JP2002105615A - Hot-dip Sn-Mg plated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Pb-free hot-dip Sn-Mg based plating which has excellent corrosion resistance and is particularly suitable as a material for an automobile fuel tank. SOLUTION: The molten Sn-Mg-based steel sheet contains 0.1 to 4% of Mg and has a plating structure in which a major axis dimension of a plated metal crystal on the outermost surface is 20.0 mm or less. Furthermore, Zn 0.5-20%, Ca 0.01-2%
In the plating layer, Ni, Co, and Cu in the alloy layer, and a post-treatment layer composed of an inorganic compound, an organic compound, or a composite thereof may be provided on the outermost surface. [Effect] The plated steel sheet has characteristics suitable as a fuel tank material not using Pb.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip Sn-Mg based steel sheet which has excellent corrosion resistance, bonding properties and workability and is suitable as a material for automobile fuel tanks, household electric machines and industrial machines. is there.

[0002]

2. Description of the Related Art Conventionally, Pb-S as a fuel tank material has been excellent in corrosion resistance, workability, solderability (weldability) and the like.
N-alloy-plated steel sheets are mainly used and widely used as fuel tanks for automobiles. On the other hand, Sn-Zn alloy plated steel sheets have been mainly produced by an electroplating method in which electrolysis is performed in an aqueous solution containing Zn and Sn ions, as disclosed in, for example, Japanese Patent Application Laid-Open No. 52-130438. BACKGROUND ART Sn-Zn alloy-plated steel sheets mainly composed of Sn are excellent in corrosion resistance and solderability and are often used for electronic components and the like. On the other hand, this Sn-Z
It has been found that an n-plated steel sheet has excellent characteristics, and is disclosed in JP-A-8-269733 and JP-A-8-269733.
In the publications, molten Sn-Z with controlled plating structure
An n-plated steel sheet is disclosed.

[0003]

SUMMARY OF THE INVENTION Pb-Sn alloy plated steel sheets that have been used as fuel tank materials for automobiles are:
Various excellent properties (for example, workability, corrosion resistance of tank inner surface, solderability, seam weldability, etc.) have been recognized and used, but are shifting to the direction of Pb-free with increasing awareness of global environment in recent years. . On the other hand, Sn-Zn electro-alloy plated steel sheets have been mainly used as electronic components requiring solderability or the like in applications where the corrosive environment is not so severe.

[0004] The above-mentioned hot-dip Sn-Zn-plated steel sheet certainly has excellent corrosion resistance, workability and solderability. However, in recent years, further improvement in corrosion resistance has been demanded. In Sn-Zn-plated steel sheets, although the cut portion having a small width (a flaw reaching the base iron) has a certain sacrificial corrosion prevention effect, the width of the cut portion is wide. The sacrificial corrosion protection ability is not sufficiently exhibited in the case or at the end portion where the plating is not coated.

In particular, in a salt spray test assuming a salt damage environment, a period until red rust is generated is short and not sufficient. In order to further improve the sacrificial corrosion protection ability, the amount of Zn added may be increased. However, if the amount of Zn is too high, the main constituent of the plating layer shifts from Sn to Zn, and Zn
Since the elution itself is much larger than Sn, the corrosion resistance of the plating layer itself is impaired. The present invention solves the above-mentioned problems and highly balances corrosion resistance, workability, and weldability,
The present invention provides a hot-dip Sn-based plated steel sheet that does not use any steel.

[0006]

DISCLOSURE OF THE INVENTION The present inventors aimed at providing a rust-preventive steel sheet which does not contain Pb and has improved rust-prevention performance at a cut portion and an end face. Have been studied in various ways, and have led to the present invention. The present invention
A Sn—Mg alloy plated steel sheet containing 0.1 to 4% Mg has a plating structure in which the major dimension of the plated metal crystal on the outermost surface is 20.0 mm or less via an alloy layer having a thickness of 3.0 μm or less. The present invention relates to a rust-preventive steel plate for a fuel tank having excellent workability and corrosion resistance.

