JPH1112709A - Dip plating method of steel product - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method of a steel product having sufficient corrosion resistance on a whole face of the steel product even in a severe corrosive environment and forming a plating layer in which a surface is flat and excellent in external appearance and which has a high product value. SOLUTION: (1) After coating flux on a surface of a steel work, drying and by dipping in a dip plating bath (first bath) consisting of Zn and inevitable impurities, pre-Zn plating of >=10 g/m<2> is applied on the surface of the steel work and then the work is dipped in a dip plating bath (second bath) consisting of 0.1-15% Mg and the balance Zn with inevitable impurities. (2) The first bath contains 0.1-0.5% Al. (3) Pre-Zn plating is conducted by electrolytic plating. (4) A steel work, in which the Zn plating layer of >=10 g/m<2> on the surface is applied, is dipped in a dip plating bath consisting of 0.1-15% Mg and the balance Zn with inevitable impurities as mentioned in (1).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for immersion plating of steel products such as steel towers and guardrails (main bodies and columns) used for construction and civil engineering, and more particularly, to a method for improving the corrosion resistance and surface quality of steel products. The present invention relates to a method for forming an excellent plating layer.

[0002]

2. Description of the Related Art In general, steel processed products are used after being galvanized for the purpose of extending their life, and the zinc plating is performed by degreasing, washing with water, pickling, washing with water, applying flux or dipping the steel processed product. After treatment and drying, the Al content is 0.1-0.
This is performed by immersing in a zinc bath containing 2%. Zinc is a metal which is inexpensive, has stable compounds formed in the atmosphere and in water, has an appropriate corrosion rate, and is suitable for corrosion prevention of steel products. However, the corrosion rate of zinc differs depending on the environment, and there is a problem that the corrosion rate sharply increases, especially in an environment containing chlorine ions such as a coast. In order to maintain corrosion resistance even in the above environment, Mg is added to the Zn plating layer,
The effectiveness of stabilizing corrosion products is shown in FIG. 3 of JP-A-56-96062.

[0003] Japanese Patent Application Laid-Open No. 56-96062 discloses that
The present invention relates to a method for producing a hot-dip steel strip by a hot-dip galvanizing line of a Sendzimer method, and comprises 0.1 to 2.5% of Mg,
A highly corrosion-resistant hot-dip galvanized steel in which an area in which a plated metal adhered to the surface of a steel strip that has passed through a zinc bath containing 0.1 to 3.0% of Al is in an unsolidified state is surrounded by a seal box and an oxygen concentration is 5000 ppm or less. Obtaining a belt. The purpose of adding Mg is to stabilize the corrosion products of the plating layer and improve the corrosion resistance.
It is disclosed that the purpose of the addition is to suppress the growth of the Fe—Zn alloy layer and to prevent oxidation of the bath and the plating layer.

[0004] For example, Japanese Patent Publication No. 60-43430 discloses degreasing, washing with water, pickling with concentrated hydrochloric acid containing a corrosion inhibitor, washing with water, pickling with concentrated hydrochloric acid containing no corrosion inhibitor,
After washing with water, flux application and drying, steel containing 0.01% or more of silicon has Al 0.01-0.5%, Mg 0.00
By immersing in a zinc bath containing 1 to 0.1% and 0.03 to 2.0% Sn, a plating layer having excellent appearance and adhesion is formed on the surface of steel containing 0.01% or more of silicon. Is disclosed. It is also disclosed that the purpose of Mg and Sn coexisting in the zinc bath is to prevent non-plating.

