JP4537894B2 - Hot Sn-Zn plated steel sheet with good corrosion resistance and weldability - Google Patents

Description

  The present invention relates to a molten Sn—Zn-based plated steel sheet that has excellent corrosion resistance and bonding properties, and is subjected to a surface treatment that does not contain hexavalent chromium as an automobile fuel tank material, household electric machine, or industrial machine material. .

  Conventionally, Pb—Sn alloy-plated steel sheets having excellent corrosion resistance, workability, solderability (weldability) and the like have been mainly used as automobile fuel tank materials and widely used. 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 Laid-Open No. 52-130438 (Patent Document 1). Sn—Zn alloy-plated steel sheets mainly composed of Sn are excellent in corrosion resistance and solderability and have been used in many electronic components. On the other hand, it has been found that this Sn-Zn plated steel sheet has excellent characteristics for use in automobile fuel tanks, and Japanese Patent Application Laid-Open No. 8-269733 (Patent Document 2), Japanese Patent Application Laid-Open No. 8-269734 (Patent Document 3), Japanese Unexamined Patent Application Publication No. 2004-131819 (Patent Document 4) and the like have disclosed a molten Sn—Zn plated steel sheet.

  Automobile fuel tanks need to be seam welded around so as to join many accessories and pipes, and to prevent fuel leakage, and the material side is required to have good and stable jointability that does not hinder continuous production. The However, although the Sn-based plated steel sheet can be subjected to resistance welding such as spot welding or seam welding, it has a property that Sn of the plating layer is easily alloyed with Cu which is a welding electrode. It will be converted to intermetallic compounds. Since this intermetallic compound is brittle, it is gradually lost, and there is a problem that sufficient heat generation cannot be obtained due to a decrease in current density accompanying an increase in surface area, resulting in inferior electrode life.

  This problem is particularly noticeable during spot welding, and the electrode life during spot welding has been extremely short. In order to solve this problem, in Japanese Patent No. 3002445 (Patent Document 5) and Japanese Patent Application Laid-Open No. 2004-360019 (Patent Document 6), the surface contact resistance value is appropriately determined by the plating surface roughness and surface coating amount of the Sn-based plated steel sheet. Techniques for improving continuous weldability by controlling have been disclosed.

Japanese Patent Laid-Open No. 52-130438 JP-A-8-269733 JP-A-8-269734 JP 2004-131819 A Japanese Patent No. 3002445 JP 2004-360019 A JP 2004-531639 A

However, in order to reduce manufacturing costs, the need for further continuous weldability improvement is still high, and the longevity of automobiles has been extended by environmental regulations such as CARB (California Air Resource Board) regulations and ELV (End of Life Vehicle) directives. There has been a strong demand. Since hexavalent chromate becomes unusable due to environmental regulations such as EU directives, various trivalent chromium type and completely chromium free type post-treatment films such as JP 2004-531639 A (Patent Document 7) have been developed. Yes. Unfortunately, the actual state of rust prevention is not as good as hexavalent chromate. Although the corrosion resistance is certainly improved by increasing the adhesion amount, the surface contact resistance is increased, so that the resistance weldability is deteriorated.
The present invention solves the above-mentioned problems, and provides a molten Sn-Zn plated steel sheet that highly balances corrosion resistance and weldability and does not use hexavalent chromium.

  In order to provide a Sn-Zn plated steel sheet with improved rust prevention and weldability, the present inventors variously examined the amount and distribution of the surface treatment film containing the plating composition, structure, and trivalent chromium compound. Thus, the present invention has been achieved. Conventionally, measures to improve corrosion resistance and weldability are trade-offs (that is, to increase rust prevention, the amount of plating or surface treatment can be increased, but both weldability deteriorates) However, it was found that both performances can be improved by optimizing the adhesion amount distribution of the surface treatment film containing the plating composition / structure / trivalent chromium compound.

