to restrain formation of metallic dusts inside the snout, an area where a material plated is led into a plating bath carried port as an area which connects a reduction annealing furnace of a plating line with a port during hot dipped metal plating.

Description

PREVENTING METHOD OF METALLIC DUST FORMATION FROM MOLTEN METAL IN SNOUT FOR A HOT DIP COATING}

The present invention relates to a method of suppressing metal dust generation during molten metal plating, and more particularly, to the metal dust inside the snout during molten metal plating such as hot dip galvanizing, alloyed hot dip galvanizing, molten aluminum plating, and hot dip zinc-aluminum alloy plating. The present invention relates to a method for suppressing generation of metal dust in a snout of a molten metal plating apparatus for inhibiting formation to prevent peeling, unplating, and surface defects of a product surface.

As a method of hot-dip metal plating, plating by Sendimir is generally performed. After annealing heat treatment in a hydrogen reducing atmosphere at a high temperature, the oxide film on the surface of the plated material is reduced and the surface is activated. The plated material is deposited in a plating bath of molten metal and passed through for several seconds to prepare a molten metal plated steel sheet by adjusting the plating adhesion amount using various gas nozzles.

However, the snout region of the plating equipment for performing the molten metal plating is a region connecting the reducing annealing furnace for maintaining a reducing atmosphere and the pot containing the molten metal, and the atmosphere gas is connected to the annealing furnace. It is a section in which a liquid molten metal is exposed to a reducing atmosphere, and a plated object is deposited into the molten metal.

In general, a black gray metal dust is generated in a snout, and the metal dust is deposited on the molten metal together with the plated material, which eventually causes defects on the surface. The metal dust is generated by the evaporation of the molten metal. It is condensed on the outer wall of and gradually accumulates, and when accumulated cannot be tolerated anymore, or the density is very light at 0.02g / ㎠, it is dropped to the molten metal surface when there is fluctuation in gas flow or minute vibration. .

In addition, it was confirmed that the metal dust is composed of a solid metal and a metal oxide, it is known that the surface is covered with a metal oxide because the metal dust is not dissolved into a liquid metal above the melting temperature.

When the metal dust is deposited on the molten metal surface inside the snout and deposited with the plated material, an alloy layer is not formed between the plated material and the molten metal at the metal dust-attached portion, thereby causing flaking or bare spots. When the metal dust is finely separated and deposited together with the plated material, the metal dust is mixed in the molten metal plating layer, and fine unevenness or fine wrinkles occur during the solidification process, causing surface defects.

As such, metal dust is not desirable because it causes flaking, bare spots, or surface defects on the surface of the plated product during the molten metal plating process.

As a related technology for preventing flaking, bare spots and surface defects caused by metal dust on the surface of such a plated product, US Patent No. 4557953 to Boston et al. (Mitch) et al. US Pat.

U.S. Pat. No. 4,455,53 to Boston et al. Discloses a series of snouts in a continuous galvanized line in which zinc or zinc alloy is used as a hot dip coating on a ferrous base metal strip. A method of controlling or removing evaporation of molten zinc, wherein the atmosphere in the snout is oxidative with respect to zinc vapor but non-oxidative with respect to the strip so that the hydrogen in the atmosphere gas has a ratio of 4: 1 or more such that the ratio of hydrogen: water vapor is at least 4: 1. The minimum content of is strictly regulated.

On the other hand, US Patent No. 4,455,722 to Mitch et al. Relates to a finishing method in which a hot dip zinc base coating is applied on a ferrous base metal strip. A technique of controlling the formation of zinc vapor in an airtight coating chamber by forming a high dew point atmosphere in the coating chamber of ˜6: 1 and more than 0.3 volume, or finishing of a strip taken out from a hot dip bath. There is a difference in that it is a technology for controlling the formation of zinc vapor in the closed coating chamber containing the equipment.

As such, the related art relates to controlling zinc vapor formation by controlling the atmosphere gas, but still peeling, unplating and surface defects are caused by metal dust on the surface of the plated product, and related to the atmosphere in which the strip is taken out. It is merely a completely different invention from the present invention regarding the atmosphere of the part where the strip is drawn in to deposit the plating bath.

Accordingly, the present invention has been made to solve the above problems of the prior art, and provides a hot-dip plating method that suppresses the generation of metal dust causing peeling or unplating or surface defects in the snout during the hot-metal plating process. The purpose is.