[0005]

SUMMARY OF THE INVENTION The present inventors immerse a normal temperature steel product after degreasing, washing with water, pickling, washing with water, applying flux and drying in a zinc bath containing Mg for the purpose of improving corrosion resistance. Then, when plating was applied (hereinafter referred to as a conventional method), when 0.1% or more of Mg was added to the zinc bath, a large number of unplated portions were generated, and the catalytic action of Mg, which is an active metal, caused Elution of processed steel products becomes intense, the amount of Zn-Fe dross generated becomes enormous, and if dross removal is not carried out at high frequency, the surface roughness becomes rough due to dross adhesion, and plating However, it was also found that there was a problem in appearance quality. The present invention
By solving the above problems, the entire surface of the steel product has sufficient corrosion resistance even in severe corrosive environment, the surface is smooth,
It is another object of the present invention to provide a method of immersion plating of a processed steel material capable of forming a plating layer having excellent blackening resistance and excellent appearance quality and a high commercial value.

[0006]

The immersion plating method for a steel product according to the present invention comprises applying a flux to the surface of the steel product, drying the flux, and forming a hot-dip plating bath (first bath) comprising Zn and unavoidable impurities. ) And pre-Zn plating of 10 g / m ² or more on the surface of the steel product, and then Mg 0.1 to 15
%, With the balance being immersed in a hot-dip plating bath (second bath) consisting of Zn and unavoidable impurities to achieve the above object. Particularly important here is the adhesion amount of the pre-Zn plating, as described later. Further, by containing 0.1 to 0.5% of Al in the first plating bath, the above object is achieved and good plating adhesion is provided. In addition, the pre-Zn plating is performed with a necessary amount of Zn.
And electrolytic plating can be employed instead of immersion plating. Further, a steel product obtained by processing a steel material having a Zn plating layer of 10 g / m ² or more on the surface may be immersed in the second bath.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 shows a conventional flux treatment method (FIG. 1) of a steel material obtained by processing a hot-rolled Al-killed steel into a column.
a) and the pre-plating method of the present invention (FIG. 1b,
The difference of the manufacturing process using c) is shown. FIG.
As described above, in the conventional method, a treatment is performed using a flux to prevent oxidation of the steel material surface, and the steel material is directly immersed in a Zn bath. The inventors performed Zn plating before immersion in the second bath in order to prevent oxidation of the steel material surface and suppress the reaction of the steel material surface (FIGS. 1B and 1C).

[0008] Table 1 shows that a steel material obtained by subjecting a hot-rolled Al-killed steel to a cylindrical process has a conventional flux (ZnCl ₂ / NH _4).
Cl = 300/100 g / l), dried at 100 ° C., and pre-Zn plated (bath temperature 450 ° C.) at 50 g / m
² After washing with water and drying, Zn containing 1% Mg at room temperature
A steel material obtained by immersion in a plating bath (bath temperature 450 ° C.) for 1 minute and a flux (ZnCl ₂ / NH ₄₎
Cl = 300/100 g / l), dried at 100 ° C., and directly immersed in a Zn plating bath (bath temperature 450 ° C.) containing Mg = 1% at room temperature for 1 minute, and a conventional steel material. Flux (ZnCl ₂ / NH ₄ Cl = 300)
/ 100 g / l), dried at 100 ° C, and dried at room temperature.
The results of comparison of the performance of steel materials obtained by immersing in a Zn plating bath (bath temperature 450 ° C.) containing no g for 1 minute are shown.

[0009]

[Table 1]

[0010] The amount of dross generated was compared relatively based on the amount of dross generated when a conventional method was used in a zinc bath containing no Mg. The appearance was evaluated using a score in consideration of the presence or absence of non-plating and the surface roughness. In the corrosion resistance test, the corrosion weight loss measurement after 72 hours of the salt spray test was performed, and 0.1 g
/ M ² · hour or less was regarded as acceptable. The appearance was evaluated by a five-point method, and three or more points were regarded as acceptable. The evaluation criteria are as follows. As shown in Table 1, a steel material subjected to Zn plating in the first bath has a small amount of dross, and has no problem in workability and appearance.

