JP2000239817A - PRE-HOT DIP Zn-Mg-Al BASED ALLOY PLATED STEEL SHEET EXCELLENT IN CORROSION RESISTANCE IN CUT EDGE FACE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the corrosion resistance of a cut edge face in a plated steel sheet by controlling the contents of Mg and Al in a pre-hot dip galvanizing based plating layer to specified ranges and moreover incorporating small amounts of Ti and B therein. SOLUTION: The compsn. of a plating layer in a hot dip Zn-Mg-Al based alloy plated steel sheet is composed of, by weight, 1 to 4% Mg, 4 to 10% Al, and the balance Zn with inevitable impurities. Or, 0.002 to 0.1% Ti and 0.001% to 0.045% B are moreover incorporated therein. Mg promotes the formation of Mg-contg. Zn based corrosion products high in protecting properties on the plating layer and reduces the corrosion rate of the plating layer itself. Moreover, a part of the corrosion products flows into the surface of the steel base in the cut edge face to suppress the corrosion of the steel base. Al in the plating layer hardly elutes from the plating layer, and Zn-Al based corrosion products are formed on the lower layer of the Zn based corrosion products high in fluidity. By the addition of Ti and B, the formation of a Zn11Mg2 phase damaging the surface appearance can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pre-fused Zn--Mg--Al alloy-plated steel sheet having excellent corrosion resistance at cut edges. The pre-hot-dip coated steel sheet refers to a steel sheet obtained by hot-dip coating a steel strip with a continuous hot-dip plating apparatus in contrast to post hot-dip coating in which hot-dip coating is applied to a molded product.

[0002]

2. Description of the Related Art When a hot-dip Zn-coated steel sheet is formed from a steel sheet into a product, a cut end face where a steel base is exposed without being covered with a hot-dip coating layer is inevitably generated. In the current pre-hot-dip Zn-coated steel sheet, when the corrosion resistance of the cut end face is concerned, a measure is taken to repair and coat only the end face portion with a Zn-containing rust preventive paint or the like.

[0003]

The repair coating of the cut end face portion of the pre-dip Zn-coated steel sheet is technically easy, but causes a reduction in productivity and an increase in cost. In addition, since there is an essential problem such as deterioration of the coating film over time, the pre-melted Z having excellent corrosion resistance at the cut end face.
Development of an n-type plated steel sheet is desired.

[0004]

SUMMARY OF THE INVENTION Accordingly, the present invention has been devised for the purpose of improving the corrosion resistance of the cut end face of a pre-dip Zn-coated steel sheet. The first gist of the present invention is that the components of the plating layer are Mg: 1 to 4% by weight, Al: 4
To 10% by weight, the balance being Zn and unavoidable impurities. The second point is that the plating layer has an additional 0.002
About 0.1% by weight of Ti and 0.001 to 0.045% by weight of B.

[0005]

DETAILED DESCRIPTION OF THE INVENTION Fused Zn-Mg-A according to the invention
The 1-based alloy plated steel sheet has a composition of a plating layer of Mg: 1.
-4% by weight, Al: 4-10% by weight, balance consisting of Zn and unavoidable impurities, or the plating layer is further composed of 0.002-0.1% by weight of Ti and 0.001-0.1%.
It is characterized by containing 0.045% by weight of B.

[0006] Mg in the plating layer promotes the formation of a very protective Mg-containing Zn-based corrosion product containing Mg on the plating layer, and has the effect of significantly reducing the corrosion rate of the plating layer itself. In addition, the inventors have found that some of these corrosion products flow into the steel substrate at the cut end face, and have a remarkable effect of suppressing corrosion of the steel substrate. If the Mg content is less than 1% by weight, such an effect is not sufficient, while if it exceeds 4% by weight, the effect is saturated. Therefore, Mg is preferably contained in an amount of 1 to 4% by weight.

While Zn and Mg in the plating layer elute from the plating layer to form a Mg-containing Zn-based corrosion product,
Al in the plating layer hardly elutes from the plating layer, and a Zn-Al-based corrosion product is formed in the portion that was originally the plating layer. Therefore, the Zn-Al-based corrosion product is formed in a lower layer of the Mg-containing Zn-based corrosion product having relatively high fluidity. The present inventors have found that the Zn-Al-based corrosion product has extremely strong adhesion, so that even if the Mg-containing Zn-based corrosion product in the upper layer flows out to the cut end face steel substrate,
-It has been found that an Al-based corrosion product does not act as a barrier to rapidly corrode the underlying plating layer and steel substrate. If the plating layer does not contain Al and the plating layer is a Zn-Mg-based alloy, and the Mg-containing Zn-based corrosion product flows out, there is no corrosion product having strong adhesion to the underlayer, so that the plating layer Corrosion will occur rapidly. In this case, Mg-containing Zn plays an effect of suppressing corrosion of the steel base.
Since the system corrosion products are depleted at an early stage, the corrosion of the cut end face steel base material starts early. In order to form a Zn—Al-based corrosion product having a strong adhesion and a barrier effect on the base, 4 wt% or more of Al is required. However, when the content exceeds 10% by weight, since the Fe-Al intermetallic compound is locally generated at the interface between the plating layer and the steel base, there is a concern that the plating layer may be partially peeled during the secondary processing. Also, from the viewpoint of weldability, the addition of 10% by weight or more should be avoided. Therefore, Al is 4 to 10
% By weight.

