JP2012017488A - Sn-zn plated steel plate with excellent corrosion resistance, film adhesion and spot weldability, and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an Sn-Zn plated steel plate with excellent corrosion resistance, film adhesion and spot weldability which is subjected to rust prevention treatment using a chromate-free aqueous metal surface treatment agent.SOLUTION: The Sn-Zn steel plate with excellent corrosion resistance, film adhesion and spot weldability includes a steel plate, an Sn-Zn plating layer formed on one surface or both surfaces of the steel plate, and a composite film formed on the Sn-Zn plating layer. The composite film contains a predetermined organic silicon compound, at least one fluoro compound selected from a titanium hydrofluoric acid and a zircon hydrofluoric acid, a phosphoric acid, and a heteropoly acid compound. The organic silicon compound, the fluoro compound and the phosphoric acid are in a predetermined mass ratio.

Description

The present invention relates to a Sn-Zn plated steel sheet excellent in corrosion resistance, coating film adhesion, and spot weldability, and a method for producing the same.

  Conventionally, Pb-Sn plated steel sheets with excellent corrosion resistance, workability, solderability (weldability), etc. have long been used as materials for automobile fuel tanks. Yes. Various steel plates have been proposed as alternatives. Among them, Sn—Zn plated steel plates are widely used because of their excellent corrosion resistance, workability, and economic efficiency.

  Patent Documents 1 and 2 disclose a surface-treated steel sheet for a fuel tank in which a chromate treatment containing hexavalent chromium is performed on a Zn-Ni alloy plating. Patent Documents 3 and 4 show a material obtained by chromate treatment of molten aluminum plating. However, the treatment method using hexavalent chromium is excellent in corrosion resistance and economic efficiency, but since hexavalent chromium is an environmentally hazardous substance, regulations are becoming strict and its use has been restricted. Therefore, a method for reducing hexavalent chromium shown in Patent Document 5, a method using trivalent chromium shown in Patent Document 6, and a method using Si-based drug without using chromium as shown in Patent Document 7 Etc. have been proposed.

 However, depending on the corrosion resistance evaluation under severe conditions and the welding conditions, the conventional technology that does not include chromium does not sufficiently achieve the purpose, and a technology using trivalent chromium is used. However, as will be described later, the prior art is a process invented based on a galvanized steel sheet, and even when applied as it is to a Sn-based plated steel sheet having a different surface state, the coating film adhesion was insufficient.

 On the other hand, in the case of automotive fuel tank materials, Pb-Sn plated steel sheet called turn plating was used, but Pb may not be used due to European regulations, so molten aluminum plated steel sheet or molten Sn-Zn plated steel sheet Has come to be used.

In the prior art, Patent Documents 8 to 14 show methods for obtaining a treatment liquid having a good liquid stability by a combination of trivalent chromium and an organic acid. These are the amounts of hexavalent chromium in the treatment liquid. The main focus is on reducing the amount, and the ratio of hydroxyl group / carboxyl group in the organic acid molecule described later is insufficient, so the coating film adhesion and water resistance are not necessarily sufficient.
In addition to the above-described reasons, the inventions disclosed in Patent Documents 15 and 16 are based on the premise that the coating film is applied and washed with water after drying. Is disadvantageous. With respect to the aforementioned Patent Document 6, no component having a hydroxyl group is contained, and with respect to Patent Document 17 as well, no examination for improving the adhesion of the coating film has been made. Even in Patent Document 18, the coating film adhesion and the corrosion resistance are certainly good, but the weldability, in particular, the continuous spotting property of spot welding is insufficient. Also in patent document 19, since the phenol resin which is an insulator is mixed, the continuous spot property of spot welding is inadequate.

JP 58-45396 A JP-A-5-106058 Japanese Patent Laid-Open No. 10-168581 JP-A-11-217682 JP 11-256354 A International Publication WO 2002 / 20874A2 JP 2001-32085 A Japanese Patent Laid-Open No. 10-81977 Japanese Patent Laid-Open No. 10-81976 JP-A-10-176279 Japanese Patent Laid-Open No. 10-212586 JP 11-256354 A JP 2001-181855 A JP 2002-146550 A JP 2002-256447 A JP 2004-346360 A Japanese Patent Laid-Open No. 2002-226981 International Publication WO2008 / 029953 Japanese Patent No. 3897487

  The present invention is intended to solve the problems of the prior art, and uses a hexavalent chromium-free water-based metal surface treatment agent that is excellent in corrosion resistance, coating film adhesion and spot weldability. It aims at providing the Sn-Zn plating steel plate which was excellent in corrosion resistance, coating-film adhesiveness, and spot weldability which processed the rust prevention, and its manufacturing method.

