JP2002317233A - Hot dip tin-zinc based plated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide Pb-free hot dip Sn-Zn based plating which has excellent corrosion resistance, and is particularly suitable as an automotive fuel tank material. SOLUTION: The hot dip Sn based plated steel sheet is obtained by forming a hot dip plated layer containing, by weight, 1 to 20% Zn and 0.001 to 0.5% of one or more kinds selected from the group 4A elements (Ti, Zr and Hf), and containing 79.5 to 98.0% Sn on the surface of a steel sheet. The plated layer has a metallic structure in which an Sn single phase or a Zn single phase is coexistent in an Sn-Zn binary eutectic structure, and the major axis of the Zn single phase is <=5 μm.

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-Zn coated steel sheet having excellent corrosion resistance, bonding properties and workability and suitable as a material for automobile fuel tanks, domestic electric machines and industrial machines. is there.

[0002]

2. Description of the Related Art Conventionally, as a fuel tank material, Pb-S which has been excellent in corrosion resistance, workability, solderability (weldability), etc.
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 carried out in an aqueous solution containing Zn and Sn ions, as disclosed in, for example, JP-A-52-130438. BACKGROUND ART Sn-Zn alloy-plated steel sheets mainly composed of Sn are excellent in corrosion resistance and solderability, and have been frequently used for electronic components and the like.
On the other hand, it has been found that this Sn-Zn-plated steel sheet has excellent properties for use in automobile fuel tanks.
JP-A-69733, JP-A-8-269934, and the like have disclosed hot-dip Sn-Zn plated steel sheets.

[0003]

SUMMARY OF THE INVENTION Pb-Sn alloy plated steel sheets that have been used as fuel tank materials for automobiles are:
Various excellent characteristics (for example, workability, corrosion resistance of tank inner surface, solderability, seam weldability, etc.) have been recognized and used. . 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, and in Sn-Zn plated steel sheets, pitting corrosion due to Zn segregation easily occurs even in a flat part that has not been processed, and particularly, salt water spray assuming a salt damage environment. In the test, the period until the occurrence of red rust is short, and the corrosion resistance in a salt damage environment is not sufficient. In order to further improve the sacrificial corrosion protection, it is sufficient to increase the amount of Zn added. 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.

[0005]

DISCLOSURE OF THE INVENTION The present inventors have studied variously the plating composition, film structure, constitution, etc. with the aim of providing a rustproof steel sheet which does not contain Pb and has an improved rustproof ability. This has led to the present invention. In the present invention, Zn is added in an amount of 1 to 20 wt.
Hot-dip coating layer containing one or more of the 4A group elements (Ti, Zr, Hf) in total of 0.001 to 0.5 wt% and Sn: 79.5 to 98.0 wt% A molten Sn-based plated steel sheet formed on the surface of a steel sheet, wherein the plating layer has a metal structure in which a Sn single phase or a Zn single phase is mixed in a base of a Sn-Zn binary eutectic structure. And a hot-dip Sn—Zn-based plated steel sheet, wherein the major axis of the Zn single phase is 5 μm or less.

[0006] In addition, S in the plating layer of the Sn-based plated steel sheet.
When the n-Zn binary eutectic structure has a spheroidized structure of Zn, the maximum grain size of Zn in the binary eutectic is 1 μm or less, and the Sn—Zn binary eutectic structure has a layered structure. When the maximum interlayer distance between Sn and Zn in the binary eutectic is 1
μm or less. Sn for the plating layer
0.1 to 4.0 wt% Mg in addition to Zn and 4A group elements
%, And Ni, C
o, having an alloy layer having a thickness of 3.0 μm or less containing at least 0.5 wt% of one or more of Cu,
The plating layer surface may have a post-treatment layer made of an inorganic compound, an organic compound, or a composite thereof.

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, Sn-Zn alloy plating is basically performed by a hot-dip plating method. The main reason for adopting the hot-dip plating method is to secure 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 a relatively thick region of 20 to 150 g / m ² (one side), and the hot-dip plating method is most suitable. Further, when the potential difference between the plating elements is large, it is difficult to appropriately suppress the composition. Therefore, the hot-dip plating method is optimal for the Sn—Zn alloy.