In order to achieve the above object, in the present invention, metal dust is generated inside a snout which is a part for introducing a plated material into a port accommodating a plating bath as a part for connecting a reducing annealing furnace and a port of a plating facility during molten metal plating. In order to suppress this, the molten metal plating apparatus is characterized by injecting an atmosphere gas into the snout so that the ratio of hydrogen: water vapor is less than 4: 1 and the water content is not less than -40 ° C and less than -5 ° C at the dew point. Provided are methods for inhibiting metal dust generation in snouts.

In the present invention, the atmospheric gas injected into the snout is water vapor, an inert gas including water vapor, or a mixture of these gases.

Hereinafter, the present invention will be described in detail with reference to preferred embodiments.

In a typical molten metal plating process, the snout is spatially connected to the annealing furnace, or even if a shielding device such as a seal roll is installed in the snout, the snout is completely spaced from the annealing furnace and gas atmosphere. Not blocked

In addition, the annealing furnace maintains a reducing atmosphere. The iron oxide remaining on the surface of the plated material is reduced to activate the surface of the plated material, and the plated material is heated even at high temperature in order to secure a mechanical material. The reducing atmosphere is maintained so as not to be oxidized, and nitrogen, 99 to 70, hydrogen 1 to 30, and other impurity gases contain extremely small amounts of water vapor, argon and methane.

Therefore, if the atmosphere gas is not artificially produced and injected, the atmosphere of the snout is almost the same gas atmosphere as the annealing furnace, and the dew point is usually -40 ° C to -70 ° C.

In the present invention, the atmosphere in the snout is less than 4: 1 ratio of hydrogen: water vapor, and in order to keep the content of water vapor in the snout atmosphere higher than -40 ° C and below -5 ° C at the dew point, steam or steam Inert gas (preferably nitrogen gas or argon gas), or a mixture thereof, is injected into the snout.

The injection position may be injected at any position of the snout, but preferably the closer to the molten metal surface, the more the same flow rate is injected at both sides of the snout.

In the present invention, in order to suppress the generation of metal dust in the snout, the content of water vapor should be lower than -40 ° C to -5 ° C as the dew point.

According to the experimental results of investigating the metal dust with various kinds of dew point changes, the metal dust rapidly decreases from the dew point temperature of -40 ° C or higher, and preferably at -30 ° C or higher, the metal dust can be well suppressed. However, at -5 ° C or higher, plating adhesion starts to deteriorate.

In addition, in this invention, the ratio of hydrogen: water vapor is comprised less than 4: 1 in the atmosphere in a snout.

The above-described U.S. Pat.No.4557953 defines the ratio of hydrogen: steam to 4: 1 or higher, which is based on thermodynamic theory. When the atmosphere in the snout maintains at least 4: 1 in the ratio of hydrogen: steam, zinc vapor ( It is suggested that the conditions remain oxidative for zinc vapor but remain non-oxidative for strips. In other words, defining the ratio of hydrogen to water vapor is a condition that prevents the strip from oxidizing, and the reason for strictly inhibiting the oxidation of the strip is that the wettability between the strip and the molten zinc is poor, causing unplated defects. It is understood that. In other words, if the hydrogen: water vapor ratio presented in US Pat. No. 4,455,53 is less than 4: 1, it means that the molten metal product cannot be plated or unplated to produce a defect.

However, as a result of several experiments by the present inventors, it has been found that metal dust is not generated even when the ratio of hydrogen: water vapor is less than 4: 1, and defects such as plating peeling or unplating do not occur on the surface of the plated steel sheet.

The iron-based metal strip proposed in the U.S. patent does not oxidize and the molten metal surface is oxidized.Therefore, metal dust does not occur even when the ratio of hydrogen to water vapor, which is a thermodynamic condition, is less than 4: 1. It is understood that there is a large variation that cannot be equally applied to iron metal strips moving at high speed because they are derived from the maintained state, and the length of the snout even if the reaction occurs with free oxygen from the iron metal strip and water vapor. Since it is typically about 10 meters, it is understood that the exposure time of the reaction is very short, so that the thickness of the iron oxide on the surface of the strip is extremely thin, so that the wettability between the molten zinc and the strip may not be lost during the molten metal plating process.

In addition, in the present invention, the oxygen content is not defined in the atmosphere of the snout because it is confirmed that it does not affect the generation of metal dust, in the present invention does not artificially include oxygen in the injection gas, the trace amount of oxygen Is an impurity inevitably contained in nitrogen gas.

In addition, the metal for plating in the molten metal plating is all metals that can be melted and plated, and typical platings are hot dip galvanizing, alloyed hot dip zinc-aluminum alloy plating (galfan, galvalume), hot dip aluminum plating, Turn plating etc. are mentioned.

Further, the plated material of the molten metal plating may include all ferrous and nonferrous metals, alloys of ferrous and nonferrous metals thereof, or nonmetals.