As a result of intensive studies to solve these problems, the present inventors have adopted the following configuration.
[1] A steel plate, a Sn—Zn plating layer formed on one or both surfaces of the steel plate, and a composite coating formed on the Sn—Zn plating layer, The silane coupling agent (A) containing one group and the silane coupling agent (B) containing one glycidyl group have a mass ratio [(A) / (B)] of 0.5 to 1.7. And an organosilicon compound (W) having a number average molecular weight of 1000 to 10,000, at least one fluoro compound (X) selected from titanium hydrofluoric acid or zircon hydrofluoric acid, and phosphoric acid ( Y) and a heteropolyacid compound (Z), and the mass ratio [(X) / (W)] of the organosilicon compound (W) to the fluoro compound (X) is 0.02. ≦ [(X) / (W)] ≦ 0.07 The mass ratio [(Y) / (W)] of the organosilicon compound (W) and the phosphoric acid (Y) is in the range of 0.03 ≦ [(Y) / (W)] ≦ 0.12. Sn-Zn plated steel sheet with excellent corrosion resistance, coating film adhesion and spot weldability.
[2] The heteropolyacid compound (Z) is silicotungstic acid hydrate, phosphotungstic acid hydrate, phosphomolybdic acid hydrate, phosphorus tungstomolybdate hydrate or phosphovanadomolybdate hydrate. The Sn-Zn plated steel sheet having excellent corrosion resistance, coating film adhesion, and spot weldability according to [1].
[3] Corrosion resistance, coating film adhesion according to [1] or [2], wherein the mass per unit area after drying of the composite film is 0.4 to 2.0 g / m ² . Sn-Zn plated steel sheet with excellent spot weldability.
[4] An organosilicon compound (W) comprising a silane coupling agent (A) containing one amino group and a silane coupling agent (B) containing one glycidyl group, a fluoro compound (X), phosphorus The acid (Y) and the heteropolyacid compound (Z) have a mass ratio [(X) / (W)] of the organosilicon compound (W) and the fluoro compound (X) of 0.02 ≦ [(X ) / (W)] ≦ 0.07, the mass ratio [(Y) / (W)] of the organosilicon compound (W) to the phosphoric acid (Y) is 0.03 ≦ [(Y) / (W)] An aqueous metal surface treatment agent contained in a range of ≦ 0.12 is applied on the Sn—Zn plating layer of a steel sheet having an Sn—Zn plating layer, and then the aqueous metal surface treatment agent is applied. Is formed at a maximum plate temperature of 50 ° C. to 250 ° C. to form a composite film. Edible, coating adhesion method excellent Sn-Zn plated steel sheet spot weldability.

  ADVANTAGE OF THE INVENTION According to this invention, the Sn-Zn plating steel plate which satisfies corrosion resistance, coating-film adhesiveness, and spot weldability can be provided.

  The Sn-Zn plated steel sheet excellent in corrosion resistance, coating film adhesion and spot weldability of the present invention and the production method thereof will be described in detail below.

First, the Sn—Zn plated steel sheet used in the present invention will be described.
The steel plate used in the present invention is preferably a component system in which the components of the steel plate can be processed into a complicated shape of the fuel tank, and further, the progress of corrosion in the fuel tank inside and outside environment is suppressed. Moreover, it is preferable to use the steel plate which the thickness of the alloy layer formed in the interface of a steel plate and a Sn-Zn plating layer is thin, and can prevent plating peeling.