Next, the reasons for limiting the amount of Zn in the plating composition will be described. The amount of Zn in the plating composition 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-preventive ability, but as a raw material, red rust is prevented by the anticorrosive effect of the plating layer. In particular, the anticorrosion effect of this plating layer is extremely important in a portion where the coating is difficult to rotate. The addition of Zn in the Sn-based plating lowers the potential of the plating layer and provides sacrificial corrosion protection. For that, 1wt
% Or more is necessary, but excessive addition causes an increase in the melting point and leads to excessive growth of the intermetallic compound layer under the plating, so it must be 20 wt% or less.

[0010] On the other hand, corrosion on the inner surface of the tank is not a problem when only normal gasoline is used. However, the corrosion is extremely severe due to mixing of water, mixing of chlorine ions, generation of organic carboxylic acid due to oxidation 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-mentioned corrosion promoting component was prepared and its performance under various conditions was examined, it was confirmed that the Sn—Zn alloy plating film containing Zn at 20 wt% or less exhibited extremely excellent corrosion resistance. .

When the content of pure Sn or Zn containing no Zn is less than 1% by weight, the plating metal does not have a sacrificial anticorrosion ability to the ground iron from the initial stage of exposure to a corrosive environment. Pitting at the pinholes and the early occurrence of red rust on the outer surface of the tank are problems. Zn is 20w
If it is contained in a large amount exceeding t%, Zn is preferentially dissolved, and a large amount of corrosion products are generated in a short period of time. Also, Z
As the n content increases, the workability of the plating layer also decreases, deteriorating the good press formability characteristic of Sn-based plating. Further, the solderability is significantly reduced by increasing the Zn content. Therefore, Sn- in the present invention
Zn content in Zn alloy plating is 1 to 20 wt%
In order to obtain an even more sufficient sacrificial anticorrosive action, suppress the dissolution of Zn, and make clogging of the carburetor less likely to occur, the content should be in the range of 5.0 to 8.8 wt%. desirable.

Next, a 4A group element (Ti, Z
The reason for limiting r, Hf) is that it is the most important additive element in the present invention, and is limited by the balance between corrosion resistance and productivity on the inner and outer surfaces of the gasoline tank.
As described above, Zn controls corrosion on the inner and outer surfaces of the tank by imparting a sacrificial anti-corrosion ability in Sn-based plating. However, in such a corrosive environment, Zn itself originally elutes at a high rate. If there is a Zn segregation part in the layer, only that part is eluted preferentially, and the state is liable to cause pitting corrosion at that part. Group 4A element (T
i, Zr, Hf) are added for the purpose of suppressing Zn segregation during plating and forming a uniform plating film. Although the mechanism of the homogenization is not clear, the group 4A element (Ti, Zr, H
It is presumed that f) acts to increase the nucleation in the plating layer and suppresses the coarsening of the specific plating component (Zn in this system).

This homogenization has a remarkable effect on the major axis of the Zn single phase in the plating structure, and the 4A group element (Ti, Zr,
The addition of Hf) tends to decrease the major axis of the Zn single phase. Similarly, in the Sn-Zn binary eutectic, Z
In the case of having a spherical structure of n, the maximum grain size of Zn, and in the case of having a layered structure, both of the maximum interlayer distance between Sn and Zn tend to be reduced by the addition of a Group 4A element (Ti, Zr, Hf). Also, the elements of the 4A group (Ti, Zr, Hf) act on the refinement of the binary eutectic structure.

Such a homogenizing effect is achieved by using a group 4A element (T
i, Zr, Hf) does not appear when added at less than 0.001 wt%. On the other hand, when the total amount of the 4A group elements (Ti, Zr, Hf) exceeds 0.5 wt%, not only the homogenization effect is saturated, but also a large amount of dross is generated from the plating bath during operation and the melting point is reduced. The rise causes deterioration of plating quality and cost. Therefore, the content of the Group 4A element (Ti, Zr, Hf) in the present invention is in the range of 0.001 to 0.5 wt% in total of one or two or more kinds. Within this range, the structure of the plating layer described below can be controlled by appropriately adjusting the amount of addition based on the above findings.