[0011]

[Table 2]

FIG. 2 shows the relationship between the amount of Mg added and the corrosion resistance (corrosion loss) by examining the corrosion loss of steel obtained by changing the amount of Mg added in the range of 0 to 20% in the above method. It is a thing. As shown in FIG.
If it is less than 1%, sufficient corrosion resistance cannot be obtained, and if it exceeds 15%, sufficient corrosion resistance cannot be obtained. When the plating adhesion is required, the first bath may be added with 0.1% Al.
It is preferably set to 1 to 0.5%, particularly preferably 0.2 to 0.5%. This is because, as shown in FIG. 3, if it is less than 0.1%, sufficient plating adhesion cannot be obtained, and if it exceeds 0.5%, a blackening phenomenon occurs and the product value deteriorates in appearance.

As described above, according to the present invention, Japanese Patent Application Laid-Open No. 56-96
It can be seen that immersion plating is possible with a wider Mg concentration than the production method using a hot-dip plating line of the Sendzimer method described in JP-A-062. It is presumed that this is because the temperature and time during bath immersion are different from the Sendzimer-type manufacturing method. In other words, in the immersion plating method, steel materials are usually
Since it is immersed at room temperature and the presence of the Zn plating layer, the reactivity of Fe with Mg and Fe seems to be greatly different.

The amount of the pre-Zn plating is important, and the amount of the coating in the present invention is 10 g / m ² or more. If it is less than 10 g / m ² , the pre-Zn is eluted instantaneously in the second bath, and immersed in the plating bath in a state where Fe is partially exposed. As a result of further studies, it has been found that the pre-plating method is not particularly limited, and even if an electroplating method or the like is used as long as the required amount of Zn is present, no problem occurs. That is,
After cleaning the Fe surface in the degreasing step, plating can be performed by electrolysis at 1 to 100 A / dm ² in a zinc sulfate bath using a steel plate as a cathode.

[0015] Based on the same concept, 10 g / m
A steel product obtained by processing a steel material having a Zn plating layer containing ^two or more unavoidable impurities contains 0.1 to 15% of Mg, and the remainder is immersed in a hot-dip plating bath containing Zn and unavoidable impurities. It has also been found to have excellent performance. In addition, Sn and Sb are 3% in the second plating bath.
When added below, a shiny plating appearance with a spangle pattern was obtained.

In the pre-plating method of the present invention, the mechanism for suppressing the generation of dross and improving the appearance quality is considered as follows. Mg is an element having high reactivity, which promotes the elution of Fe and is considered to cause a large amount of dross. Therefore, by performing pre-Zn plating, it has become possible to prevent Fe from directly reacting.
This is the point of the present invention. That is, the adhesion amount of the pre-Zn plating is important, and the pre-Zn plating of 10 g / m ² or more is required in the range of the present invention.

[0017]

【Example】

(Example 1) Table 3 shows examples of the present invention, comparative examples, and conventional examples. This is a result of a case where hot rolled Al-killed steel is subjected to immersion plating using a steel material processed into a column. In the examples, unless otherwise specified, the flux (Zn) was used for the pretreatment before the first bath immersion.
Cl ₂ / NH ₄ Cl = 300/100 g / l)
After drying at 100 ° C., it is immersed in a predetermined second bath. The steel temperature during bath immersion is room temperature, and the plating bath immersion time is
One minute. In the test of black discoloration, the test was left standing indoors for 2 weeks, and the occurrence of black discoloration was visually observed. The plating adhesion was evaluated by a scoring method based on the following criteria.
Other evaluations (dross generation amount, appearance, corrosion resistance) were performed based on the evaluation criteria described above.

[0018]

[Table 3]

Nos. 1 to 8 were immersed in a zinc bath (450 ° C.) containing unavoidable impurities for 10 seconds as a first bath, adjusted to the adhesion amount in the table by gas wiping, washed with water,
After drying, it was immersed in a second bath (450 ° C.) containing Mg for 1 minute. The obtained product performance is good within the range of the present invention except for the plating adhesion. For example, when the pre-Zn plating amount is small (No. 1), the appearance and the amount of dross are inferior. When the Mg concentration is low in the second bath (No.
4) On the contrary, when it is too high (No8), the corrosion resistance is poor. In this case, since the plating adhesion is assumed to be a final processed product, the plating adhesion is not a necessary characteristic to be provided.