Next, the composition of the Sn—Zn plating layer will be described.
The Zn content in the Sn—Zn plating layer is desirably in the range of 1.0 to 8.8% by mass, and further in the range of 3.0 to 8.8% by mass in order to obtain a more sufficient sacrificial anticorrosive action.
The reason for the limitation of the Zn content in the Sn-Zn plating layer is limited by the balance of corrosion resistance between the inner surface and the outer surface of the fuel tank. The outer surface of the fuel tank is painted after the fuel tank is molded because perfect rust prevention capability is required. Therefore, although the coating thickness determines the rust prevention ability, the material prevents red rust by the anticorrosion effect of the plating layer. In particular, the anticorrosive effect of the plating layer is extremely important in the part where the coating is not good. Therefore, the addition of Zn to the Sn-based plating lowers the potential of the plating layer and imparts sacrificial anticorrosive ability. For that purpose, addition of 1.0 mass% or more of Zn is necessary.
On the other hand, the addition of excess Zn exceeding the Sn-Zn binary eutectic point of 8.8% by mass does not cause Sn dendrite to crystallize, causing the melting point to rise, leading to excessive growth of the intermetallic compound layer under the plating, etc. There is a risk. In addition, Zn in the plating layer is preferentially dissolved, and a large amount of corrosion products are generated in a short period of time, so that there is a problem that the carburetor is easily clogged. In addition, when the Zn content increases, the workability of the plating layer also decreases, and the good press formability, which is a feature of Sn-based plating, may be impaired. Furthermore, there is a problem that the solderability is significantly lowered due to the rise in melting point of the plating layer and Zn oxide. Therefore, the Zn content in the Sn—Zn plating layer is desirably 8.8% by mass or less.

  In addition, corrosion on the inner surface of the fuel tank is not a problem with normal gasoline alone, but there is a fairly severe corrosive environment due to water contamination, chlorine ion contamination, and the formation of organic carboxylic acids due to oxidative degradation of gasoline. Appear. If gasoline leaks outside the fuel tank due to piercing corrosion, it can lead to serious accidents, and these corrosions must be completely prevented. A deteriorated gasoline containing the above corrosion promoting components was prepared, and the performance under various conditions was examined. As a result, it was confirmed that the Sn-Zn plated layer containing 8.8% by mass or less of Zn exhibits extremely excellent corrosion resistance. .

Furthermore, in the case of a pure Sn plating layer that does not contain any Zn, or a Sn-Zn plating layer with a Zn content of less than 1.0% by mass, the plated metal is sacrificed to the iron from the beginning when exposed to the corrosive environment. Since it does not have anti-corrosion capability, pitting corrosion at the plating pin hole portion on the inner surface of the fuel tank and early red rust generation on the outer surface of the fuel tank are problematic.
Therefore, the Zn content in the Sn—Zn plating layer in the present invention is desirably in the range of 1.0 to 8.8% by mass, and further in the range of 3.0 to 8.8% by mass in order to obtain a further sufficient anticorrosive action.

Next, the composite film in the present invention will be described.
The composite film is formed on the Sn—Zn plating layer and is composed of an organosilicon compound (W), a fluoro compound (X), a phosphoric acid (Y), and a heteropoly acid compound (Z).
This composite film is obtained by applying a water-based metal surface treatment agent having a composition described later and drying. At this time, it is desirable to adjust the mass per unit area of the composite film after drying to be 0.5 to 1.0 g / m ² . If the mass per unit area of the composite coating is less than 0.4 g / m ² , the coverage of the composite coating on the Sn—Zn plating layer is low, and the corrosion resistance and spot weldability are inferior. On the other hand, if the mass per unit area of the composite coating exceeds 2.0 g / m ² , the specific resistance of the composite coating becomes high, so that spot weldability decreases, which is not preferable.

  The organosilicon compound (W), which is an essential component of the composite film, includes a silane coupling agent (A) containing one amino group in the molecule and a silane coupling agent (B) containing one glycidyl group in the molecule. ) At a mass ratio [(A) / (B)] of 0.5 to 1.7. As a compounding ratio of the silane coupling agent (A) and the silane coupling agent (B), the mass ratio [(A) / (B)] is preferably 0.5 to 1.7. If the mass ratio [(A) / (B)] is less than 0.5, the bath stability is remarkably lowered, which is not preferable. On the other hand, if it exceeds 1.7, the coating film adhesion is remarkably lowered, which is not preferable.

Although it does not specifically limit as a silane coupling agent (A), 3-aminopropyl triethoxysilane, 3-aminopropyl trimethoxysilane, etc. can be illustrated.
Examples of the silane coupling agent (B) containing one glycidyl group in the molecule include 3-glycidoxypropyltrimethoxysilane and 3-glycidoxypropyltriethoxysilane.

  The component ratio of the organosilicon compound (W) in the composite film is preferably in the range of 65 to 95% by mass.