Next, the reason for limiting the form of the metallographic structure of the plating layer is determined by the effect of improving the corrosion resistance of the inner and outer surfaces of the tank, particularly the effect of suppressing perforation corrosion. The Sn—Zn binary system is a eutectic type having no intermetallic compound, and its eutectic point is Z
n 8.8 wt%. Therefore, the structure obtained after solidification is a Sn-Zn binary eutectic structure if it has a eutectic composition, and if the Zn content is less than 8.8 wt%, the Sn-Zn binary eutectic structure is contained in the Sn-Zn binary eutectic structure. If Zn is more than 8.8 wt%, a metal structure in which a Zn single phase is mixed in the Sn-Zn binary eutectic structure base material is obtained.

In this plating, the ground iron side of the plating is slightly affected by the ground iron, but the plated layer itself has a Sn-Zn binary system structure. As described above, Zn is an essential element from the viewpoint of imparting sacrificial corrosion protection to Sn-based plating.
Since Zn elutes relatively easily in a corrosive environment due to the nature of Zn, its distribution is particularly important, and it is preferable that it is finely and homogeneously dispersed. Above all, if the Zn single-phase portion grows very coarsely, only that portion is selectively dissolved in a corrosive environment, and as a result, it becomes a starting point of perforation corrosion.

Further, even if the corrosion does not occur, the Zn is immediately released, so that long-term sacrificial corrosion cannot be expected. In order to maintain such long-term corrosion resistance, the major axis of the Zn single phase needs to be 5 μm or less. In the present invention, the size of the Zn single phase is defined by the major axis of the crystal. Since the formed crystal is not necessarily spherical (two-dimensionally circular), the major axis and the minor axis of the crystal are not equal. Therefore, in the present invention, the crystal is defined by the major axis of the crystal. When taking the form of a needle, its length is taken as the major axis.

Although not as remarkable as the Zn single phase portion, the distribution of Zn in the Sn-Zn binary eutectic structure must be fine in order to maintain the sacrificial corrosion protection ability over the entire plating layer for a long period of time. It is preferred that they are dispersed. The Sn-Zn binary eutectic structure may have a Zn spheroidized structure or a layered structure due to a cooling method at the time of solidification and a trace amount of impurities contained in the plating layer. The maximum grain size of Zn in the binary eutectic is 1 μm or less in the case of having a eutectic, and the maximum interlayer distance between Sn and Zn in the binary eutectic is 1 μm or less in the case of having a layered structure. It is preferable to maintain. The inter-phase distance of the layered structure is defined as a distance from the center of the thickness of the Sn layer to the center of the thickness of the Zn layer, and is substantially [(Sn layer-thickness of layer) + (Zn
Layer-thickness of layer)] ÷ 2.

For improving the corrosion resistance, it is effective to add Mg to the plating layer of the Sn-based plated steel sheet in addition to the Sn, Zn, and 4A group elements. Mg has an action of stabilizing the corrosive organisms of Zn and an action of itself forming a protective Mg-based hydroxide film on the plating surface in a salt damage environment, and particularly improves the corrosion resistance of the outer surface of the tank. This effect is 0.1wt%
If less, there is no significant difference in the presence or absence of addition. On the other hand, if it is added in an amount of more than 4 wt%, a large amount of dross is generated, and the melting point of the plating bath is increased, which is disadvantageous in terms of operability and cost. This leads to a large amount of crystallization in the layer, and the workability of the plating layer is reduced. Therefore, when adding Mg,
The added amount of Mg is 0.1 to 4 wt%.

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. On the other hand, in order to process into a complicated shape like a fuel tank, it is necessary to secure a high degree of workability. The alloy layer must be formed in a small amount in order to wet well, but because it is hard and brittle, 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 exfoliation has a very large relationship with the plating type, thickness, and steel type. In the case of the present invention, the thickness of the alloy layer is desirably 3.0 μm or less.

To improve the wettability, it is 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 easily reacts with tin is plated on the surface of the steel sheet immediately before plating to improve the wettability. Ni, Co, Cu or the like 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 wt% or more, a rust-preventive steel sheet having excellent workability and corrosion resistance can be obtained.

In the present invention, perfect corrosion resistance is expected by further performing a post-treatment of the surface of the plating layer with an inorganic compound, an organic compound, or a composite thereof. This treatment is very familiar with the underlying Sn—Zn plating layer, has the effect of covering a defective portion such as a minute pinhole, or repairing the pinhole by dissolving the plating layer, and greatly improves corrosion resistance.

