JP2001355051A - Hot-dip Zn-Sn-based plated steel sheet with excellent corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a molten Zn-based alloy having excellent corrosion resistance which is applied to metal building materials such as automobile bodies and roofs / walls.
Provide an Sn-based plated steel sheet. SOLUTION: The steel sheet has a hot-dip Zn-Sn plating layer, and the plating layer composition contains Sn in an amount of less than 1 to 50%, preferably less than 1 to 40% in addition to Zn, and has excellent corrosion resistance. Hot-dip Zn-Sn plated steel sheet. In addition, molten Zn containing 1 to 25% of Cr in steel and having excellent corrosion resistance.
-Sn-based plated steel sheet. Further, the average value of the amount of plating is 25 to 100 g / m ^{2 on} one side, and the standard deviation is 4 g / m 2.
² or less. And the composition of the plating layer is Zn, Sn
In addition, one or more of Mg: 0.2 to 8%, Al: 0.02 to 5%, and Ca: 0.1
-5%, Li: 0.1-5%. In addition, Ni,
This includes having a pre-plating layer containing Co, Fe, Cr, Sn, Zn, and Cu, or having a post-treatment film on the outermost surface of the plating layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-treated steel sheet having excellent corrosion resistance, and more particularly to a surface-treated steel sheet suitable for use as a rust preventive for automobile bodies, metal construction materials such as roofs and walls, and the like. .

[0002]

2. Description of the Related Art It is well known that Zn-plated steel sheets have rust-preventive properties against steel sheets (iron sheets) due to their sacrificial corrosion protection ability. However, because of this, white rust easily occurs, and there is a potential difference from the steel plate more than necessary, which is not sufficient for long-term rust prevention. On the other hand, Sn-plated steel sheets are widely used mainly for food cans and beverage cans because of the excellent corrosion resistance and workability of Sn. However, Sn has a drawback that it does not have an effect of sacrificing corrosion protection of a steel sheet in a normal environment where oxygen is present, and that corrosion from ground iron easily proceeds.
To compensate for this, for example, S containing 20 to 40% of Zn is added.
An n-Zn plated steel sheet is mainly used in the field of post-plating for electronic parts, automobile parts and the like (Japanese Patent Laid-Open No. Hei 6-116).
No. 749). However, until now, the electroplating method has been used, and since the electroplating of Sn has a low current density, it has been difficult to achieve a high deposition amount for reasons of cost and productivity.

On the other hand, the present inventors have found that this Sn-Zn plated steel sheet has excellent characteristics for use in automobile fuel tanks and disclosed in Japanese Patent Application No. 7-69087 and the like a molten Sn having a controlled plating structure. Japanese Patent Application No. 7-132959 discloses a Zn-plated steel sheet having a solidified pattern (spangle) diameter of 20 mm or less as a rust-preventive steel sheet for fuel tanks having excellent workability and corrosion resistance. .

[0004]

SUMMARY OF THE INVENTION The aforementioned molten Sn-Z
The n-plated steel sheet certainly has excellent corrosion resistance, workability, and weldability, and is optimally used for fuel tanks that are processed into a complicated shape and require corrosion resistance on the inner and outer surfaces. However, it is improved for applications such as automobile bodies that require excellent chemical conversion properties and continuous weldability comparable to Zn plating, or for applications where construction materials such as roofs and walls do not like the occurrence of red rust on the cut end surface of steel plates. Was needed.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have studied the optimum composition of the plating layer metals Zn and Sn, and further improved the corrosion resistance in combination with the steel components of the steel sheet. Are found to be improved, leading to the present invention. That is, when the content of Zn, which is a metal constituting the plating layer, exceeds 50% of the plating layer, excellent phosphate treatment (chemical conversion treatment) required for automobile body steel materials is exhibited. Further, the potential of the plating layer is lower than that of the steel sheet, and exerts a sacrificial corrosion prevention action on the steel sheet. Further, by adding Cr to the steel sheet, the corrosion resistance of the steel sheet itself is improved, and due to the hot-dip plating, a small amount of thermal diffusion of Cr in the steel occurs in the plating layer. Thereby, a further improvement in the corrosion resistance of the plating layer can be obtained.
This effect is also advantageous when this steel sheet is used for roof and wall building materials. In other words, the generation of red rust from the cut end surface is avoided by the sacrificial corrosion prevention action of the plating layer, and the long-term edge creep resistance is secured by the excellent corrosion resistance and the improvement of the corrosion resistance of the steel sheet itself as compared with the plating layer of Zn alone. .