  The method for adjusting the organosilicon compound (W) is not particularly limited, but the silane coupling agent (A) and the silane coupling agent (B) are sequentially added to water adjusted to pH 4, The method of stirring for a predetermined time is mentioned.

In the composite film, as a functional group derived from the organosilicon compound (W), a formula —SiR ¹ R ² R ³ (wherein R ¹ , R ² and R ³ are independent of each other, an alkoxy group or At least one selected from a functional group (a) represented by a hydroxyl group, at least one representing an alkoxy group, a hydroxyl group (separate from those that can be included in the functional group (a)) and an amino group 1 type of hydrophilic functional group (b) is included. In this case, the number of functional groups (a) is preferably 2 or more per molecule, and the number of functional groups (b) is preferably 1 or more per molecule. When the number of functional groups (a) is one in one molecule, the adhesion force to the Sn surface and the film-forming property are lowered, so that the coating film adhesion is lowered. The number of carbon atoms of the alkoxy group in the definition of R ¹ , R ² and R ³ of the functional group (a) is not particularly limited, but is preferably 1 to 6, more preferably 1 to 4, more preferably 1 or 2. Most preferably.

  Furthermore, the number average molecular weight of the organosilicon compound (W) is preferably 1000 to 10,000, and more preferably 1300 to 6000. Although the number average molecular weight here is not particularly limited, either direct measurement by TOF-MS method or conversion measurement by chromatography method may be used. When the number average molecular weight is less than 1000, the water resistance of the formed composite film is remarkably lowered. On the other hand, when the number average molecular weight is larger than 10,000, it is difficult to stably dissolve or disperse the organosilicon compound (W).

Next, about the fluoro compound (X) which is an essential component of the composite film, at least one selected from titanium hydrofluoric acid or zircon hydrofluoric acid can be exemplified.
The component ratio of the fluoro compound (X) in the composite film is preferably in the range of 1 to 7% by mass.

  The component ratio of phosphoric acid (Y), which is an essential component of the composite film, is preferably in the range of 2 to 10% by mass.

Examples of the heteropolyacid compound (Z) that is an essential component of the composite film include silicotungstic acid · 26 water (SiO ₂ · 12WO ₃ · 26H ₂ O), phosphotungstic acid n hydrate (H ₃ (PW ₁₂ O ₄₀ ) · nH ₂ O), phosphomolybdic acid n hydrate (H ₃ [PMo ₁₂ O ₄₀ ] · nH ₂ O, n≈30), phosphorus tungsto molybdic acid (H ₃ [PW ₁₂ -XMo _X O ₄₀ ] · nH ₂ O, (0 <X <12, n≈30)), phosphorus vanadomolybdic acid (H ₁₅ -X [PV ₁₂ -XMo _X O ₄₀ ] · nH ₂ O, (6 <X < 12, n≈30)).
The component ratio of the heteropolyacid compound (Z) in the composite film is preferably in the range of 0.5 to 25% by mass.

Next, each compounding ratio of the organosilicon compound (W), the fluoro compound (X), the phosphoric acid (Y), and the heteropolyacid compound (Z), which are each component of the aqueous metal surface treatment agent, will be described.
First, regarding the compounding ratio of the fluoro compound, the mass ratio [(X) / (W)] of the organosilicon compound (W) and the fluoro compound (X) is preferably 0.02 to 0.07. If the mass ratio [(X) / (W)] is less than 0.02, the effect of adding to the corrosion resistance is not exhibited, which is not preferable. On the other hand, if the mass ratio [(X) / (W)] is larger than 0.07, spot weldability is lowered, which is not preferable.

  Next, regarding the compounding ratio of phosphoric acid (Y) which is an essential component of the composite film, the mass ratio [(Y) / (W)] of the organosilicon compound (W) and phosphoric acid (Y) is 0.03 to 0.03. It is preferably 0.12. If the mass ratio [(Y) / (W)] is less than 0.03, the effect of addition to corrosion resistance is not exhibited, which is not preferable. On the other hand, if the mass ratio [(Y) / (W)] is greater than 0.12, the resulting composite coating will become so water-soluble that the coating film adhesion deteriorates, which is not preferable.