Hereinafter, the present invention will be described in detail with reference to Examples.

Example

In a hot-dip galvanizing line working at a line speed of 120 to 170 mm for a steel strip with a thickness of 0.3 to 0.7 mm and a width of 800 to 1234 mm, it is injected into the snout by artificially adjusting the atmospheric gas state using a dew point controller. It was. At this time, the state of nitrogen gas, which is the raw material gas entering the dew point controller to control the atmosphere, is below dew point -70 ° C, inevitably included oxygen is below 5ppmv, and finally, the state of nitrogen gas prepared for injection into the snout is dew point- 50 ~ 0 ℃, the atmosphere gas was injected into the snout, and then the metal dust generated inside the snout was visually observed, collected and evaluated, and the rock forming test was performed as a method of evaluating the adhesion of the plating. The results are shown in Table 1.

As a result, as can be seen from Inventive Examples 1 to 5, the effect of suppressing metal dust generation began to appear from the dew point of -40 ° C or higher, and the plating adhesion was poor at the dew point of -5 ° C or higher. In addition, no change with hydrogen and oxygen content was observed.

Example Atmospheric gas state in snout Metal dust generation amount (visual observation) Adhesion of Plating Products Remarks vapor Hydrogen content (ppmv) Hydrogen / vapor ratio Oxygen content (ppmv) Dew point (℃) Content (ppmv) Comparative Example 1 -67 4 95000 23750 3.5 ×× ○ Normal line condition Comparative Example 2 -50 39 100 2.5 3.4 × ○ Inventive Example 1 -40 127 500 3.9 3.4 △ ○ Inventive Example 2 -30 376 1000 2.6 3.6 ◎ ○ Inventive Example 3 -20 1021 1100 1.1 3.2 ◎ ○ Inventive Example 4 -10 2566 1000 0.4 3.5 ◎ ○ Inventive Example 5 -5 3964 1200 0.3 3.4 ◎ △ Comparative Example 3 0 6025 1300 0.2 3.8 ◎ × Conventional Example 1 -50 39 25500 654 3.2 × ○ Conventional Example 2 -40 127 26000 205 3.5 △ ○ Conventional Example 3 -30 376 24700 66 3.5 ◎ ○ Conventional Example 4 -20 1021 28500 28 3.7 ◎ ○ Conventional Example 5 -10 2566 23400 9 3.7 ◎ ○ Conventional Example 6 -5 3964 29700 7 3.8 ◎ △ Conventional Example 7 0 6025 22200 3.7 3.3 ◎ ×

Metal dust generated from the snout was collected and visually evaluated and evaluated according to the following criteria.

◎: not generated at all

○: Extremely small amount does not affect defects

△: slightly occurs and causes defect

×: considerable amount generated and defect occurred

××: A large amount of defects occurred.

Plating adhesion evaluation, after taking the product, was processed by using a rock forming test machine, the processed edges were visually evaluated and the degree was evaluated by the following criteria.

○: Good adhesion due to no plating peeling at the edge of machined part

△: Only a small amount of plating peeling occurs at the edge of the machined part

×: Bare spots occur on the plated surface of the plated product, resulting in no product value

As described above, according to the present invention, the gas atmosphere is artificially controlled and injected into the snout so that the ratio of hydrogen: water vapor is less than 4: 1, and the water vapor content is lower than -40 ° C and lower than -5 ° C at the dew point. By forming the atmosphere in the nut, it is possible to provide a technology for suppressing the generation of metal dust in the snout and to suppress the occurrence of surface peeling, unplating or surface defects caused by metal dust in the molten metal plating process. In addition, it is expected to improve the surface quality of GI steel.

Claims (3)

In hot metal plating, the ratio of hydrogen to water vapor is reduced in order to prevent metal dust from forming inside the snout, which is a part of connecting the reduction annealing furnace and the port of the plating equipment to the port where the plating bath is accommodated. To suppress the generation of metal dust in the snout of the molten metal plating equipment, characterized by injecting an atmospheric gas into the snout so that the water content is less than 4: 1 and the water vapor content is -40 ° C or more and less than -5 ° C at the dew point. Way. The method for suppressing metal dust generation in a snout of a molten metal plating apparatus according to claim 1, wherein an atmosphere gas is injected into the snout such that the water vapor content is less than -30 ° C and less than -5 ° C at a dew point. . The method of claim 1 or 2, wherein the atmosphere gas injected into the snout is water vapor, an inert gas including water vapor, or a mixture of these gases to generate metal dust in the snout of the molten metal plating facility. Method for suppression.