Further, Zn 0.5 to 20%, Ca 0.0
When 1 to 2% is contained in the plating layer, Ni, Co, Cu
That at least 0.5% is contained in the alloy layer, or that a post-treatment layer made of an inorganic compound, an organic compound, or a composite thereof is provided on the outermost surface. The present invention relates to a rust-preventive steel plate for a fuel tank having excellent workability and corrosion resistance.

Hereinafter, the present invention will be described in detail. The steel slab is subjected to a series of steps such as hot rolling, pickling, cold rolling, annealing, temper rolling, etc. After performing the pretreatment, plating is performed. Regarding steel components, it is a component system that can be processed into a complicated shape of the fuel tank, the alloy layer at the steel-plating layer interface is thin and the plating can be prevented from peeling, and the corrosion progress in the fuel tank internal and external environment is suppressed. Component system.

In the present invention, the Sn-Mg alloy plating is desirably performed by a hot-dip plating method. The greatest reason why the hot-dip plating method is desirable is that the hot-dip plating method is more suitable for securing the coating weight. Even in the electroplating method, if the electrolysis is performed for a long time, the amount of plating can be secured, but it is not economical. The range of the amount of plating applied in the present invention is 20 to 150.
g / m ² (one side), which is a relatively thick area, and the hot-dip plating method is optimal. Furthermore, when the potential difference between the plating elements is large, it is difficult to appropriately control the composition, and therefore, the hot-dip plating method is optimal for the Sn—Mg alloy.

Next, the reason for limiting the plating composition is that it is limited by the balance of corrosion resistance between the inner surface and the outer surface of the gasoline tank. The outer surface of the tank is painted after forming the tank because perfect rust prevention is required. Therefore, the coating thickness determines the rust prevention ability. As the material, the greater the amount of deposited Mg in the plating layer, the greater the effect of preventing red rust.

[0011] On the other hand, corrosion on the inner surface of the tank is not a problem when only normal gasoline is used. However, considerably intense corrosion is caused by mixing of water, mixing of chlorine ions, generation of organic carboxylic acid by oxidative deterioration of gasoline, and the like. The environment appears.
If gasoline leaks out of the tank due to pitting corrosion, serious accidents may occur, and these corrosions must be completely prevented. When deteriorated gasoline containing the above-described corrosion promoting component was prepared and its performance under various conditions was examined, it was confirmed that the Sn—Mg alloy plating film containing 4% or less of Mg exhibited extremely excellent corrosion resistance. .

If the content of pure Sn containing no Mg or the content of Mg is less than 0.1%, the plating metal does not have a sacrificial anticorrosion ability to the base iron, so that the pitting corrosion at the plating pinhole portion on the tank inner surface, Early red rust generation on the tank outer surface becomes a problem. When Mg is contained in a large amount exceeding 4%, Mg is preferentially dissolved, and a large amount of corrosion products is generated in a short period of time, so that there is a problem that the carburetor is likely to be clogged. In addition, the workability of the plating layer also decreases as the Mg content increases. Further, as the Mg content increases, the solderability significantly decreases.

Therefore, the Mg content in the Sn—Mg alloy plating in the present invention is in the range of 0.1 to 4%,
Further, in order to obtain a more sufficient sacrificial anticorrosion effect, suppress the dissolution of Mg, and make it more difficult for the carburetor to be clogged, the content is preferably in the range of 0.2 to 2%. Rust prevention ability is improved by the addition of Mg.
a 0.01 to 2% may be added. The addition of Zn lowers the potential of the plating layer and provides sacrificial corrosion protection. For this purpose, the addition of 0.5% or more is desirable, and an excessive addition causes an increase in the melting point and leads to an excessive growth of the intermetallic compound layer under the plating. Ca contributes to the improvement of the rust prevention ability similarly to Mg. However, since the contribution to the rise of the melting point is large, it is not preferable to add excessively, and it is desirable that the content is 0.01 to 2%.