  Next, regarding the compounding quantity of heteropoly acid compound (Z), mass ratio [(Z) / (W)] of organosilicon compound (W) and heteropoly acid compound (Z) is 0.01-0.3. It is preferable. If the mass ratio [(Z) / (W)] is less than 0.01, the corrosion resistance cannot be sufficiently improved, which is not preferable. On the other hand, if the mass ratio [(Z) / (W)] is greater than 0.3, the heteropolyacid is an elution component, and if the elution component ratio is increased, the shielding property (protective property) of the coating itself is lowered, resulting in corrosion resistance. Since it gets worse, it is not preferable.

In addition, a leveling agent, a water-soluble solvent, a metal stabilizer, an etching inhibitor, a pH adjuster, and the like for improving the coatability may be added to the composite film according to the present invention.
Examples of the leveling agent include nonionic or cationic surfactants such as polyethylene oxide or polypropylene oxide adducts and acetylene glycol compounds.
Water-soluble solvents include alcohols such as ethanol, isopropyl alcohol, t-butyl alcohol and propylene glycol, cellosolves such as ethylene glycol monobutyl ether and ethylene glycol monoethyl ether, esters such as ethyl acetate and butyl acetate, acetone , Ketones such as methyl ethyl ketone and methyl isobutyl ketone.
Furthermore, examples of the metal stabilizer include chelate compounds such as EDTA and DTPA, and examples of the etching inhibitor include amine compounds such as ethylenediamine, triethylenepentamine, guanidine, and pyrimidine. In particular, those having two or more amino groups in one molecule are more preferable because they are effective as metal stabilizers.
Examples of the pH adjuster include organic acids such as acetic acid and lactic acid, inorganic acids such as hydrofluoric acid, ammonium salts and amines.

    Next, the manufacturing method of the Sn-Zn plated steel sheet of this invention is demonstrated. First, pre-treatment such as removal of rolling oil or oxide film using steel plate or rolled material that has undergone a series of processes such as hot rolling, pickling, cold rolling, annealing, temper rolling, etc. as a material to be plated I do.

Next, an Sn—Zn plating layer is formed on the steel plate surface. The Sn-Zn plating layer is formed by performing Sn-Zn plating. Sn-Zn plating is basically performed by the hot dipping method in order to secure the coating amount. Even if electroplating is performed for a long time, the amount of plating can be secured, but it is not economical. Moreover, the plating adhesion amount range aimed at in the present invention is a relatively thick area of 20 to 150 g / m ² (single side), and if the potential difference of the plating elements is large, it is difficult to control the composition appropriately. Therefore, Sn-Zn plating is optimally performed by hot dipping.

Next, a composite film is formed on the Sn—Zn plating layer.
First, a water-based metal surface treatment agent, which is a constituent component of the composite film, is prepared by blending an organosilicon compound (W), a fluoro compound (X), a phosphoric acid (Y), and a heteropolyacid compound (Z).
The aqueous metal surface treatment agent can be obtained by blending each component in an aqueous solvent such as water or alcohol at a predetermined blending ratio. The obtained aqueous metal surface treatment agent is applied on the Sn—Zn plating layer of the steel sheet.

  Thereafter, the aqueous metal surface treatment agent is dried to form a composite film on the Sn—Zn plating layer. The drying temperature at this time is preferably 50 ° C. to 250 ° C. as the maximum plate temperature, and more preferably 110 ° C. to 200 ° C. at which the siloxane reaction sufficiently occurs. When the maximum plate temperature is less than 50 ° C., the cross-linking reaction in the composite film is insufficient, which adversely affects the coating film adhesion. On the other hand, if the maximum plate temperature exceeds 250 ° C., the composite coating becomes hard and the elongation deteriorates, so that the spot weldability deteriorates. The heating method is not particularly limited, and any method such as hot air, direct fire, induction heating, infrared, near infrared, electric furnace, etc. may be used.

  The Sn—Zn plated steel sheet of the present invention satisfies all of the corrosion resistance, coating film adhesion and spot welding continuous spotting properties. This makes it possible to provide automobile fuel tank materials, household electrical machines, and industrial machine materials that have excellent corrosion resistance, coating film adhesion, and spot weldability. The reason for such an effect is presumed as follows, but the present invention is not limited to such presumption.