The present inventors have previously described Japanese Patent Application Laid-Open No. 10-226865.
And Japanese Patent Application No. 10-57446, have proposed a hot-dip Zn-Al-Mg-based plated steel sheet having improved corrosion resistance and surface appearance. In particular, Japanese Patent Application No. 10-57446 discloses a method for improving the surface appearance of a plating layer.
l: 4 to 10% by weight, Mg: 1 to 4% by weight, balance of Zn and unavoidable impurities in a plating bath further containing Ti:
0.002 to 0.1% by weight, B: 0.001 to 0.04
It was proposed to add 5% by weight. That is, Ti,
The addition of B suppresses the generation of a Zn ₁₁ Mg ₂ phase that impairs the surface appearance, and facilitates the use of substantially only Zn ₂ Mg as the Zn—Mg based intermetallic compound crystallized in the plating layer.

Specifically, 0.002% by weight or more of Ti
, The generation of a Zn ₁₁ Mg ₂ phase can be suppressed. However, if the Ti content exceeds 0.1% by weight, a Ti—Al-based precipitate grows in the plating layer, and irregularities (bubbles) occur in the plating layer, and the appearance is impaired. Therefore, it is preferable to contain 0.002 to 0.1% by weight of Ti.

When B is contained in an amount of 0.001% by weight or more, generation of a Zn ₁₁ Mg ₂ phase can be suppressed. However, when the B content exceeds 0.045% by weight, Ti-B-based and Al-B-based precipitates grow in the plating layer, and irregularities (thumps) are generated in the plating layer, and the appearance is impaired. Therefore, it is preferable to contain 0.001 to 0.045% by weight of B.

[0011]

Example 1 Plated steel sheet thickness: 3.2 mm Al content in plating layer: 3.1, 4.2, 10.0% by weight Mg content in plating layer: 0 for each of the above Al contents , 0.5, 0.8, 1.2, 2.0, 3.0,
4.3% by weight Corrosion acceleration test: Salt spray test (JIS Z2371) Plating weight: 120 g / m ² (per side) Chemical conversion treatment: General chromate treatment Cutting method: Shearing

Under the above conditions, the red rust resistance of the cut end faces of various plated steel sheets was evaluated. The change in the appearance of the cut end face was as follows. At the beginning of the test, a small amount of iron is corroded and slightly red rust is generated. However, a Zn-based corrosion product (white rust) flows over the cut end surface and covers the red rust. Further, when the test time is prolonged, red rust is generated again from under the white rust, and thereafter the red rust monotonically increases. The evaluation of red rust resistance was performed at the second red rust occurrence time at which iron substantially started corrosion. In the evaluation, an index G defined by the following calculation was used to clarify the effect of Mg. When the Al content is the same, the index G in the Mg content: X wt% is (Mg
The second red rust generation time of the cut end face at a content of X% by weight) ÷ (the second red rust generation time of the cut end face at a Mg content of 0% by weight). Table 1 shows the evaluation results.

[0013]

[Table 1]

As shown in Table 1, when the Al content is within the range of the present invention, the index G is significantly increased when the Mg content is about 1% by weight or more, that is, the red rust resistance of the cut end surface is improved. It is clear that. However, even if Mg is added in excess of 4% by weight, the effect is saturated. Also, Al
If the content is less than 4% by weight, no remarkable improvement in red rust resistance is observed even if Mg exceeds 1% by weight.

[0015]

Example 2 Al content in plating layer: 6.2% by weight Mg content in plating layer: 0, 0.8, 1.2, 2.0,
3.0, 4.4% by weight Ti content in the plating layer: 0.03% by weight B content in the plating layer: 0.006% by weight The conditions other than the above are the same as in Example 1, and the red rust resistance of the cut end surface is the same. investigated. Table 2 shows the obtained results.

[0016]

[Table 2]

As shown in Table 2, when Al, Ti, and B are within the range of the present invention, the effect of improving the red rust resistance when Mg is increased in the plating layer is the same as that when Ti and B are not contained. It can be considered similar.

[0018]

As described above, the molten Zn of the present invention
-The Mg-Al alloy plated steel sheet shows excellent corrosion resistance at the cut end face.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Toshiharu Tachibana Takashi Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd. Technology Research Laboratory Surface Treatment Laboratory F term (reference) 4K027 AA02 AA05 AA22 AB05 AB44 AC82 AE21

Claims (2)

[Claims] The composition of the plating layer is as follows: Mg: 1 to 4% by weight;
l: A pre-melted Zn-Mg-Al-based alloy-plated steel sheet having excellent corrosion resistance at cut end faces, characterized by comprising 4 to 10% by weight, with the balance being Zn and unavoidable impurities. 2. The pre-melted Zn— excellent in cut end face corrosion resistance according to claim 1, wherein the plating layer further contains 0.002 to 0.1% by weight of Ti and 0.001 to 0.045% by weight of B. Mg-Al alloy plated steel sheet.