That is, the present invention is a hot-dip Sn-based plated steel sheet in which a hot-plated layer comprising 1 to 8.8% by mass of Zn and the balance Sn: 91.2 to 99.0% by mass and inevitable impurities is formed on the steel sheet surface. The plating layer is composed of an Sn dendrite crystal extending in the shape of an arm and a structure in which a Sn-Zn binary eutectic structure is buried between the arms of the Sn dendrite. It is a molten Sn—Zn plated steel sheet excellent in corrosion resistance and weldability, characterized in that a surface treatment film made of 3 to 50 mg / m ² of a trivalent chromium compound is formed.

  In addition, the chromium in the surface treatment film remains on the Sn dendrite crystal of the plating layer, and the chromium adhesion amount on the Sn-Zn binary eutectic structure is more than three times that on the Sn dendrite crystal. Furthermore, it is possible to achieve a high balance of weldability and corrosion resistance. As described above, in order to optimize the chromium adhesion distribution of the surface treatment film, the physical properties of the surface treatment agent having a trivalent chromium compound are set such that the surface tension is 30 to 80 mN / m and the viscosity is 1.0. It is desirable to set it to ˜2.5 mP · s.

  As described above, according to the present invention, a hexavalent chromium-free, lead-free rust-proof steel sheet for a fuel tank that has excellent corrosion resistance and weldability and can withstand a long period of time against deteriorated gasoline is obtained industrially. It has an extremely excellent effect.

The present invention is described in detail below.
Steel strip that has been subjected to a series of processes such as hot rolling, pickling, cold rolling, annealing, temper rolling, etc., and using rolled material as a material to be plated, removing rolling oil or oxide film, etc. After the pretreatment, plating is performed. Regarding steel components, it must be a component system that can be processed into a complex shape of the fuel tank, the thickness of the alloy layer at the steel-plating layer interface can be thin to prevent plating peeling, and the progress of corrosion inside and outside the fuel tank is suppressed. It must be a component system.

In the present invention, Sn—Zn alloy plating is basically performed by a hot dipping method. The biggest reason for adopting the hot dipping method is to secure the plating adhesion amount. Even if electroplating is performed for a long time, the amount of plating can be secured, but it is not economical. The plating adhesion range targeted in the present invention is a relatively thick area of 20 to 150 g / m ² (single side), and the hot dipping method is optimal. Further, when the potential difference between the plating elements is large, it is difficult to appropriately control the composition. Therefore, the Sn—Zn alloy is most suitable for the hot dipping method.

  Next, the reason for limiting Zn of the plating composition is that it 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 tank is painted after the tank is molded because it requires perfect rust prevention ability. 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. Addition of Zn for Sn-based plating lowers the potential of the plating layer and provides sacrificial anticorrosive ability. For that purpose, it is necessary to add 1% by mass or more of Zn. Addition of excess Zn exceeding the Sn-Zn binary eutectic point of 8.8% by mass does not cause Sn dendrite to crystallize, raises the melting point, and leads to excessive growth of the intermetallic compound layer under the plating. For this reason, it must be 8.8% by mass or less.

  On the other hand, corrosion inside the tank is not a problem with normal gasoline alone, but a fairly severe corrosive environment appears due to water contamination, chlorine ion contamination, and the formation of organic carboxylic acids due to oxidative degradation of gasoline. To do. If gasoline leaks outside the tank due to piercing corrosion, it can lead to serious accidents, and these corrosions must be completely prevented. When a deteriorated gasoline containing the above-mentioned corrosion promoting component was prepared and the performance under various conditions was examined, the Sn—Zn alloy plating film containing Zn of 8.8% by mass or less exhibits extremely excellent corrosion resistance. confirmed.

  When the pure Sn or Zn content is less than 1% by mass, the plated metal has no sacrificial anti-corrosion ability against the iron from the beginning when exposed to the corrosive environment. There is a problem of pitting corrosion at the part and early red rust generation on the outer surface of the tank. On the other hand, when Zn is contained in a large amount exceeding 8.8% by mass, Zn is preferentially dissolved, and a large amount of corrosion products are generated in a short time, so that there is a problem that the carburetor is easily clogged.