Hereinafter, the present invention will be described in detail. Zn in the plating layer is added for imparting a sacrificial anticorrosion effect to the steel sheet, and depending on the environment to be exposed, leads to a plating layer potential lower than that of the steel sheet by several percent or more. However, the addition of a small amount of Zn causes the sacrificial anticorrosion action to be lost early after the elution of Zn is completed, so that a certain amount of Zn must be contained, and the sacrificial anticorrosion period is lengthened accordingly. When the Zn-Sn plated steel sheet, which is the present steel sheet, is used for an automobile body, important factors include phosphoric acid used as a pretreatment for coating, in addition to bare corrosion resistance (unpainted corrosion resistance) that requires corrosion resistance of a plating layer. Has salt treatment properties. If the phosphatability is not enough,
Insufficient appearance of the electrodeposited finish and the adhesion of the coating film after the intermediate coating + overcoating performed thereafter become unstable. Recently, various excellent paints have been developed, and it seems that they are less susceptible to phosphatability than before. There is no doubt that it is an important factor in ensuring corrosion resistance. Experiments by the present inventors have revealed that a sound phosphate film is formed when Zn exceeds 50%.

[0007] On the other hand, as for rust prevention of building material end faces, there is a tendency to dislike the generation of red rust which tends to give an impression that it is corroded from the appearance. The hot-dip galvanized steel sheet of 100% Zn generates white rust, but hardly generates red rust. Just Zn
The leaching rate alone is so high that long-term rust prevention cannot be expected. The present invention controls the properties of Zn and the elution rate of the plating layer, and has better end face corrosion resistance than Zn alone. From the above, the Sn amount in the plating layer is limited to 1 to less than 50%. Desirably, it is less than 1 to 40%.

When the steel sheet is used for various purposes, resistance welding such as spot welding is performed. At this time, since the Cu of the electrode and the plating metal, particularly Sn, easily form a compound, the adhesion amount of the plating greatly affects the weldability. In addition, the amount of plating has a large effect on corrosion resistance. The larger the amount of adhesion, the more advantageous in terms of corrosion resistance and disadvantageous in terms of weldability, but the influence of the amount of adhesion is particularly large. That is, from the balance between weldability and corrosion resistance, the amount of adhesion is preferably 20 to 100 g / m ^{2 on} one side (selected according to application), but the variation is 4
g / m ² or less aims to stabilize corrosion resistance and weldability. In general, the gas wiping method is used to control the amount of adhesion, but the flapping of the plate at this time has the greatest effect on the variation of the amount of adhesion.For example, the variation of the amount of adhesion is suppressed by suppressing the flutter of the plate with a support roll or an electromagnet in a bath. Can be reduced.

This Zn-Sn plated steel sheet has a beautiful glossy appearance and is promising as a metal building material. In that case, long-term durability is required. It is effective to add Cr to steel, and the effect appears when Cr is added at 1% or more.
By adding 3% or more, the generation of red rust can be significantly suppressed.
Therefore, for applications requiring long-term corrosion resistance, 3% Cr
It is desirable to add above. The addition of Cr naturally increases the cost and also impairs the workability. For this reason, it is desirable that the amount of Cr added be 25% or less. For applications requiring high workability, it is desirable to use IF steel excellent in workability, and furthermore, B was added at a few ppm in order to ensure weld airtightness after welding, secondary workability, and the like. Steel plates are preferred. In particular, the use of low carbon steel is desirable for applications that do not require workability.

Next, the plating layer is based on Zn and Sn, but the addition of Mg is preferable from the viewpoint of corrosion resistance. Mg forms a compound called Sn and Mg ₂ Sn in the plating layer, which preferentially dissolves in a corrosive environment, and the Mg-based film covers the plating layer and the base iron to exhibit an anticorrosion effect. Mg is Sn,
Since it is a lighter element than Zn, even if it is a small amount by weight, the atomic concentration becomes several times as large, so the effect is exhibited from a relatively small amount. The addition of 0.2% or more improves the corrosion resistance, and the more the addition, the better the corrosion resistance. However, the melting temperature also increases, so the upper limit is preferably 8%. Further, Mg is known to also have a stabilizing effect on Zn-based corrosion products, and exhibits an effect of improving corrosion resistance in combination with the above-described action.