[0023]

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) An annealed and pressure-regulated steel sheet having a thickness of 0.8 mm was coated with a flux for plating containing zinc chloride, ammonium chloride and hydrochloric acid, and then an Sn-Zn plating bath containing Ti (bath melting point +80) ° C). After the plating bath and the steel sheet surface were sufficiently reacted, the steel sheet was drawn out of the plating bath, the amount of the steel sheet was adjusted by a gas wiping method, and cooled. The plated steel sheet had an alloy layer mainly composed of FeSn ₂ , and the amount of plating (the total amount of Sn + Zn) was controlled to 40 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 metallographic structure of the steel sheet, after the cross section was polished, the distribution of Sn and Zn was analyzed by an EPMA (Electron Probe Microanalyzer). The corrosion resistance of the outer surface of the tank in a salt damage environment was evaluated by the corrosion form and the pit depth after 960 hours of SST. The corrosion form was good for uniform corrosion.
The corrosion resistance of the inner surface of the tank was determined by adding 10 vol% water to forcedly degraded gasoline left at 100 ° C. for 24 hours in a pressure vessel to prepare a corrosion liquid. In 350 ml of this corrosive liquid, a plated steel plate (30 × 35
mm end face / back face seal) was subjected to a corrosion test at 45 ° C. × 3 weeks, and the ionic species and the amount of metal ions eluted were measured. The elution amount was determined to be 200 ppm.

In the present invention examples (Nos. 1 to 9) shown in Table 1, both the Zn single phase and the binary eutectic structure are finely controlled, and the alloy layer does not grow thick. In the external environmental corrosion test,
The corrosion mode was uniform corrosion, no red rust was generated, and Zn
Pitting corrosion due to segregation was not observed. In the internal environmental corrosion test, the detected metal species are Zn and Sn constituting the plating layer, and the amount of elution is small and very good corrosion resistance is shown.

In Comparative Example (No. 14), Ti was not added, and the Zn single phase and binary eutectic structure were not refined / homogenized. Similarly, in Comparative Examples (Nos. 10 and 12), the binary eutectic structure is not refined / homogenized. In such non-homogenized plating, local dissolution progresses in the segregated portion of Zn in the corrosive environment on both the inner and outer surfaces, and as a result, pitting corrosion that immediately reaches the base iron occurs.

The comparative examples (Nos. 11, 13, and 15) were made of Ti
Is excessively added, and the melting point of the plating bath rises, so that operation exceeding 500 ° C. is forced, and as a result, the thickness of the alloy layer is increased. Since the hard and brittle alloy layer grew, the good workability of the Sn-based plating was impaired, and when the beaded drawing was performed, many cracks were formed in the plating layer,
As a result, elution from the ground iron was observed in the internal environmental corrosion test. In Comparative Example (No. 16), no Zn was added at all, so there was no sacrificial anticorrosive ability, pitting occurred from the pinholes that had to be present, and corrosion resistance was poor in a corrosive environment on both the inner and outer surfaces. In Comparative Example (No. 17), Zn was added excessively, the Ti homogenization effect was no longer exerted, the properties of the entire plating layer approached Zn, and good corrosion resistance was not obtained.

[0028]

[Table 1]

Example 2 An annealed and pressure-regulated steel sheet having a thickness of 0.8 mm is coated with a plating flux containing zinc chloride, ammonium chloride and hydrochloric acid, and then Ti, Zr, H
f was introduced into a Sn-15 wt% Zn plating bath containing various wt% (bath melting point + 80 ° C.). After the plating bath and the steel sheet surface were sufficiently reacted, the steel sheet was drawn out of the plating bath, the amount of the steel sheet was adjusted by a gas wiping method, and cooled. The plated steel sheet has an alloy layer mainly composed of FeSn ₂ , and has a plating adhesion amount (total adhesion amount of Sn + Zn) of 40.
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 metallographic structure of the steel sheet, the distribution of Sn and Zn was analyzed by EPMA (Electron Probe Microanalyzer) after polishing the cross section. The corrosion resistance of the outer surface of the tank in a salt damage environment was evaluated by the corrosion form and the pit depth after 960 hours of SST. The corrosion form was good for uniform corrosion.
The corrosion resistance of the inner surface of the tank was determined by adding 10 vol% water to forcedly degraded gasoline left at 100 ° C. for 24 hours in a pressure vessel to prepare a corrosive liquid. In 350 ml of this corrosive liquid, a plated steel plate (30 × 35
mm end face / back face seal) was subjected to a corrosion test at 45 ° C. × 3 weeks, and the ionic species and the amount of metal ions eluted were measured. The elution amount was determined to be 200 ppm.