  Moreover, when Zn content increases, the workability of a plating layer also falls and the good press moldability which is the characteristics of Sn group plating is impaired. Further, the increase in the Zn content causes a significant increase in the melting point of the plating layer and the Zn oxide, resulting in a significant decrease in solderability. Therefore, the Zn content in the Sn—Zn alloy plating in the present invention is in the range of 1 to 8.8% by mass, and further in the range of 3.0 to 8.8% by mass to obtain a more sufficient sacrificial anticorrosive action. It is desirable.

  In the plating composition region of the present invention, the molten Sn—Zn plated structure is usually a solidified structure in which primary Sn and spangled binary eutectic structures are mixed. At this time, Zn is particularly easily segregated at the spangle-spangle grain boundary. The reason why Zn tends to segregate at the spangle-spangle grain boundary is not clear, but it is thought that a small amount of impurities having high affinity with Zn have an influence. Zn segregated at the spangle-spangle grain boundary becomes a starting point of corrosion as described above, and causes a state in which piercing corrosion is likely to occur.

  In order to eliminate such segregation of Zn, it is necessary to positively develop primary crystal Sn as an arm-shaped dendrite as shown in FIG. 1 to suppress spangle growth. In the composition range of the present invention, Sn crystallizes as an initial crystal. Therefore, if the arm-shaped Sn dendrite is spread over the plating layer in the initial stage of solidification in the form of a network, the spangle-like binary eutectic formed by the eutectic reaction. The dendrite arm is restrained from growing and cannot be developed greatly. Therefore, the spangles do not collide with each other, Zn that segregates at the spangle-spangle grain boundary disappears, and the corrosion resistance on the inner and outer surfaces of the tank is remarkably improved.

In order to positively develop Sn dendrite, the growth start point of Sn dendrite may be increased. In the solidification process of this hot dipping, the heat removal on the steel plate side is large and solidifies from the interface side of the plating / base metal. Therefore, a dendrite growth starting point can be created by providing fine irregularities on the alloy layer under the hot-dip plating layer or fine irregularities on the base iron itself. In order to give fine irregularities to the alloy layer, the alloying reaction between the hot dipping and the steel plate may be controlled. Specifically, the kind of pre-plating, the plating bath temperature, and the immersion time may be controlled. The kind of pre-plating may be a simple substance of Ni, Co, Cu, an alloy with Fe, or an alloy of these metals. A pre-plating amount of about 0.01 to 2.0 g / m ² is sufficient. Moreover, what is necessary is just to provide surface roughness in the rolling process before hot dipping in order to give an unevenness | corrugation to the surface of a ground iron.

In the present invention, the surface of the plating layer is further coated with a chemical mainly composed of a trivalent chromium compound, and the corrosion resistance and good weldability are obtained by securing ³ to 50 mg / m ^{2 in} terms of metal chromium in the dry film. It is done. If it is less than 3 mg / m ² , sufficient corrosion resistance cannot be expected due to insufficient rust prevention, and if it exceeds 50 mg / m ² , the surface contact resistance increases rapidly, and resistance weldability deteriorates rapidly. Therefore, it is necessary to secure a surface treatment film adhesion amount of ³ to 50 mg / m ^{2 in} terms of metal chromium in the dry film.

  In addition, as a result of intensive studies on the reason why the corrosion resistance and weldability of the material composed of the above-described plating structure and surface treatment adhesion amount are balanced, arm-shaped Sn dendrite crystals that preferentially grow as primary crystals at the time of hot dipping are solidified. And the difference in chromium adhesion due to the difference in plating surface height from the Sn-Zn binary eutectic part, which is the final solidified part and volume shrinkage (pseudo "shrinkage" phenomenon) at the time of solidification is greatly in weldability and corrosion resistance. I found out that it had an effect. That is, the Sn—Zn binary eutectic structure that becomes the starting point of corrosion hits the concave portion on the plating surface and becomes a liquid pool when the surface treatment agent is applied. As a result, on the Sn-Zn binary eutectic structure, the Cr adhesion amount increases, so that excellent corrosion resistance can be ensured.