In Nos. 9 to 11, the first bath was immersed in a zinc bath containing Al and then immersed in a second zinc bath containing Mg. Al is used for ensuring plating adhesion, and becomes an indispensable characteristic when light processing is performed after this processing. That is, when the Al concentration is less than 0.1% (No.
9) is inferior. Further, when the content exceeds 0.5% (No. 11), black discoloration occurs and the commercial value is impaired. No. 12 passed a current of 1 A / m ^{2 in} a bath composed of zinc sulfate, and electrogalvanized 20 g.
/ M ² pre-plated and immersed in a bath containing Mg. Good product performance is obtained as in the case of hot-dip galvanizing. Further, since the electrogalvanized itself has excellent adhesion, good plating adhesion can be obtained regardless of whether or not Al is contained. No. 14 is a conventional method, in which the amount of dross generated is small, but the corrosion resistance is not sufficient. The bath containing Mg and using the conventional flux (No. 13) has a poor product appearance, generates a large amount of dross, and has a large work load.

[0021]

[Table 4]

(Example 2) Table 5 shows that various commercially available Zn-plated steel sheets were subjected to columnar processing and then immersed in a bath (450 ° C) containing 1% Mg and the balance consisting of Zn at a steel material temperature of room temperature. The results are shown. See FIG. 1D for the manufacturing flow. No. 1 is a bath (450 ° C.) in which an electrogalvanized steel sheet (Al-killed steel, 1.6 mm, adhesion amount: 20 g / m ² ) is cylindrically processed to an inner diameter of 30 mm as a material, and contains 1% Mg and the balance is Zn. In this case, the steel was immersed at room temperature. Both performances are good. No2
Is a hot-dip galvanized steel sheet (Al-killed steel,
This is a case in which a cylindrical member having an inner diameter of 1.6 mm and an adhesion amount of 60 g / m ² ) is processed into a bath (450 ° C.) containing 1% of Mg and the balance of Zn at a room temperature of 450 ° C. Both performances are good. Incidentally, as a result of the analysis, the hot-dip galvanized steel sheet as the material contained 0.2% of Al.

[0023]

[Table 5]

[0024]

As described above, according to the present invention, the entire surface of a steel material processed product has a sufficient corrosion resistance even in a severe environment, has a smooth surface and is excellent in appearance quality, and has a high commercial value. It is possible to provide a method of immersion plating of a steel product on which a layer can be formed, and the effect is large.

[Brief description of the drawings]

FIG. 1 is a simplified illustration of the difference between the conventional process and the manufacturing process of the present invention.

FIG. 2 shows the relationship between the amount of added Mg and corrosion resistance.

FIG. 3 shows the relationship between the amount of Al added and plating adhesion.

Claims (4)

[Claims] 1. A flux is applied to the surface of a steel product,
After being dried, it is immersed in a hot-dip plating bath (first bath) composed of Zn and unavoidable impurities, and is immersed in the surface of the processed steel product at 10 g / m ²
After the above pre-Zn plating, containing Mg 0.1-15%,
An immersion plating method for a steel product, wherein the remainder is immersed in a hot-dip plating bath (second bath) comprising Zn and unavoidable impurities. 2. The immersion plating method for a steel product according to claim 1, wherein the first bath contains 0.1 to 0.5% of Al. 3. The method according to claim 1, wherein the pre-Zn plating is performed by electrolytic plating. 4. A steel product obtained by processing a steel material having a Zn plating layer of 10 g / m ² or more on its surface, containing 0.1 to 15% of Mg according to claim 1, and the balance being Zn and unavoidable. An immersion plating method for a steel product, wherein the immersion plating is immersed in a hot-dip plating bath containing impurities.