  The composite film formed using the aqueous metal surface treatment agent in the present invention is mainly composed of an organosilicon compound. The excellent corrosion resistance obtained by the present invention is that the organosilicon compound reacts with each other to form a continuous film when a part of this organosilicon compound is concentrated by drying after application of the aqueous metal surface treatment agent, And -Si-OH group formed by hydrolysis of a part of the organosilicon compound exerts a remarkable barrier effect by forming a Si-OM bond (metal element on the surface of the object to be coated) with the steel sheet surface. It is estimated that.

  On the other hand, the composite film using the aqueous metal surface treatment agent in the present invention is formed on the basis of silicon. In addition, the structure of the silicon-organic chain is regular and the organic chain is relatively short, so that the silicon-containing part and the organic part are regularly and densely arranged in a very small area in the composite film. That is, an inorganic substance and an organic substance are arranged. Therefore, it is presumed that it becomes possible to form a new composite film having both the heat resistance and conductivity usually possessed by the inorganic film and the processing followability and paint film adhesion usually possessed by the organic film.

In addition, the Sn—Zn plating layer in the present invention exhibits excellent corrosion resistance, but this Sn—Zn plating layer is likely to deteriorate in spot weldability (continuous spot welding spot performance). The reason is estimated as shown in the following 1) to 3).
1) Cu, which is an electrode for spot welding, and Sn, which accounts for more than 90% of the plating, form a brittle alloy easily by the heat of resistance heating, and the alloy layer generated by impact during electrode pressurization is lost, resulting in a surface area However, since the welding is performed at a constant current, the current density is substantially reduced.
2) If the current density is below a certain level, the welding strength cannot be maintained, so it is necessary to replace it with a new electrode.
3) In order to shorten electrode costs and electrode replacement time, continuous spot spotting of spot welding of 400 points or more is desired, but the spot spot welding spot spot of the plating is about 200 points.

  Then, spot weldability can be improved by forming the composite film which concerns on this invention. That is, in the present invention, direct contact between Cu as an electrode and Sn during plating is prevented due to the ductility of the organic component in the composite film during electrode pressurization in spot welding. Furthermore, the heat generation on the plating surface is suppressed by the heat of vaporization accompanying the evaporation of the organic film during the resistance heat generation. Moreover, the inorganic component in the composite film is transformed into a glass state. This inorganic component transformed into glass functions as a barrier film that prevents alloy formation between Cu as an electrode and Sn during plating. As described above, spot weldability can be improved. However, this effect was originally observed only for Sn-Zn plated steel sheets that are inferior in spot weldability. Spots such as electrogalvanized steel sheets (EG), hot dip galvanized steel sheets (GI), and hot galvanized steel sheets (GA) The effect is hardly obtained with a material having a continuous spotability of 1000 or more in welding. This is presumably because the glassy film formed during the continuous spotting covers the electrode surface and the energization point cannot be secured.

  EXAMPLES The present invention will be specifically described below with reference to examples and comparative examples of the present invention, but the present invention is not limited thereto.

First, the Sn-Zn plated steel sheet shown in the following (1) was prepared, and the degreasing process shown in the following (2) was applied to the Sn-Zn plated steel sheet to perform a pretreatment. Next, various components shown in Tables 1 to 3 were blended to prepare an aqueous metal surface treatment agent and applied to the surface of the Sn-Zn plated steel sheet. Further, the aqueous metal surface treatment agent was dried to form a composite film on the surface of the Sn—Zn plated steel sheet. Thereafter, the following evaluation tests were performed to evaluate corrosion resistance, coating film adhesion, and spot weldability.
Tables 4 and 5 show formulation examples of the water-based metal surface treatment agent, the mass per unit area of the composite film, and the drying temperature. Table 6 shows the result of the evaluation test.

Preparation of test plate (1) Test material Commercially available materials shown below were used.
Sn-Zn plated steel sheet (SZ: Zn content 7% by mass): Plate thickness = 0.8 mm, plating basis weight = 30/30 (g / m ² )

(2) Degreasing treatment The above-described Sn-Zn plated steel sheet is subjected to a silicate alkaline degreasing agent Fine Cleaner 4336 (registered trademark: manufactured by Nihon Parkerizing Co., Ltd.) for 2 minutes at a concentration of 20 g / L and a temperature of 60 ° C. The test plate was spray-treated, washed with pure water for 30 seconds and then dried.