Examples of the present invention shown in Table 2 (Nos. 18 to 29)
In this case, both the Zn single phase and the binary eutectic structure are finely controlled, and the alloy layer is not grown thick. In the external surface corrosion test, the corrosion mode was uniform corrosion, no red rust was generated, and no pitting corrosion due to Zn segregation was observed. In the internal environmental corrosion test, the detected metal species are Zn and Sn constituting the plating layer, and the amount of elution is small and very good corrosion resistance is shown.

[0032]

[Table 2]

(Example 3) A plating flux containing zinc chloride, ammonium chloride and hydrochloric acid was applied to an annealed and pressure-regulated steel sheet having a thickness of 0.8 mm, and then Ti was added to 0.05 w.
It was introduced into a Sn-8 wt% Zn plating bath containing various wt% of t% and Mg. After the plating bath and the steel sheet surface were sufficiently reacted, the steel sheet was drawn out of the plating bath, the amount of the steel sheet was adjusted by a gas wiping method, and cooled. The steel sheet after plating is Fe
It had an alloy layer mainly composed of Sn ₂ . The plating weight (Sn + Zn total weight) was controlled at 40 g / m ² (per one side). 15m as chrome on this surface
The product plate was subjected to chromate treatment with an adhesion amount of g / m ² .

Examples of the present invention are shown in Table 3 (Nos. 30 to 33). To examine the metallographic structure of this steel sheet,
When the distribution states of n, Zn, and Mg were analyzed by EPMA (Electron Probe Microanalyzer), a fine and uniform structure was observed. The corrosion resistance of the outer surface of the tank in a salt-damaged environment was good after corrosion for 960 hours after SST with uniform corrosion and no pitting. The corrosion resistance of the inner surface of the tank was such that the metal ions eluted were Sn, Zn, and Mg in the plating layer, and the elution amounts were all less than 200 ppm, which was good. On the other hand, in the comparative example (No. 34) in which Mg was excessively added, the melting point was increased, so that the alloy layer grew thickly and workability was deteriorated due to the introduction of a large amount of brittle Mg-based intermetallic compound into the plating layer. Especially, it was inferior in the corrosion test of the internal environment.

[0035]

[Table 3]

Example 4 An annealed and pressure-adjusted steel sheet having a thickness of 0.8 mm was subjected to electric Ni plating in a Watt bath so as to be 1.0 g / m ² . Subsequently, a plating flux containing zinc chloride, ammonium chloride and hydrochloric acid was applied, and then introduced into a Sn-8 wt% Zn plating bath containing 0.05 wt% of Ti. After the plating bath and the steel sheet surface were sufficiently reacted, the steel sheet was drawn out of the plating bath, the amount of the steel sheet was adjusted by a gas wiping method, and cooled. After plating,
It has an alloy layer mainly composed of FeSn ₂ ,
i was contained at 20 wt%, and the thickness was 2 μm. The plating weight (total Sn + Zn weight) is 40 g /
m ² (per 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 metallographic structure of this steel sheet, the distribution of Sn and Zn was analyzed by EPMA (Electron Probe Microanalyzer) after section polishing, and a fine and uniform structure was observed. The corrosion resistance of the outer surface of the tank in a salt-damage environment was excellent after SST 960 hours with uniform corrosion and no pitting. The corrosion resistance of the inner surface of the tank was good because the eluted metal ions were Sn and Zn in the plating layer and the elution amount was 50 ppm.

Example 5 An annealed and pressure-regulated steel sheet having a thickness of 0.8 mm was subjected to electric Ni plating in a Watt bath so as to have a thickness of 1.0 g / m ² . Subsequently, a plating flux containing zinc chloride, ammonium chloride and hydrochloric acid was applied, and then introduced into a Sn-8 wt% Zn plating bath containing 0.05 wt% of Ti. After the plating bath and the steel sheet surface were sufficiently reacted, the steel sheet was drawn out of the plating bath, the amount of the steel sheet was adjusted by a gas wiping method, and cooled. After plating,
It has an alloy layer mainly composed of FeSn ₂ and is made of Ni
Was contained at 20 wt%, and the thickness was 2 μm. The plating weight (total Sn + Zn weight) is 40 g /
m ² (per side). Silica on this surface,
A post-treatment consisting of phosphoric acid and an acrylic resin was performed at 40 mg / m ² as SiO ₂ to obtain a product plate.