In the hot-dip plating method, formation of an alloy layer cannot be avoided. This is because it is important that the surface to be plated and the plating metal are well wetted (alloyed) in order to prevent the occurrence of plating pinholes and obtain a uniform and good corrosion-resistant plating film. In order to process into a complicated shape like a fuel tank, it is necessary to ensure a high degree of workability. The alloy layer must be formed in a small amount in order to get wet well, but it is hard and brittle, so cracks are likely to occur during processing, and when it is thicker than a certain thickness, cracks propagate to the plating layer outside the alloy layer and Cracking is caused, which causes deterioration of corrosion resistance due to plating peeling or damage to the plating layer. Such plating peeling is caused by
There is a very large relationship with thickness and steel type, and in the case of the present invention,
The thickness of the alloy layer needs to be 3.0 μm or less.

To improve the wettability, it is also effective to change the surface of the steel sheet. In the steel sheet manufacturing process, oxides formed on the steel sheet surface are difficult to remove,
Inhibits plating properties. In order to eliminate this influence, Ni, Co, Cu, or the like, which readily reacts with tin, is plated on the surface of the steel sheet immediately before plating to improve wettability. Ni, Co, Cu, etc. may be plated alone, or may be an alloy with Fe or an alloy of these metals. The plating amount is such that the surface of the steel sheet is uniformly covered, for example, 0.1 to 2.0 g.
/ M ² is sufficient. The product after plating is N
By containing one or more of i, Co, and Cu in the alloy layer in an amount of 0.5% or more, a rust-proof steel sheet excellent in workability and corrosion resistance can be obtained.

In the present invention, the plating layer is mainly composed of Sn, and Mg is present therein. The Sn and Mg are
Plated on the surface of the steel sheet in the molten state.
And Mg ₂ Sn phase. The eutectic point is about 2% Mg, and after that point, there are a case where Sn crystallizes as a primary crystal at the beginning of cooling and a case where Mg ₂ Sn crystallizes as a primary crystal. The performance of a plated steel sheet largely depends on the degree of formation of crystals in the cooling process, that is, the manner of aggregation. The solidification state of the plating crystal can be visually observed by lightly corroding the surface after plating with a weak acid or weak alkali.

In general, a small crystal structure (hereinafter, referred to as a spangle) appears when extremely rapid cooling is performed. However, when a large strain is incorporated in the structure, corrosion resistance and workability are poor. There is. On the other hand, when cooled slowly after plating, large spangles are formed and the problem of thermal distortion is eliminated. However, in the Sn-Mg alloy plating, there is a case where Mg ₂ Sn will grow significantly. In such a plating film, only Mg tends to be rapidly dissolved in a corrosive environment, and a long-term anticorrosion effect due to Mg cannot be expected. As a result, the corrosion resistance is deteriorated. Also, during processing, the Mg ₂ Sn crystal is not preferable because it becomes a starting point of crack generation. For this reason, the present invention mainly considers limiting the size of spangles.

The size of the spangle is determined by the length of the major axis of the crystal.
Can be more defined. Usually round spangles
Often formed, but not necessarily the major and minor diameters of the crystal
Since the lengths are not equal, the present invention depends on the length of the major axis of the crystal.
I decided to define. In the present invention, corrosion resistance, workability
From the viewpoint, the long diameter of the crystal as spangle after plating
Is 20 mm or less, more preferably 10 mm or less.
It is necessary to be single. The major axis length of the crystal is 20
mm for coarse crystals of _TwoSn connection
The crystals are large and easy to grow.
g_TwoSn is a good starting point for cracks during processing.
Not good.

Fine crystals having a major axis length of 1.0 mm or less are worried because large thermal strains are incorporated in the structure. However, the fact that Mg ₂ Sn is very uniformly dispersed and processed as a fuel tank. In the process of heating, heat such as baking of paint is applied, so that distortion is expected to be released, so that excellent performance is expected as practical performance. Therefore, it is not necessary to set the lower limit dimension of the spangle. In the present invention, thorough corrosion resistance is expected by further performing a post-treatment of the plating layer surface with an inorganic compound, an organic compound, or a composite thereof. This process is based on the underlying Sn-M
g It is very familiar with the plating layer, has the effect of covering a defective portion such as a minute pinhole, and has the effect of dissolving the plating layer to repair the pinhole, thereby greatly improving the corrosion resistance.