〔Evaluation test〕
1. A salt spray test (SST) according to SST plane test JIS Z 2371 was conducted for 980 hours, and the occurrence of white rust was observed. ○ and ◎ were accepted.
<Evaluation criteria>
◎ = White rust generation is less than 3% of total area ○ = White rust generation is 3% or more and less than 10% △ = White rust generation is 10% or more and less than 30% of total area × = White rust generation is total area More than 30%

2. Coating adhesion test Melamine alkyd paint was applied by bar coating so that the film thickness after baking and drying was 25 μm, baked at 120 ° C. for 20 minutes, then cut into 1 mm grids, and the coating adhesion Was evaluated based on the remaining number ratio (remaining number / number of cuts: 100). ○ and ◎ were accepted.
<Evaluation criteria>
◎ = 100%
○ = 95% or more Δ = 90% or more and less than 95% × = less than 90%

3. Spot weldability (continuous spotting)
Appropriate current range Electrode: Cr-Cu 16mmφ-Tip 6mmφ-40R Pressurization: 220kgf
Welding conditions: pre-pressurization time; 50 cycles energization time; 10 cycles hold time; 3 cycles Evaluation method: nugget diameter after peeling nugget part peel lower limit ⇒ 3.6 mm upper limit ⇒ generation of dust

The continuous dot was investigated at the center current value in the appropriate current range obtained above. The end point was a point where the nugget diameter after peeling of the nugget portion was less than 3.6 mm. ○ and ◎ were accepted.
◎ = Number of hits is 600 points or more ○ = Number of hit points is 400 points or more and less than 500 points Δ = Number of hit points is 100 points or more and less than 400 points x = Number of hit points is 100 points or less

  As shown in Table 6, it can be seen that Examples 1 to 52 satisfy all of good corrosion resistance, coating film adhesion and spot weldability.

Claims (4)

Steel sheet,
A Sn-Zn plating layer formed on one or both surfaces of the steel sheet;
A composite film formed on the Sn-Zn plating layer,
In the composite film, the silane coupling agent (A) containing one amino group and the silane coupling agent (B) containing one glycidyl group are 0 in mass ratio [(A) / (B)]. An organosilicon compound (W) blended at a ratio of 0.5 to 1.7 and having an average molecular weight of 1000 to 10,000;
At least one fluoro compound (X) selected from titanium hydrofluoric acid or zircon hydrofluoric acid;
Phosphoric acid (Y),
A heteropolyacid compound (Z), and
The mass ratio [(X) / (W)] of the organosilicon compound (W) and the fluoro compound (X) is in the range of 0.02 ≦ [(X) / (W)] ≦ 0.07. The mass ratio [(Y) / (W)] of the organosilicon compound (W) and the phosphoric acid (Y) is in the range of 0.03 ≦ [(Y) / (W)] ≦ 0.12. A Sn-Zn plated steel sheet excellent in corrosion resistance, coating film adhesion, and spot weldability.   The heteropolyacid compound (Z) is either silicotungstic acid hydrate, phosphotungstic acid hydrate, phosphomolybdic acid hydrate, phosphotungstomolybdate hydrate or phosphovanadomolybdate hydrate. The Sn-Zn plated steel sheet having excellent corrosion resistance, coating film adhesion, and spot weldability according to claim 1, wherein the number is one. The mass per unit area after drying of the composite film is 0.4 to 2.0 g / m ^2. Corrosion resistance, coating film adhesion, and spot weldability according to claim 1 or 2. Excellent Sn-Zn plated steel sheet.   An organosilicon compound (W) comprising a silane coupling agent (A) containing one amino group and a silane coupling agent (B) containing one glycidyl group, a fluoro compound (X), and phosphoric acid (Y ) And the heteropolyacid compound (Z) have a mass ratio [(X) / (W)] of the organosilicon compound (W) and the fluoro compound (X) of 0.02 ≦ [(X) / ( W)] ≦ 0.07, the mass ratio [(Y) / (W)] of the organosilicon compound (W) to the phosphoric acid (Y) is 0.03 ≦ [(Y) / (W) The aqueous metal surface treatment agent contained in the range of ≦ 0.12 is applied on the Sn—Zn plating layer of the steel sheet having the Sn—Zn plating layer, and then the aqueous metal surface treatment agent is added at 50 ° C. to 50 ° C. Corrosion resistance characterized by forming a composite film by drying at a maximum plate temperature of 250 ° C, Film adhesion method excellent Sn-Zn plated steel sheet spot weldability.