Mg is an element having an extremely strong affinity for oxygen. When Mg is added to Zn-Sn, severe oxidization of Mg occurs on the bath surface, and the operability is greatly reduced. is there. However, Al is effective in suppressing the oxidation of Mg, and the operability is improved by adding about 1/10 of the amount of Mg. Therefore, when Mg is added, Al is also added at the same time. Al is Sn, Z
It is also effective in suppressing the oxidation of n itself, and even when Mg is not added, the appearance of plating is improved by adding Al. In order to exhibit such an effect, Al is desirably 0.02 to 5%. The lower limit is based on the effect on oxidation suppression.
The upper limit is determined from the melting temperature.

As an element having the same action as Mg, C
a and Li. These also form a compound which easily dissolves with Sn, and the dissolved Ca and Li form a film to have an anticorrosive effect. Therefore, addition of these elements is also effective for improving corrosion resistance, and it is possible to add 0.1 to 5% each. The lower limit of these element concentrations is determined by the effect on corrosion resistance, and the upper limit is determined by the melting temperature. All of these are elements having a strong affinity for oxygen, and Al is also effective for suppressing its oxidation. A trace amount of Fe may be present as an impurity element of the plating layer. If necessary, Mg, Al, misch metal, Sb, etc. may be added.

At the time of plating, it is naturally possible to directly plate the steel sheet, and it is also possible to perform a pre-plating treatment before the plating. The pre-plating is performed to improve the plating property. Ni, Co, Fe, Cr,
There can be Sn, Zn, Cu, or a metal containing these. The thickness is usually about 0.1 μm, but is not particularly limited. As a hot-dip plating method, a flux method and a sendzimer method can be roughly used, and both manufacturing methods are possible. In general, the Sendzimer method has high productivity, and production by this method is more desirable.

Next, as a post-treatment film after plating, there is, for example, a chromate film, which affects properties such as corrosion resistance, weldability, and corrosion resistance after painting. Chromate film has excellent corrosion resistance and paintability and has been used ^frequently.
Is harmful to the human body, and in recent years many post-treatment films have been devised. In the present invention, these post-treatment films can be applied. As the post-treatment film, a non-black film is preferable, and for example, a silane coupling agent-phenol resin-phosphoric acid-based film may be used.

Next, the present invention will be described in more detail by way of examples.

EXAMPLES (Example 1) Steels having the components shown in Table 1 were melted by ordinary converter-vacuum degassing to obtain steel slabs, and then hot-rolled, cold-rolled, and continuously under ordinary conditions. An annealing step was performed to obtain an annealed steel sheet (sheet thickness 0.8 mm). After applying 1 g / m ² of Ni plating to this steel sheet in a watt bath, Z was applied by a flux method.
n-Sn plating was performed. The flux was used by applying a ZnCl ₂ aqueous solution on a roll, and the composition of Sn was changed between 0 and 60%. The bath temperature was set to 280 to 320 ° C., and the coating weight was 50 g / m ² on one side by air wiping after plating.
Was adjusted. These performances were evaluated. The evaluation method at this time was based on the method described below. Table 2 shows the plating conditions and performance evaluation results.

[0016]

[Table 1]

(1) Chemical conversion property Phosphoric acid treatment property PB-3020 manufactured by Nippon Parkerizing Co., Ltd. was performed, and the presence or absence of chemical conversion film unevenness after the processing was observed. [Evaluation Criteria] ○: No unevenness in chemical appearance △: Slight unevenness X: Some unevenness and no formation of chemical crystals

(2) Corrosion resistance evaluation Salt damage corrosion resistance After the above-mentioned phosphate chemical conversion treatment, a cationic electrodeposition paint U-80 manufactured by Nippon Paint Co., Ltd. was coated to a thickness of 20 μm.
After making a cross-cut flaw with a cutter on a sample having a size of 70 × 150 mm painted with JIS Z 2371
A salt spray test was performed on the 35th day in accordance with the standards described above, and swelling of the coating film from the flaws (maximum swelling on one side) was measured. [Evaluation Criteria] A: Coating swelling 1 mm or less O: Coating swelling 1 <Swelling ≤2.5 mm Δ: Coating swelling 2.5 <Swelling ≦ 4 mm ×: Coating swelling more than 4 mm

