JP2002038250A - Hot-dip Sn-Zn plated steel sheet with excellent corrosion resistance - Google Patents

Abstract

(57) [Summary] PROBLEM TO BE SOLVED: To provide a more stable corrosion resistance than the conventional one.
The present invention provides a hot-dip Sn—Zn-based plated steel sheet. SOLUTION: A steel sheet has a hot-dip Sn-Zn plating layer.
And the plating layer composition contains 1 to 50% of Zn in addition to Sn
And (Zn% of surface layer composition / total plating layer)
(Zn% of the body) is 0.95 or less
Hot-dip Sn-Zn plated steel sheet with excellent corrosion resistance. As building material
When using steel sheets, use steel sheets containing Cr: 3 to 25%.
It can be used as a fuel tank,
50g / m ^Two , Standard deviation 4g / m^Two The following is desirable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention provides a surface-treated steel sheet having excellent corrosion resistance. The present invention is, for example, automotive fuel tank materials, metal building materials such as roof walls, homes,
It is suitable as a steel sheet for industrial electronic equipment.

[0002]

2. Description of the Related Art Sn-plated steel sheets are widely used mainly for food cans and beverage cans because of their excellent corrosion resistance and workability. However, in an environment without dissolved oxygen, such as inside a food can, Sn is known to sacrifice and protect ground iron. However, in a normal environment where oxygen is present, corrosion from the ground iron tends to proceed. There is. To compensate for this, Sn-Zn plated steel sheets containing 20 to 40% of Zn are also mainly used in the field of post-plating of electronic parts, automobile parts and the like (Japanese Patent Laid-Open No. 6-116749). 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 Laid-Open No. 8-269733 and others a molten Sn having a controlled plating structure. Japanese Patent Application Laid-Open No. 8-325692 discloses a Zn-plated steel sheet as a rust-preventive steel sheet having excellent workability and corrosion resistance.
An n-plated steel sheet has been disclosed.

[0004]

SUMMARY OF THE INVENTION The aforementioned molten Sn-Z
The n-plated steel sheet certainly has excellent corrosion resistance, workability, and weldability. However, during the course of development, it was found that corrosion resistance after painting, especially peeling of paint from a cross cut after painting, was likely to occur slightly. This is because when a corrosion test such as a salt spray test is performed with a flaw that reaches the ground iron such as a cross cut after painting, the Sn-Zn plated steel sheet sacrifices and protects the ground iron, so the corrosion of Zn near the surface layer has priority. This is considered to be a phenomenon in which Zn is corroded under the coating film and the adhesion at the location is reduced.

In this case as well, the corrosion of the plating layer is only on the outermost surface, and it is thought that there is no significant effect on the durability life of the plating layer. Would. To prevent this phenomenon, it is effective to lower the Zn concentration near the surface. As the means, for example, it is possible to dissolve Zn near the surface by pickling or the like, but if the pickling and water washing steps are provided, the line configuration becomes complicated, and the spangles emerge due to the pickling, so that Sn-Zn plating is performed. There is a problem that the original gloss appearance is lost.

[0006]

Means for Solving the Problems The present inventors have diligently studied a method for reducing the amount of Zn near the surface during the solidification process in order to solve the above-mentioned problems, and have achieved this by optimizing the plating conditions. Things. That is, as a result of examining the solidification process of the plated metal in detail, for example, cooling conditions after plating, mist spraying, powder spraying, etc., accelerate the solidification near the surface and reduce the spangle, thereby reducing the Zn concentration near the surface. Was obtained.

[0007] When the Zn concentration near the surface is reduced in this manner, the above-mentioned preferential corrosion of Zn under the coating film and the peeling phenomenon of the coating film due to this can be prevented, and the corrosion of Zn when used without coating can be prevented. Is unlikely to be caused by whitening. But,
Since the amount of Zn in the plating layer is secured, it is not inferior to those manufactured under normal cooling conditions in terms of long-term corrosion resistance and red rust resistance. That is, the present invention makes it possible to achieve both corrosion resistance after coating, initial white rust resistance, and red rust resistance when corrosion progresses.

In order to obtain the effect of the present invention, it is effective to reduce the spangle diameter as described above.
This effect is insufficient to obtain a spangle diameter of about 20 mm or less as disclosed in Japanese Unexamined Patent Publication No.
It is necessary to make the diameter even smaller, that is, 3 mm or less. The present inventors have found that by setting the spangle diameter to 3 mm or less, the Zn concentration on the surface is reduced (Sn becomes rich) and the corrosion resistance is remarkably improved, thereby completing the present invention. .

