JPS6138259B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for hot-dip galvanizing steel materials, and particularly to a method for hot-dip galvanizing zinc alloys containing 0.1% or more of aluminum in order to impart high corrosion resistance to steel materials. In recent years, the uses of galvanized steel products have been expanding rapidly, and along with this, a high degree of corrosion resistance has been required. Furthermore, as the thickness of structural materials decreases as the weight of structural materials decreases, the amount of rust is reduced accordingly, so there is a need for a galvanizing method that provides even more improved corrosion resistance. Generally, a method of increasing the amount of zinc plating deposited is adopted as a method for improving the corrosion resistance of galvanized products. To this end, it is necessary to increase the immersion time of the steel material in the galvanizing bath and to increase the rate of withdrawal from the galvanizing bath.
Increasing the soaking time increases the thickness of the alloy layer and increasing the pulling rate increases the thickness of the zinc layer. In addition to complicating process control, such methods
The following problems also arise with the resulting galvanized products: (1) The plating adhesion and workability of the galvanized products deteriorate. (2) Since the immersion time is long, the amount of alloy layer formed by the reaction between Fe-Zn increases, and the proportion of the Fe-Zn alloy layer in the entire galvanized layer increases. This presents serious drawbacks in the following points: (a) The Fe-Zn alloy layer reaches the surface of the plating layer, causing a phenomenon called so-called burnt plating.
It impairs the plating appearance and reduces the product value. (b) Corrosion resistance deteriorates as the proportion of the Fe-Zn alloy layer increases. For these reasons, conventional approaches to increasing the amount of zinc deposited are unsatisfactory, and their intended improvement in corrosion resistance is limited. Therefore, there is a need for a new hot-dip galvanizing method that does not present the above-mentioned problems and can increase corrosion resistance. The present inventor has discovered that by forming a zinc alloy layer containing 0.1% or more of aluminum on a steel material, a tamped layer with excellent corrosion resistance can be obtained. Furthermore, corrosion resistance is further improved by including 0.01 to 5% of one or more of magnesium, copper, titanium, and zirconium in addition to aluminum. However, it is impossible to perform zinc alloy plating containing 0.1% or more of aluminum using the conventional hot-dip plating method that has been used industrially. That is, in the conventional method, the steel material is subjected to pretreatment steps such as degreasing, water washing, pickling, water washing, and fluxing, and then immersed in a molten zinc bath for a predetermined period of time, and then the material is pulled out of the bath. However, even if an attempt is made to obtain aluminum-zinc alloy plating by adding aluminum to the molten zinc bath, if the aluminum concentration exceeds 0.1%, some parts of the plating layer will not adhere, making it unusable for practical use. Become. Therefore,
Formation of zinc alloy plating containing more than 0.1% aluminum cannot be achieved by simply immersing the steel material in a bath of that composition. Thus, other measures must be taken to stably form an aluminum-zinc alloy plated layer on an industrial machine. In the present invention, this problem has been solved by employing a two-step method in which the steel material is galvanized by a conventional method and then immersed in a zinc alloy bath containing the required amount of aluminum. By doing so, the above-mentioned problems do not occur, and a matted layer is formed which is extremely corrosion resistant and has excellent appearance, adhesion, workability, etc. The invention thus provides a first step of hot-dip galvanizing a steel material in a zinc bath consisting of distilled zinc, electrolytic zinc or purest zinc or containing less than 0.1% aluminum, followed by A second step of hot-dipping in a zinc bath containing aluminum or a zinc bath containing 0.1% or more aluminum and 0.01 to 5% at least one of magnesium, copper, titanium and zirconium. Provides a hot-dip galvanizing method for steel materials. In the past, a small amount of aluminum was added to the zinc bath in order to apply zinc plating thinly and uniformly to items with complex shapes and to improve the luster, but this was done only when the amount of aluminum was less than 0.05%. However, unlike the present invention, it is not intended to improve the corrosion resistance of zinc plating. In addition, in order to avoid the generation of harmful gases associated with the use of flux, a two-step method has been proposed in which the surface of the steel piece to be plated is electrogalvanized and then immersed in a molten zinc bath. , its purpose and formed plating layer are completely different from the present invention. In the present invention, the melt gluing pretreatment and the melt gluing operation itself can be performed as conventionally and their explanation will be omitted. The first step, hot dip galvanizing, is carried out by immersing the material in a high purity zinc bath, such as 99.9% or higher distilled zinc, electrolytic zinc or purest zinc. Aluminum may be added to the bath if it is less than 0.1%. The addition of a trace amount of aluminum, less than 0.1%, has little effect on zinc adhesion;