[0020]

EXAMPLES The quality characteristics of the rustproof steel plate for a fuel tank according to the present invention will be shown in examples. (Example 1) After applying a plating flux containing zinc chloride and hydrochloric acid to an annealed and pressure-regulated steel sheet having a thickness of 0.8 mm, an Sn-Mg plating bath containing 1.5% Mg (temperature of 350 ° C) ). After sufficient reaction between the plating bath and the steel sheet surface, the steel sheet was drawn out of the plating bath, the amount of coating was adjusted by a gas wiping method, and the steel sheet was rapidly cooled. The steel sheet after plating had an alloy layer mainly composed of 0.7 μm FeSn ₂ and a plating layer having an adhesion amount (total adhesion amount of Sn + Mg) of 32 g / m ² (per one side). The surface was subjected to a chromate treatment with an adhesion amount of 15 mg / m ² as chromium to obtain a product plate.

In order to examine the crystal structure of the steel sheet, when the surface was slightly corroded with 1% hydrochloric acid, a crystal structure recognized by the naked eye appeared, and the average value of the major axis dimension was 6.5 mm. After the cross-section polishing, the distribution of Sn and Mg was analyzed by EPMA (Electron Probe Microanalyzer), and a uniform distribution was confirmed. 1 in a pressure vessel
10 vol. For forcedly degraded gasoline left overnight at 00 ° C
% Water was added to prepare a corrosion liquid. In this etchant,
When a corrosion test was performed at 45 ° C. for 3 weeks, the eluted metal ions were mainly Mg, and an elution of 2000 ppm was observed. However, it was determined that the metal ions exhibited good corrosion resistance.

(Embodiment 2) Annealing and pressure regulation for a sheet thickness of 0.8 mm
0.8g / m on already used steel plate^Two Electric nickel
Plating flux containing zinc chloride and hydrochloric acid
After coating, Sn-Mg plating bath containing 3% Mg
(At a temperature of 350 ° C.). Make sure the plating bath and steel plate surface
After the reaction, remove the steel sheet from the plating bath,
The coating amount was adjusted by the iping method, and the mixture was rapidly cooled. Me
0.5 μm FeSn _TwoThe subject
Gold layer (containing 17% Ni) and adhesion amount (Sn + Mg total attachment)
33g / m^Two Has (one side) plating layer
Was something. 12 mg / m as chromium on this surface
^Two Was subjected to chromate treatment to obtain a product plate.

In order to examine the crystal structure of the steel sheet, when the surface was slightly corroded with 1% hydrochloric acid, a crystal structure recognized by the naked eye appeared, and the average value of the major axis dimension was 12.0 mm. After the cross section polishing, the distribution of tin and zinc was analyzed by EPMA (Electron Probe Microanalyzer).
Although a slightly larger Mg ₂ Sn crystal was observed than in Example 1, an almost favorable distribution was confirmed. Forcibly degraded gasoline left overnight at 100 ° C in a pressure vessel
0 vol% water was added to prepare a corrosive liquid. When a corrosion test was performed at 45 ° C. for 3 weeks in this corrosive liquid, the eluted metal ions were mainly composed of Mg and 3000 ppm
Was eluted, but it was judged to show good corrosion resistance.

(Embodiment 3) Annealing and pressure regulation of 0.8 mm in plate thickness
0.8g / m on already used steel plate^Two Electric nickel
Plating flux containing zinc chloride and hydrochloric acid
After coating, Sn-Mg plating bath containing 2% Mg
(Temperature 400 ° C.). Make sure the plating bath and steel plate surface
After the reaction, remove the steel sheet from the plating bath,
The coating amount was adjusted by the iping method, and the mixture was rapidly cooled. Me
The steel sheet after welding is 0.7 μm FeSn _TwoThe subject
Gold layer (containing 12% Ni) and adhesion amount (Sn + Mg total attachment)
30g / m^Two Has (one side) plating layer
Was something. 10 mg / m as chromium on this surface
^Two Chromate treatment of the adhesion amount of
A non-processed material not subjected to heat treatment was produced. These two types of steel
The plate was subjected to the same corrosion test as in Examples 1 and 2.
However, the chromate treated material elutes 2100 ppm of Mg.
In comparison with the untreated material, Mg elution of 4700 ppm was observed.
In order to further improve corrosion resistance, post-treatment is better.
It was recognized as good.