  Further, since the surface treatment agent hardly stays on the arm-shaped Sn dentrite crystal corresponding to the convex portion of the plating surface and the amount of Cr adhesion decreases, the surface contact resistance can be lowered and excellent weldability can be ensured. FIG. 2 shows a schematic cross-sectional view of a plated-surface-treated coated steel sheet according to the present invention. As shown in this figure, the portion of the arm-shaped Sn dentrite crystal 3 becomes convex and the chromium adhesion amount difference decreases, and the Sn-Zn binary eutectic part 1 becomes concave, so the chromium adhesion amount difference is small. Become more. Since corrosion resistance deteriorates when the trivalent Cr compound of the surface treatment film 2 is not coated on the Sn dendrite, it is desirable that Cr remains on the Sn dendrite crystal. Further, it has been found that by making the chromium adhesion amount on the Sn-Zn binary eutectic structure more than 3 times that of the Sn dendrite crystal, both excellent corrosion resistance and weldability can be achieved.

  Furthermore, in order to obtain the microscopic amount distribution of the surface treatment film containing the trivalent chromium compound as described above, it is desirable to optimize the liquid properties of the drug, and the surface tension is 30 to 80 mN / m, And it is desirable for a viscosity to be 1.0-2.5 mP * s. When the surface tension of the chemical is less than 30 mN / m or the viscosity is less than 1.0 mP · s, improvement in weldability cannot be expected because the surface treatment chemical is uniformly applied to the plating surface. Further, when the surface tension is more than 80 mN / m or the viscosity is more than 2.5 mP · s, a portion that is not applied at all on the plating surface appears, so that the corrosion resistance tends to deteriorate.

  Examples of the trivalent chromium compound in the drug include chromium (III) sulfate, chromium (III) nitrate, chromium (III) biphosphate, chromium (III) fluoride, and chromium (III) halide. As the water-dispersible silica in the drug, colloidal silica, vapor phase silica or the like is desirable. Addition of metal nitrate is desirable as an additional component for improving corrosion resistance and paint adhesion, and the metal in the metal nitrate is at least one selected from the group consisting of alkaline earth metals, Co, Ni, Zr and Ti It is desirable to contain.

  Further, phosphonic acid or a phosphonic acid compound is desirable as an additional component in order to improve the adhesion between the surface treatment film and the plating surface. Although it does not specifically limit as a phosphonic acid compound, Methyl diphosphonic acid, a methylene phosphonic acid, an ethylidene diphosphonic acid etc., or these ammonium salts, alkali metal salts, etc., and the chelate which has one or more phosphonic acid groups or its salt in a molecule | numerator Among these phosphonic acid chelating agents, those having a nitrogen atom in the molecule thereof are oxidized to form an N-oxide. Furthermore, for the purpose of improving corrosion resistance and paintability, it is desirable to add a water-soluble resin as an additional component. The water-soluble resin is not particularly limited, but a water-soluble acrylic resin is desirable.

The quality characteristic of the antirust steel sheet for fuel tanks of the present invention will be shown in Examples.
An annealed and pressure-adjusted steel sheet having a thickness of 0.8 mm was subjected to Ni plating by 0.1 g / m ² (per one side) from a watt bath by electroplating. After applying a plating flux containing zinc chloride, ammonium chloride and hydrochloric acid to this steel plate, it was introduced into a Sn-Zn hot dipping bath. The plating bath and the steel sheet surface drawn out steel sheet from a plating bath After reaction, the adhesion amount adjusted by gas wiping method, (total coating weight of Sn + Zn) coating weight was controlled at 40 g / m ² (per one side). After gas wiping, the cooling rate was variously changed with an air jet cooler to solidify the hot-dip plated layer, and the area ratio of the Sn dendrite and the arm interval were changed. In order to investigate the metallographic structure of this steel sheet, the distribution state of Sn and Zn is analyzed by EPMA (Electron Probe Microanalyzer) from the plating surface layer. Calculated.