On the contrary, it is effective in applying zinc uniformly and suppressing the expansion of the iron-zinc alloy layer. However, if the amount of aluminum added exceeds 0.1%, the above-mentioned problems will occur, so the amount of aluminum added should be kept sufficiently low at less than 0.1%. The immersion time should be sufficient to bring about adhesion of the zinc layer, and about 1 to 3 minutes is sufficient. The material pulled from the bath in the first stage is 0.1
% or more of aluminum in a zinc bath. If the aluminum content is 0.1% or more, the effect of increasing corrosion resistance will be exhibited. The upper limit of the aluminum content is not critical; more than half, for example up to 55%, and in some circumstances even up to about 70%, is possible. However, in consideration of plating humidity, economical efficiency, etc., it is practically desirable to set the content to 1 to 5%. In addition to aluminum in the second stage bath, magnesium,
At least 0.01 to 5% of a covalent material selected from copper, titanium, and zirconium may be added. This further improves corrosion resistance. The immersion time and pulling speed vary depending on the intended use, but are 1 to 3
Immersion times of the order of minutes are typical. After the two-stage treatment is completed, the product is cooled and dried in the conventional manner and then used or stored. The resulting hot-dip galvanized steel material prevents the occurrence of iron rust over a long period of time, even under adverse environmental conditions in industrial areas, and exhibits excellent corrosion resistance. The corrosion resistance is almost twice as long as conventional galvanized products. When corrosion resistance of 30 years or more is required in heavy industrial areas, the conventional method uses ¹⁰⁰⁰
This method requires a plating amount of more than g.
The amount of plating can be halved to 500 to 600g. The ability to make the plating layer thinner means (1) a reduction in the consumption of zinc metal for plating, (2) elimination of thermal deformation of the component by shortening the plating time, and (3) elimination of discoloration and plating. This is extremely beneficial in terms of improved appearance. In addition, there is no problem with plating adhesion, workability, etc. As described above, the present invention establishes a plating method that is far more corrosion resistant than the conventional method for hot-dip galvanized steel products, which will be increasingly in demand in the future, and has a very thin plating layer that improves plating adhesion, etc. It is extremely significant industrially in that it enables the production of steel materials that exhibit no problems with corrosion and have extremely excellent corrosion resistance. Example 1 Ordinary steel was immersed in an alkaline bath at a temperature of 80°C for 60 seconds to degrease it, washed with water, and then immersed in a 15% sulfuric acid solution (60°C) for 5 minutes to remove rust. After washing with water, the parts are immersed in a solution of ZnCl ₂ -NH ₄ Cl to apply a chloride fluxing coating to prevent oxidation of the parts, and then soaked in a normal high-purity zinc bath (zinc
99.99%) for 2 minutes at a temperature of 460°C. This first stage immersion produced a Fe--Zn alloy layer and a Zn layer on the part, and the part was gently pulled out of the zinc bath. This member was then immersed in a zinc bath (430°C) containing 5% Al for 3 minutes to form a zinc alloy layer containing Al. The appearance of the plating was beautiful. Examples of corrosion rate test results under several environments for galvanized steel materials by the conventional method (hot-dip galvanizing of the same thickness) and zinc alloy-plated steel materials by the present method are shown in Table 1 below (unit: g/ ^m2 /year):

[Table] Example 2 A plated steel material similar to that of Example 1 was prepared except that 1% copper was added in addition to aluminum to the second stage galvanizing bath. When the obtained product was tested for corrosion rate in a heavy industrial area, the corrosion rate was 3.5 per year.
g/m ² and was confirmed to provide excellent corrosion resistance. In addition to copper, magnesium, titanium, and zirconium were also tested in the same amounts, and similar corrosion prevention effects were confirmed. Example 3 The procedure of Example 1 was repeated, except that the high purity zinc bath in the first stage soak of Example 1 was replaced by a high purity zinc bath with 0.05% aluminum added. The results were the same as in the example.

Claims (1)

[Claims] 1. A first step of hot dip galvanizing the steel material in a zinc bath consisting of distilled zinc, electrolytic zinc or purest zinc or containing less than 0.1% aluminum, followed by a zinc bath containing more than 0.1% aluminum or 0.1% aluminum. % or more aluminum and 0.01~5
% of magnesium, copper, titanium and zirconium.