In order to examine the metallographic structure of the steel sheet, the distribution of Sn and Zn was analyzed by EPMA (Electron Probe Microanalyzer) after section polishing, and a fine and uniform structure was observed. The corrosion resistance of the outer surface of the tank in a salt-damaged environment was good after corrosion for 960 hours after SST with uniform corrosion and no pitting. The corrosion resistance of the inner surface of the tank was good because the eluted metal ions were Sn and Zn of the plating layer, and the elution amount was 70 ppm.

(Example 6) A cold-rolled steel sheet having a thickness of 0.8 mm was used.
After removing the rolling oil by the Sendzimer method,
Reduced, Sn-8 wt% Zn containing 0.05 wt% Ti
It was introduced into the plating bath. Sufficient reaction between plating bath and steel sheet surface
After steel sheet is pulled out from plating bath, gas wiping
The coating amount was adjusted by the method and cooled. Plated steel sheet
Is FeSn _TwoHaving an alloy layer mainly composed of
Its thickness was 1 μm. Plating weight (Sn +
Total amount of Zn) is 40 g / m^Two Control (per side)
did. 15 mg / m as chromium on this surface^Two Adhesion of
A quantity of chromate treatment was performed to obtain a product plate.

In order to examine the metallographic structure of this steel sheet, the distribution of Sn and Zn was analyzed by EPMA (Electron Probe Microanalyzer) after section polishing, and a fine and uniform structure was observed. The corrosion resistance of the outer surface of the tank in a salt-damaged environment was good after corrosion for 960 hours after SST with uniform corrosion and no pitting. Further, the corrosion resistance of the inner surface of the tank was good because the eluted metal ions were Sn and Zn of the plating layer and the elution amount was 40 ppm.

[0042]

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.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masahiro Fuda 1-1-1, Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works (72) Inventor Teruaki Izaki Tobata-ku, Kitakyushu-shi, Fukuoka No. 1-1, Nippon Steel Corporation Yawata Works F-term (reference) 4K027 AA05 AA22 AB02 AB05 AB12 AB13 AB46 AC03 AC15 AC52 AE03 AE23 4K044 AA02 AB02 BA06 BA10 BB01 BB03 BC02 BC05 BC08 CA11 CA18

Claims (6)

[Claims] 1. 1 to 20 wt% of Zn and Ti, Zr, H
One or more elements selected from the group 4A elements of f, 0.001 to 0.5 wt% in total, Sn: 7
A hot-dip Sn-based coated steel sheet having a hot-dip coating layer containing 9.5 to 98.0 wt% formed on the surface of a steel sheet, wherein the plating layer is formed of a Sn-single-phase Sn-Zn binary eutectic base. Alternatively, it has a metal structure in which Zn single phase is mixed,
A hot-dip Sn—Zn-based plated steel sheet, wherein the long diameter of n single phase is 5 μm or less. 2. The method according to claim 1, wherein the Sn—
2. The Sn—Zn-based plating according to claim 1, wherein the Zn binary eutectic structure has a spheroidized structure of Zn, and the maximum particle diameter of Zn in the binary eutectic is 1 μm or less. 3. steel sheet. 3. The method according to claim 1, wherein the Sn—
2. The molten Sn—Zn according to claim 1, wherein the Zn binary eutectic structure has a layered structure of Sn and Zn, and the maximum interlayer distance between Sn and Zn in the binary eutectic is 1 μm or less. 3. System plated steel sheet. 4. The method according to claim 1, wherein Sn is contained in the plating layer of the Sn-based plated steel sheet.
The hot-dip Sn-Zn-based plated steel sheet according to any one of claims 1 to 3, further comprising 0.1 to 4 wt% of Mg in addition to Zn and a 4A group element. 5. The steel sheet according to claim 1, further comprising an alloy layer having a thickness of 3.0 μm or less containing at least 0.5 wt% of one or more of Ni, Co and Cu. 4. The hot-dip Sn-Zn-based plated steel sheet according to any one of 4. 6. The molten metal according to claim 1, further comprising a post-treatment layer made of an inorganic compound, an organic compound, or a composite thereof on the surface of the plating layer.
n-Zn plated steel sheet.