Hereinafter, the present invention will be described in detail. The present invention optimizes the conditions in the cooling step after plating, as described above, to suppress initial white rust, suppress corrosion under the coating film, and achieve long-term red rust resistance. In order to sufficiently exert the effect, it is possible to suppress the preferential corrosion of Zn near the surface by setting the ratio (Zn% of the surface layer composition / Zn% of the entire plating layer) of the plating layer to 0.95 or less. . Since the paint adhesion becomes better as the Zn content of the surface layer decreases and the Sn ratio increases, no lower limit is set. The surface layer composition refers to EPMA (electro
n probe microanalyzer) refers to the depth (about 1 μm) into which the electron beam penetrates, and this EPM
By A, the surface layer composition can be measured. Alternatively, map analysis of the cross section by EPMA and 1μ
The determination may be made based on the average composition up to a depth of about m.

[0010] Zn in the plating layer is added for imparting a sacrificial anticorrosion effect to the steel sheet, and its effect is exhibited at 1% or more. When a small amount of Zn is added, the sacrificial anticorrosion action is lost when the Zn is completely eluted, but when the amount of Zn is increased, the period of the sacrificial anticorrosion is prolonged. When Zn is 50% or more, the main component of the plating layer is Zn, and the elution rate of Zn itself is S.
Since it is much larger than n, the corrosion resistance of the plating layer itself is impaired. Therefore, the amount of Zn is limited to 1 to 50%.

[0011] When Sn-Zn is used for fuel tank materials, high resistance weldability (spot welding, seam welding, projection welding, etc.) is required. At this time, since Cu and Sn of the electrode easily form a compound, the adhesion amount of plating greatly affects the weldability. Also,
Of course, the amount of plating has a large effect on corrosion resistance. It has been found that the larger the amount of adhesion, the more advantageous in terms of corrosion resistance, and the more disadvantageous in terms of weldability, but that the effect of variations in the amount of adhesion is particularly large.

That is, from the balance between the weldability and the corrosion resistance, the amount of adhesion is suitably ²⁰ to 50 g / m ^{2 on} one side, but the balance between the corrosion resistance and the weldability is particularly achieved by setting the variation to 4 g / m ² or less as a standard deviation. It will be good. 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.

The Sn-Zn plated steel sheet has a beautiful glossy appearance, and is considered to be promising as a metal building material. In that case, long-term durability is required. Since the Sn—Zn-based plated steel sheet preferentially dissolves Zn, considering the long-term use, the sacrificial anticorrosion effect of Zn is lost, and corrosive substances (red rust) of the base iron are likely to be generated. In order to suppress this, it is effective to add Cr to steel. Since addition of 3% or more of Cr can significantly suppress the generation of red rust, 3% or more of Cr is used for applications requiring long-term corrosion resistance. It is desirable to do. The addition of Cr naturally increases the cost and impairs the workability. For this reason, the content of Cr is desirably 25% or less. For applications that require high workability, it is desirable to use IF steel with excellent workability, and furthermore, a steel sheet containing a few ppm of B to ensure welding airtightness after welding and secondary workability. Is desirable. In particular, the use of low carbon steel is desirable for applications that do not require workability.

Next, the plating layer is based on Sn and Zn, but the addition of Mg is preferable from the viewpoint of corrosion resistance. Mg forms a compound called Mg ₂ Sn in hot-dip Sn-based plating, which is preferentially dissolved in a corrosive environment and
The g-based coating covers the plating layer and the base iron, and exhibits an anticorrosion effect. M
Since g is a lighter element than Sn and Zn, even if it is a small amount by weight, the atomic concentration becomes several times as large, so that 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%.

Mg is an element having an extremely strong affinity for oxygen. When Mg is added to Sn-Zn, intense oxidation of Mg occurs on the bath surface, thereby greatly reducing the operability. 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.
There may be r, Sn, Zn, Cu, or metals 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.

As a post-treatment film of Sn—Zn plating,
For example, there is a chromate film or the like, which affects properties such as corrosion resistance, weldability, and corrosion resistance after painting. Chromate film has excellent corrosion resistance and paintability and has been widely used.
r ⁶⁺ is harmful to the human body, and in recent years, alternative 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. INDUSTRIAL APPLICABILITY The steel of the present invention is extremely suitable as a fuel tank material for automobiles, and also as a metal building material such as a roof wall, and a steel plate for home and industrial electronic devices.

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, S was applied by flux method.
n-Zn plating was performed. The flux was used by applying a ZnCl ₂ aqueous solution on a roll, and the composition of Zn was changed between 0 and 60%. The bath temperature was set to 280 to 320 ° C., and the coating weight was 40 g / m ² on one side by air wiping after plating.
Was adjusted. During cooling, the spangle diameter was adjusted by changing cooling conditions such as mist cooling, air cooling, and cooling. At this time, the Zn% of the surface layer was measured by EPMA using map analysis in a 5 × 5 mm range, and the average value was used as the measured value. The Zn% in the plating layer was measured by subjecting the plating layer to electrolytic decomposition in a NaOH solution, subjecting the solution to acid decomposition treatment, and performing ICP spectroscopy (inductively coupled high frequency plasma spectroscopy). 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.