(3) Weldability Spot welding was performed under the following welding conditions, and the number of continuous hits until the nugget diameter fell below 4 t (t: plate thickness) was evaluated. [Welding conditions] Welding current: 95% of the current generated by dust. Pressing force: 200 kg. Welding time: 12 cycles Electrode: Dome type, 6φ-40R tip, made of chromium copper [Evaluation criteria] ○: More than 750 continuous hit points △: 400 continuous hit points -750 points ×: Continuous hit points less than 400 points

(4) Workability Using a hydraulic forming tester, cup forming was performed at a drawing ratio of 2.20 using a cylindrical punch having a diameter of 50 mm. The test was performed by applying oil, and the wrinkle suppressing force was 1000 kg. The evaluation of workability was based on the following index. [Evaluation criteria] ○: No abnormality △: Cracking of plating ×: Peeling of plating

[0021]

[Table 2]

Table 2 shows the results of evaluating various characteristics mainly for use in automobile bodies. Zn imparts sacrificial corrosion protection to the plating layer and suppresses red rust of iron. No. In a system containing a small amount of Zn, such as 14, the phosphatability is poor, and as a result, the swelling of the coating film of the coating material is large. Further, since Sn reacts with the welding electrode Cu, the welding point property is inferior. As the amount of Zn increases, the swelling of the coating film tends to increase.
% (No. 3) and 45% (No. 4) are less likely to cause film swelling. However, when Sn increases (No. 3), the phosphatability decreases. In addition, No. As shown in Fig. 15, when the Zn content is 45%, the formation of a phosphate film is inferior and the swelling of the coating film is large and the corrosion resistance is lowered. No. 9,
11 and 12 are systems to which Mg, Ca and Li are added,
Due to the effects of these additional elements, good corrosion resistance is shown despite the large amount of Zn. No. As shown in FIGS. 6 and 7, it can be confirmed that the corrosion resistance is improved by adding Cr to steel.

(Example 2) Steels having the components shown in Table 3 were melted by ordinary converter-vacuum degassing to obtain steel slabs, which were then hot-rolled, cold-rolled, and continuously annealed under ordinary conditions. Perform the process,
An annealed steel plate (plate thickness: 08 mm) was obtained. After applying 1 g / m ² of Ni plating to this steel sheet in a watt bath, Z was applied by flux method.
n-Sn plating was performed. The flux was used by applying a ZnCl ₂ aqueous solution by roll coating, and the composition of Zn was 82%. The bath temperature was 280 ° C., and the amount of plating was adjusted to 70 g / m ² on one side by air wiping after plating. The obtained Zn-Sn plated steel sheet was evaluated by the following evaluation method. Table 4 summarizes the evaluation results. That is, Table 4 shows the evaluation results in the use of building materials. The salt corrosion resistance is excellent. In addition, Zn has good rust prevention on the end face, but C
By using the r-containing steel as a base, the generation of red rust on the end face is further suppressed, and a high corrosion resistance is exhibited overall.

[0024]

[Table 3]

[0025]

[Table 4]

(1) Corrosion resistance evaluation Salt damage corrosion resistance Phosphated PB-30 manufactured by Nippon Parkerizing Co., Ltd.
After carrying out No. 20, a cation type electrodeposition paint U-80 manufactured by Nippon Paint Co., Ltd. was coated at a film thickness of 20 μm.
After a 50 mm sample was cut with a cross-cut flaw by a cutter, a salt spray test in accordance with JISZ2371 was performed for 35 days, and the swelling of the coating film from the flaw (one-sided maximum swelling) was measured. [Evaluation Criteria] 塗膜: Coating swelling 1 mm or less ：: Coating swelling 1 <Swelling ≦ 2.5 mm △: Coating swelling 2.5 <Swelling ≦ 4 mm ×: Coating swelling more than 4 mm Salt corrosion resistance

(2) Outdoor exposure test After the non-color post-treatment shown in Table 5, coating was performed. The coating was made of a polyethylene wax-containing acrylic resin (clear: 5 μm). Shearing to dimensions 50 x 200 mm,
An outdoor exposure test was performed. After 3 months, the occurrence rate of red rust from the end face and the discoloration of the surface were observed. [Evaluation Criteria] 赤: Red rust occurrence rate from the end face is less than 10% ○: Red rust occurrence rate from the end face is 10% or more and less than 30% △: Red rust occurrence rate from the end face is 30% or more and less than 70% ×: From the end face Red rust occurrence rate of over 80%

