JP3073679B2 - Hot-dip Zn alloy coated steel sheet with excellent initial white rust resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip Zn alloy plated steel sheet having excellent initial white rust resistance.

[0002]

2. Description of the Related Art There have been many proposals for hot-dip Zn alloy coated steel sheets having excellent corrosion resistance. A method for producing a Zn—Mg—Al plated steel sheet that is significantly better than a Zn plated steel sheet is proposed.

[0003]

Recently, in the field of construction materials and civil engineering, hot-dip galvanized steel sheets which are unpainted or coated after processing are used, but these steel sheets are in a chromate-treated state or in a state without chromate. In the processing state, the coil or the cut plate is stored outside for a certain period of time outdoors or in a warehouse, and then processed into a guardrail member, a deck plate, or the like, and used. Recently, during this storage,
Particularly in the rainy season with high humidity, etc., white rust may occur in the early stage of corrosion, and improvement of the white rust resistance has been a new solution. The Zn-Mg-Al plated steel sheet,
It is a steel sheet having excellent corrosion resistance. Although the white rust resistance in the initial stage of the corrosion is excellent as compared with a normal hot-dip galvanized steel sheet, it cannot be said that it is still sufficient, and further improvement is required. Therefore, an object of the present invention is to provide a hot-dip galvanized steel sheet having excellent initial white rust resistance.

[0004]

Means for Solving the Problems The present inventors have proposed Zn-M
The composition and structure of the plating layer of the g-Al plated steel sheet are analyzed in detail, and various experiments are repeated by further considering the added metal. We considered to develop. The present inventors first proposed a Z method proposed in Japanese Patent Application Laid-Open No. 4-147555.
Add trace amounts of various metals to the n-Mg-Al plating layer,
The initial white rust generation state in the salt water corrosion test (SST) was investigated. As a result, it was found that by adding a trace amount of Ni to the plating layer, the distribution of Mg and Al in the plating layer became more uniform, and the white rust resistance in the initial stage of corrosion was improved without chromate treatment. In addition, in the chromate-treated material, it was also found that the effect on white rust resistance became more remarkable by including Ni in the Zn-Mg-Al plating layer. Then, as a result of investigating the optimum composition including Ni in detail, it was found that the Zn alloy plated steel sheet of the present invention was found,
The present invention described below has been completed.

That is, according to the present invention, Mg0.0
5-3% by weight, Al 0.1-1% by weight, Ni 0.01-
A hot-dip Zn alloy-plated steel sheet having excellent initial white rust resistance, characterized by having a Zn alloy-plated layer consisting of 0.2% by weight, the balance being Zn and unavoidable impurities;
A hot-dip Zn alloy-plated steel sheet having excellent initial white rust resistance characterized by having a Ni-plated layer under the n-alloy-plated layer, and a steel sheet obtained by subjecting these hot-dip-plated layers to a chromate treatment.

[0006]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
This will be described in detail. FIG. 1 shows that 0.5 wt% of Mg, Al
135 g / m adhesion amount containing 0.25% by weight ^Two Zn
-In the case of Mg-Al plated steel sheet,
It is the figure which showed the relationship between weight and white rust resistance. Chromate
Without treatment, chromate treatment (20 mg / m
^Two ) Are shown together. As a test method for initial white rust,
200 hours salt spray test (SST) based on JIS
Was used. A 35%, 5% NaCl solution was used. Evaluation group
The criterion is as follows, and a score of 3 or more was judged as passing. For comparison
Also evaluated normal hot-dip galvanized steel sheet (Al 0.25%)
did.

Rating of initial white rust rating 5: White rust less than 5% 4: White rust 5% or more and less than 20% 3: White rust 20% or more and less than 50% 2: White rust 50% or more and less than 80% 1 : White rust 80% or more

[0008] The Zn-Mg-Al plated steel sheet is a normal Z
Better white rust resistance than n-Al plated steel sheet. On the other hand, when Ni is added to the Zn-Mg-Al plated steel sheet, white rust resistance is higher than that of the Zn-Mg-Al plated steel sheet when the Ni in the plating layer is in the range of 0.01 to 0.2% by weight. Significantly improved. The effect is even more remarkable in the case of chromate treatment. Although the reason why the generation of initial white rust is suppressed by the addition of Ni is not yet clear, the distribution of Mg and Al in the plating layer is uniform because the Zn-Mg-Al plating layer contains an appropriate amount of Ni. This is presumed to be due to the improvement in the corrosion resistance of the plating layer itself. If the Ni content in the plating layer is less than 0.01% by weight, white rust resistance deteriorates. It was also found that when Ni exceeds 0.2% by weight, white rust resistance is rather deteriorated. This is thought to be because if the amount of electrochemically noble Ni is excessively increased, a local battery with Zn is likely to be formed and corrosion is likely to occur.

In the Zn—Mg—Al—Ni plating layer of the present invention, the content of Mg is 0.05% to 3% by weight,
Was 0.1 to 1% by weight. Mg in plating layer
The lower limit of the content is set to 0.05% by weight in order to exert an effect on the corrosion resistance of the flat plate and the processed portion. Up to 3
The reason for setting the weight percentage is that if it exceeds this, the plating layer becomes too hard and too brittle to deteriorate the plating adhesion. The reason why the lower limit of the Al content in the plating layer is set to 0.1% by weight is that the plating adhesion becomes good.
The reason why the upper limit is set to 1% by weight is that if it exceeds 1% by weight, the corrosion resistance of the processed portion deteriorates. The effects of the present invention can be maintained even if Fe, Sb, or Pb is contained alone or in a composite within 1% by weight in the plating layer.