[0020]

[Table 1]

(1) Evaluation of Corrosion Resistance Corrosion Resistance to Salt Damage JIS Z 23 for a sample having a size of 70 × 150 mm
The salt spray test according to No. 71 was performed for 30 days, and the corrosion products were peeled off and the corrosion weight loss was measured. The indication of the corrosion weight loss is a value for one side of the plating. [Evaluation Criteria] ◎: corrosion weight loss 5 g / m ² or less ○: less than corrosion loss 10g / m ² △: Corrosion weight loss 10~25g / m ² ×: corrosion weight loss 25 g / m ² greater

Corrosion resistance after coating First, a chromic acid-silica type treatment was applied to the metal C as a chemical conversion treatment.
20 mg / m ^{2 was} treated on one side in terms of r. Then size 70 ×
A melamine black coating of 20 μm was applied to a 150 mm sample and baked at 140 ° C. for 20 minutes. Thereafter, a cross cut was made and subjected to a salt spray test. After 40 days, the peeling width of the coating film from the cross cut by taping was evaluated. [Evaluation criteria] ：: Peeling width 2 mm or less ○: Peeling width 2 to 4 mm △: Peeling width 4 to 6 mm ×: Peeling width more than 6 mm

Corrosion resistance to fuel The corrosion resistance to gasoline was evaluated. The method is as follows, using the above-mentioned hydraulic forming tester, flange width 20 mm, diameter 50 m
m, a flat-bottomed cylindrical drawing with a depth of 25 mm
The test solution was charged and covered with glass via a ring made of silicone rubber. The corrosion state after this test was visually determined. [Test conditions] Test liquid: gasoline + distilled water 10% + formic acid 200 ppm Test period: Left at 40 ° C for 3 months [Evaluation criteria] ○: Less than 0.1% of red rust △: 0.1 to 5% of red rust or white Rusted ×: Red rust generation is more than 5% or white rust is noticeable

(2) Weldability After the chemical conversion treatment described in the item (1), spot welding is performed under the following welding conditions until the nugget diameter becomes less than 4√t (t: plate thickness). Was evaluated for the number of continuous hits. [Welding conditions] Welding current: 95% of the current generated by dust: Pressing force: 240 kg Welding time: 12 cycles Electrode: Dome type, tip 6φ-40R, made of chromium copper [Evaluation criteria] ○: Continuous hitting point more than 600 △: Continuous hitting point 400 Up to 600 points ×: Less than 400 continuous hit points

(3) Workability Using a hydraulic forming tester, cup forming was performed at a drawing ratio of 2.25 using a cylindrical punch having a diameter of 50 mm. The test was performed by applying oil, and the wrinkle suppressing force was 500 kg. The evaluation of workability was based on the following index. [Evaluation criteria] ○: No abnormality △: Cracking of plating ×: Peeling of plating

[0026]

[Table 2]

Table 2 shows the results of evaluating various characteristics mainly for use in automobile fuel tanks. Zn imparts sacrificial corrosion protection to the plating layer and suppresses red rust of iron. No. In a system containing no Zn such as 14, the corrosion resistance after processing or the corrosion resistance from flaws decreases. No. 1 low Zn
The amount is slightly poor in corrosion resistance. When the amount of Zn is increased, the effect of suppressing red rust is obtained, but when it is too large, white rust due to Zn tends to be easily generated. Nos. 3 and 4 tend to decrease in salt corrosion resistance and corrosion resistance after painting.
As shown in Fig. 15, when Zn reaches 65%, the corrosion resistance is greatly reduced. No. No. 16 is when the spangle diameter was increased by slow cooling, and in this case, Zn near the surface of the plating layer was
Poor corrosion resistance after painting due to high concentration. Other systems show good characteristics. No. 9, 11, and 12 are systems in which the amount of Zn is reduced and Mg, Ca, and Li are added.
Due to the effects of these additional elements, good corrosion resistance after processing is exhibited despite the small amount of Zn.

(Example 2) Steel having the components shown in Table 3 was melted by ordinary converter-vacuum degassing to obtain a steel slab, and then hot-rolled, cold-rolled, and continuously annealed under ordinary conditions. Perform the process,
An annealed steel sheet (sheet thickness 0.8 mm) was obtained. The steel sheet was subjected to Ni plating at 1 g / m ² in a Watt bath, and then Sn—Zn plating was performed by a flux method. The flux was used by applying a ZnCl ₂ aqueous solution by roll coating, and the composition of Zn was 8%. The bath temperature was 280 ° C., and the plating adhesion amount was adjusted to 60 g / m ² on one side by air wiping after plating. The spangle diameter of the obtained steel sheet was 1 to 2 mm, and the ratio of Zn% of the surface layer / Zn% in the plating layer was 0.60 to 0.68. The Sn-Zn plated steel sheet thus obtained was evaluated by the following evaluation method. Table 4 summarizes the evaluation results.