[0028]

[Table 5]

(Example 3) The steel shown in Table 1A of Example 1 was plated with Zn-18% Sn by the Sendzimer method using a cold rolled sheet. NOF-RF type line, reaching plate temperature of 8
Annealing was performed at 20 ° C., and the plate was cooled and immersed in the bath so that the invading plate temperature was almost the same as the bath temperature. Bath temperature is 280
° C. After plating, the amount of plating was adjusted variously by air wiping. At this time, a pair of support rolls were put in the bath, and the variation of the coating weight was adjusted by changing the rolling conditions of the rolls. Variation of adhesion amount is 1m ²
An average value and standard deviation were calculated by measuring the amount of adhering at any 10 points from the plate of Example 1 by the fluorescent X-ray method. The cooling was performed by an air cooling system. These performances were evaluated. The evaluation method at this time is the same as that of the first embodiment. Table 6 shows the performance evaluation results with various amounts of adhesion.

[0030]

[Table 6]

Table 6 shows the effects of the coating weight and the variation on the performance of the steel sheet. If the amount of adhesion is small, the corrosion resistance tends to be insufficient, and if the amount of adhesion increases, the weldability tends to decrease.
Even if the adhesion amount is in the meantime, if the variation in the adhesion amount is large, the weldability becomes slightly unstable. Therefore, it is desirable that the variation in the amount of adhesion be 4 g / m ² or less as a standard deviation.

[0032]

The present invention is directed to a molten Zn—S having excellent corrosion resistance.
It is intended to provide an n-based plated steel sheet. Zn-Sn-based plating can be used for various applications such as automobile bodies and building materials such as roofs and walls because of its stable corrosion resistance and workability.
Further, by adding Cr to the steel sheet, it is possible to further improve the corrosion resistance. This makes it a material that can be widely used in areas and members where corrosion resistance is required.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masahiro Futa 1-1, Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works (72) Inventor Yasuto Goto Tobata-ku, Kitakyushu-shi, Fukuoka No. 1-1 Tobatamachi New Nippon Steel Corporation Yawata Works F term (reference) 4K027 AA02 AA22 AB05 AB26 AB43 AB46 AC15 AC82 AE03 AE23 AE24

Claims (8)

[Claims] 1. A hot-dip Zn—S alloy having excellent corrosion resistance, characterized in that the steel sheet has a hot-dip Zn—Sn plating layer, and the plating layer composition contains 1 to less than 50% of Sn in addition to Zn.
n-based plated steel sheet. 2. A hot-dip Zn—S alloy having excellent corrosion resistance, wherein a hot-dip Zn—Sn plating layer is provided on the surface of the steel sheet, and the plating layer composition contains Sn in an amount of less than 1 to 40% in addition to Zn.
n-based plated steel sheet. 3. The hot-dip Zn—Sn-based coated steel sheet according to claim 1, wherein the steel contains 1 to 25% of Cr. 4. An average value of the amount of plating applied is 25 to 10 on one side.
The molten Zn excellent in corrosion resistance according to claim 1, wherein 0 g / m ² and a standard deviation are 4 g / m ² or less.
-Sn-based plated steel sheet. 5. The composition of the plating layer is such that, in addition to Zn and Sn,
5. A hot-dip Zn-Sn-based plated steel sheet having excellent corrosion resistance according to claim 1, wherein the steel sheet contains one or two kinds of g: 0.2 to 8% and Al: 0.02 to 5%. . 6. The plating layer further contains Ca: 0.1 to 5%, L
The molten Zn excellent in corrosion resistance according to claim 1, wherein one or two kinds of i: 0.1 to 5% are contained.
-Sn-based plated steel sheet. 7. Ni, Co, F at the interface between the plating layer and the steel sheet.
The hot-dip Zn-Sn-based plated steel sheet having excellent corrosion resistance according to claim 1, further comprising a pre-plated layer containing e, Cr, Sn, Zn, and Cu. 8. The hot-dip Zn-Sn-based plated steel sheet having excellent corrosion resistance according to claim 1, further comprising a post-treatment film on the outermost surface of the plating layer.