Example 4 An annealed and pressure-regulated steel sheet having a thickness of 0.8 mm was subjected to electrolytic cobalt plating with an adhesion amount of 0.3 g / m ² , and a plating flux containing zinc chloride and hydrochloric acid was applied. Thereafter, a tin plating bath containing 1% of Mg (temperature 34)
(0 ° C.). After sufficient reaction between the plating bath and the steel sheet surface, the steel sheet was drawn out of the plating bath, the amount of coating was adjusted by a gas wiping method, and the steel sheet was rapidly cooled. The steel sheet after plating has a 0.5 μm FeSn ₂ based alloy layer (7%
And cobalt (total Sn + Mg) 3
It had a plating layer of 5 g / m ² (per side). The surface was subjected to a chromate treatment with an adhesion amount of 12 mg / m ² as chromium to obtain a product plate. When a corrosion test similar to that of Examples 1 and 2 was performed on this steel sheet, the dissolved metal ions showed a good corrosion resistance of 1800 ppm.

Example 5 An annealed and pressure-regulated steel plate having a thickness of 0.8 mm was subjected to electrolytic copper plating with an adhesion amount of 0.6 g / m ² , and a plating flux containing zinc chloride and hydrochloric acid was applied. Then, it was introduced into a Sn-Mg plating bath containing 1.5% Mg (concentration: 360 ° C). After sufficient reaction between the plating bath and the steel sheet surface, the steel sheet was drawn out of the plating bath, the amount of coating was adjusted by a gas wiping method, and the steel sheet was rapidly cooled. The plated steel sheet has an alloy layer (containing 10% of copper) mainly composed of 0.6 μm FeSn ₂ and an adhesion amount (total adhesion amount of Sn + Mg).
It had a plating layer of 45 g / m ² (per side). The surface was subjected to a chromate treatment with an adhesion amount of 18 mg / m ² as chromium to obtain a product plate. When a corrosion test similar to that of Examples 1 and 2 was performed on this steel sheet, the eluted metal ions showed a good corrosion resistance of 1400 ppm.

(Example 6) An annealed and pressure-regulated steel sheet having a thickness of 0.8 mm is coated with a plating flux containing zinc chloride and hydrochloric acid, and then is coated with S containing 1.5% Mg and 8% Zn.
It was introduced into an n-Mg-Zn plating bath (temperature: 350 ° C).
After sufficient reaction between the plating bath and the steel sheet surface, the steel sheet was drawn out of the plating bath, the amount of coating was adjusted by a gas wiping method, and the steel sheet was rapidly cooled. The plated steel sheet has an F of 0.7 μm.
The alloy layer mainly composed of eSn ₂ and the adhesion amount (Sn + Mg + Z
n) 32 g / m ² (per one side). 15 on this surface as chrome
The product plate was subjected to chromate treatment with an attached amount of mg / m ² .

In order to examine the crystal structure of the steel sheet, when the surface was slightly corroded with 1% hydrochloric acid, a crystal structure recognized by the naked eye appeared, and the average value of the major axis dimension was 6 mm. After the cross section was polished, the distribution of Sn, Mg and Zn was analyzed by an EPMA (Electron Probe Microanalyzer), and a uniform distribution was confirmed. 1 in a pressure vessel
10 vol. For forcedly degraded gasoline left overnight at 00 ° C
% Water was added to prepare a corrosion liquid. In this etchant,
When a corrosion test was performed at 45 ° C. for 3 weeks, the eluted metal ions were mainly composed of Mg and Zn.
Although elution of pm was recognized, it was judged that good corrosion resistance was exhibited.

(Comparative Example 1) A material having an adhesion amount of 40 g / m ² of a turn sheet (lead-tin alloy plated steel sheet) conventionally used as a gasoline tank material was subjected to the same corrosion test as in the example. , 9700 ppm lead and 12 iron
It eluted at 00 ppm and was found to be inferior to the steel sheet of the present invention.