[0029]

[Table 3]

[0030]

[Table 4]

(1) Corrosion resistance to salt damage JIS Z 23 for a sample having a size of 70 × 150 mm
The salt spray test according to No. 71 was performed for 30 days, and the corrosion products were peeled off and the corrosion weight loss was measured. The indication of the corrosion weight loss is a value for one side of the plating. [Evaluation Criteria] ◎: corrosion weight loss 5 g / m ² or less ○: less than corrosion loss 10g / m ² △: Corrosion weight loss 10~25g / m ² ×: corrosion weight loss 25 g / m ² greater

(2) Outdoor exposure test After the non-colored 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] ○: Red rust occurrence rate from the end face is less than 30% Δ: Red rust occurrence rate from the end face is 30 to 80% ×: Red rust occurrence rate from the end face is more than 80%

[0033]

[Table 5]

Table 4 shows the results of evaluations for building materials. Although salt corrosion resistance is excellent, Sn-Zn-based plating does not have a long-term sacrificial corrosion protection effect, so that red rust is generated at the end face. By using a Cr-containing steel as a base material, it is possible to suppress the generation of red rust on the end face portion, and exhibit high corrosion resistance overall. Therefore, in order to suppress the generation of red rust on the end face as a building material, it is desirable to use a Cr-containing steel.

Example 3 The steel shown in Table 1A of Example 1 was subjected to Sn-8% Zn plating using a cold rolled sheet by the Sendzimer method. NOF-RF type line, reaching plate temperature of 82
Annealing was performed to 0 ° C., and the plate was cooled and immersed in the bath so that the intruding plate temperature was almost equal to the bath temperature. Bath temperature is 280 ° C
And 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. The variation in the adhesion amount was obtained by measuring the adhesion amount at any 10 points from a 1 m ² plate by the fluorescent X-ray method and calculating the average value and the standard deviation. The cooling is air cooling, the spangle diameter is 1 to 2 mm, Zn% of the surface layer / Zn in the plating layer.
The% ratio was 0.60 to 0.68. 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.

[0036]

[Table 6]

Table 6 shows the amount of plating and the effect of the variation on the performance as an automobile fuel tank. 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 middle, if the variation in the adhesion amount is large, both the corrosion resistance and the weldability become somewhat unstable. Therefore, the variation of the adhesion amount is 4 g as the standard deviation.
/ M ² or less.

[0038]

According to the present invention, a molten Sn-Z having excellent corrosion resistance is provided.
The present invention provides an n-plated steel sheet. Sn-Zn-based plating is promising as a material for automobile fuel tanks because of its stable corrosion resistance, workability, and the like, and can be expected to be used as home electric appliance materials and building materials. The present invention provides a method for further stabilizing the corrosion resistance of a Sn—Zn plated steel sheet, and has a great industrial contribution.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Teruaki Izaki 1-1 Niwahata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Yawata Works (72) Inventor Yasuto Goto Tobata-ku, Kitakyushu-shi, Fukuoka 1-1 Futabacho Nippon Steel Corporation Yawata Works F-term (reference) 4K027 AA02 AA05 AA22 AA23 AB02 AB05 AB09 AB13 AB46 AC15 AC82 AE03 AE21 AE23

Claims (7)

[Claims] 1. A steel sheet having a molten Sn—Zn plating layer on its surface, wherein the plating layer composition contains 1 to 50% of Zn in addition to Sn, and (Zn% of surface composition / total plating layer) of the plating layer (% Of Zn) is 0.95 or less. 2. The molten Sn having excellent corrosion resistance according to claim 1, wherein the steel contains Cr: 3 to 25%.
-Zn-based plated steel sheet. 3. The average value of the amount of plating applied is 20 to 50 on one side.
The molten S having excellent corrosion resistance according to claim 1 or 2, wherein the molten S has a g / m ² and a standard deviation of 4 g / m ² or less.
n-Zn plated steel sheet. 4. The composition of the plating layer is such that M
The hot-dip Sn-Zn-based plating excellent in corrosion resistance according to claim 1, wherein one or two or more of g: 0.2 to 8% and Al: 0.02 to 5% are contained. steel sheet. 5. The plating layer further contains Ca: 0.1 to 5%, L
The hot-dip Sn-Zn-based plated steel sheet having excellent corrosion resistance according to claim 1, wherein one or more kinds of i: 0.1 to 5% are contained. 6. Ni, Co, F at the interface between the plating layer and the steel sheet.
The hot-dip Sn-Zn-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. 7. The hot-dip Sn—Zn 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.