[0010] There is no particular limitation on the amount of Zn-Mg-Al-Ni-based plating, but from the viewpoint of corrosion resistance, it is not limited.
0 g / m ² or more, 350 g / m ² from the viewpoint of workability
It is desirable that it be ² or less. In the present invention, when further improving the corrosion resistance of the processed portion, Zn-Mg-A
An Ni plating layer is provided below the l-Ni-based plating layer. This pre-Ni plating amount is preferably ² g / m ² or less. 2g
/ M ² , the plating adhesion deteriorates. The lower limit of the amount of Ni plating is preferably 0.2 g / m ² . The reason why the corrosion resistance of the processed portion is good when the Ni plating layer is provided below the plating layer is that Ni-Al-F generated at the plating layer-ground iron interface.
It is presumed that the e-Zn compound phase plays a role of one kind of binder.

In order to further improve the initial white rust resistance, a chromate layer is provided on the Zn-Mg-Al-Ni-based plating layer. There are no particular restrictions on the composition and amount of the chromate-treated layer, but they are within the normal practical range of 1
About 0 to 50 mg / m ² is desirable. When the chromate treatment layer is provided as an upper layer, the white rust resistance of the Zn—Mg—Al—Ni plating layer is particularly improved because the Mg and N in the plating layer are different.
It is presumed that i is taken in a trace amount into the chromate film to form a film with higher corrosion resistance. As the base steel sheet of the present invention, both hot-rolled steel sheet and cold-rolled steel sheet can be used, and the type of steel is ultra-low carbon steel sheet added with Al-killed steel, Ti, Nb, etc. Various materials such as high-strength steel to which elements are added can be applied.

Next, the method for producing the plated steel sheet of the present invention is not particularly limited. For example, a conventional hot-dip galvanizing method using a non-oxidizing furnace with Ni added to a bath can be applied. After plating using an ordinary alloying furnace, an alloying treatment may be performed. In the case of applying Ni pre-plating as the lower layer, a pre-plating method which is usually performed may be applied. As described above, after applying a pre-Ni plating amount of 0.2 to ² g / m ² , non-oxidizing or reducing atmosphere is applied. A method of performing rapid low-temperature heating followed by hot-dip plating is preferred. The method for applying the chromate treatment layer to the upper layer may be in accordance with a conventional method.

[0013]

EXAMPLE Zn-Mg in which the amount of Ni in a bath at 450 ° C. was changed on the basis of an Al-killed hot-rolled steel sheet (sheet thickness 1.6 mm).
Performed 3s molten plating -al-Ni plating bath was adjusted coating weight to 135 g / m ² by N ₂ wiping.
Table 1 shows the composition in the plating layer of the obtained plated steel sheet. For some samples, a Ni pre-plating layer was
Alternatively, a 20 mg / m ² chromate treated layer was applied to the upper layer. With respect to these plated steel sheets, the evaluation of the above-mentioned white rust resistance and the corrosion resistance evaluation of the overhang processing portion were performed.

(1) Evaluation of White Rust Resistance An SST test in accordance with the above-mentioned JIS was performed, and the occurrence rate of white rust was evaluated. The scores are as described above. (2) Corrosion resistance of processed part A cylinder with a ball head punch diameter of 10 mm and a height of 10 mm is formed,
SST6hr → Dry 3hr → Wet 14hr → Dry 1hr
Was used as one cycle, and a 60-cycle CCT test was performed. Then, the corrosion resistance was determined by scoring the state of occurrence of red rust as shown below. A score of 3 or higher was considered a pass. 5: Less than 5% 4: 5% or more and less than 10% 3: 10% or more and less than 20% 2: 20% or more and less than 30% 1: 30% or more The evaluation results are shown in Table 1.

[0015]

[Table 1]

The plated steel sheet having the composition of the plating layer of the present invention has excellent initial white rust resistance and also excellent corrosion resistance of the processed portion. On the other hand, the comparative example in which the plating layer composition deviates from the range of the present invention is inferior in the initial white rust resistance and the corrosion resistance of the processed portion as compared with the steel sheet of the present invention.

[0017]

The Zn-Mg-Al-Ni-based alloy-coated steel sheet of the present invention has a significantly improved white rust resistance and an excellent corrosion resistance in the processed part by containing an appropriate amount of Ni in the plating layer. It is useful for applications such as building materials, civil engineering, and car bodies.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between Ni content in a Zn—Mg—Al—Ni plating layer and corrosion resistance.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-320848 (JP, A) JP-A-5-98407 (JP, A) JP-A-4-247860 (JP, A) JP-A-4- 147955 (JP, A) JP-A-2-298243 (JP, A) JP-A-61-199063 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/00-2 / 40

Claims (3)

(57) [Claims] 1. A Zn alloy plating layer composed of 0.05 to 3% by weight of Mg, 0.1 to 1% by weight of Al, 0.01 to 0.2% by weight of Ni and the balance of Zn and unavoidable impurities is formed on the surface of a steel sheet. A hot-dip Zn alloy-plated steel sheet having excellent initial white rust resistance characterized by having: 2. A hot-dip Zn alloy coated steel sheet having excellent initial white rust resistance, characterized by having a Ni plating layer below the Zn alloy plating layer according to claim 1. 3. A hot-dip Zn alloy-plated steel sheet having excellent initial white rust resistance, characterized in that a chromate layer is provided on the hot-dip Zn coating layer according to claim 1 or 2.