(Comparative Example 2) In the same procedure as in Example 2, 0.8 g / m ² was applied to an annealed and pressure-regulated steel sheet having a thickness of 0.8 mm.
Is applied, and a flux for plating containing zinc chloride and hydrochloric acid is applied.
It was introduced into a tin plating bath containing 5% (temperature: 350 ° C.). After sufficient reaction between the plating bath and the surface of the steel sheet, the steel sheet was drawn out of the plating bath, the amount of coating was adjusted by a gas wiping method, and slowly cooled. The plated steel sheet is 0.5 μm FeSn
Alloy layer mainly composed of ₂ (containing 17% of Ni) and attached amount (total attached amount of Sn + Mg) 33 g / m ² (per side)
With a plating layer of The surface was subjected to a chromate treatment with an adhesion amount of 12 mg / m ² as chromium to obtain a product plate.

In order to examine the crystal structure of this steel sheet, when the surface was slightly corroded with 1% hydrochloric acid, a large crystal grew by slow cooling, and the average value of its major axis dimension was 30.0 mm. After the cross section polishing, the distribution state of Sn and Mg was changed to EPMA.
As a result of analysis with an (electron probe microanalyzer), a large number of large needle-like Mg ₂ Sn crystals were observed as compared with Example 2, and the segregation state of Sn and Mg was confirmed. As a result of the same corrosion test as in Example 2, Mg elution of 5200 ppm was observed, and deterioration of corrosion resistance due to huge Mg ₂ Sn crystals was observed.

[0032]

According to the present invention, a lead-free rust-preventive steel plate for a fuel tank having excellent corrosion resistance, workability, and weldability and enduring long-term against degraded gasoline or the like is obtained.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C23C 28/00 C23C 28/02 28/02 B60K 15/02 Z (72) Inventor Masahiro Fuda Kitakyushu, Fukuoka Inside Nippon Steel Corporation, Yawata Works, 1-1 1-1 Tobata-ku, Tohatsu-ku (72) Inventor Jun Maki Inside 1-1 Nippon Steel Corporation, Yawata Works, Kitakyushu-shi, Fukuoka Pref. Inventor Teruaki Izaki 1-1, Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka F-term in Nippon Steel Corporation Yawata Works (reference) 3D038 CA04 CA05 CA06 CC19 4K027 AA02 AA03 AA05 AA22 AB02 AB05 AB13 AB26 AB46 AC03 AC15 AC62 AC82 AE23 4K044 AA02 AB02 BA06 BA10 BA15 BA21 BB03 BB04 BC02 BC05 BC08 CA11 CA16 CA18

Claims (5)

[Claims] An alloy layer having a thickness of 3.0 μm or less is provided on the surface of a steel sheet.
Molten Sn-Mg containing 4%, with the balance being Sn and unavoidable impurities, and having a plating layer having a major axis dimension of the plated metal crystal on the outermost surface of 20 mm or less.
System plated steel sheet. 2. A steel sheet having a molten Sn—Mg plating layer on the surface thereof, wherein the composition of the plating layer is 74 to 99.9% by mass of Sn.
The molten Sn according to claim 1, further comprising:
-Mg-based plated steel sheet. 3. The plating layer according to claim 1, wherein the composition of the plating layer contains, in addition to Sn and Mg, one or more of Zn 0.5 to 20% and Ca 0.01 to 2% by weight. Or a hot-dip Sn-Mg-based plated steel sheet according to item 2. 4. A steel sheet having a thickness of 3.0 containing a total of 0.5% or more of one or more of Ni, Co, and Cu on the surface of the steel sheet.
μm or less alloy layer with a composition of
g of 0.1 to 4%, the balance being Sn and unavoidable impurities, and having a plating layer in which the major axis dimension of the plated metal crystal on the outermost surface is 20 mm or less. The described hot-dip Sn-Mg-based plated steel sheet. 5. The molten metal according to claim 1, wherein a post-treatment layer made of an inorganic compound, an organic compound, or a composite thereof is provided on the surface of the plating layer.
n-Mg plated steel sheet.