  As an example of the invention, No. 1 in Table 1 was used. The solidified structure of 1 is shown in FIG. Further, a surface treatment film was formed on the plating surface by applying and drying a chemical mainly composed of a trivalent chromium aqueous solution on the plating. The amount of adhesion was adjusted by appropriately changing the trivalent chromium concentration in the drug. For the amount of adhesion, an average value in the φ10 mm area was adopted by fluorescent X-ray analysis. Further, from the plating surface layer, the Cr distribution state was analyzed by EPMA (Electron Probe Microanalyzer), and the Cr adhesion amount ratio on the Sn—Zn binary eutectic structure and the Sn dendrite structure was calculated by an arbitrary 100-point average. .

  The corrosion resistance in the salt damage environment on the outer surface of the tank was evaluated by the area ratio of red rust generated after 960 hours of SST, and the area ratio of red rust was 10% or less. Corrosion resistance of the inner surface of the tank was prepared by adding 10 vol% water to forced-degraded gasoline that was allowed to stand at 100 ° C. for one day in a pressure vessel. In 350 ml of this corrosive solution, a plated steel plate with a bead drawn (plate thickness reduction rate 15%, 30 × 35 mm end face / back surface seal) is subjected to a corrosion test at 45 ° C. for 3 weeks, and the eluted metal ions Ion species and elution amount were measured. The amount of elution was considered good when the total metal amount was less than 200 ppm. Weldability was evaluated by the number of electrode life points of spot welding. Welding conditions are Cr-Cu electrode (16φ-DR8R-tip 6φ40R), pressure: 200 kg, pre-pressurization: 50 cycles, energization: 10 cycles, hold: 30 cycles (60 Hz), current: dust generation current x 95% It carried out in. 300 points or more were considered good. Various performance results are shown in Table 2.

  No. in Tables 1 and 2. In Examples 1 to 5 of the present invention, all of them have characteristics that can sufficiently withstand use. No. In Comparative Example 6, the Zn mass% is low, so that it does not have a sufficient sacrificial anticorrosive effect and is slightly inferior in outer surface corrosion resistance. No. In Comparative Examples 7 and 8, the Zn mass% was high, Sn dendrite was no longer crystallized, and Zn segregation was promoted, so that the corrosion resistance of both the inner and outer surfaces was lowered. No. No. 9 had a small amount of Cr deposited, and the corrosion resistance of both the inner and outer surfaces was lowered. No. Since No. 10 has a large amount of Cr adhesion and a high surface contact resistance value, the number of electrode life hit points is small.

It is a microscope picture figure which shows the plating solidification structure figure which concerns on this invention. It is a figure which shows the plating-surface treatment film part steel plate cross-section schematic diagram concerning this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sn-Zn binary eutectic part 2 Surface treatment film 3 Arm-shaped Sn dendrite crystal


Patent applicant: Nippon Steel Corporation
Attorney Attorney Shiina and others 1

Claims (4)

A hot-dip plated Sn-plated steel sheet in which a hot-dip plated layer consisting of 1 to 8.8% by weight of Zn and the balance Sn: 91.2 to 99.0% by weight and inevitable impurities is formed on the steel sheet surface. Has a structure in which a Sn-Zn binary eutectic structure is embedded between the Sn dendrite crystals extending in the form of arms and the arms of the Sn dendrite, and 3-50 mg / m ^{2 in} terms of metallic chromium on the plating surface. A molten Sn—Zn plated steel sheet excellent in corrosion resistance and weldability, characterized in that a surface treatment film made of a trivalent chromium compound is formed. The molten Sn-Zn plated steel sheet excellent in corrosion resistance and weldability according to claim 1, wherein chromium in the surface treatment film remains on the Sn dendrite crystals of the plating layer. 3. The molten Sn—Zn plated steel sheet having excellent corrosion resistance and weldability according to claim 1, wherein the amount of chromium deposited on the Sn—Zn binary eutectic structure is three times or more of that of the Sn dendrite crystal. . The surface treatment agent having a trivalent chromium compound has a surface tension of 30 to 80 mN / m and a viscosity of 1.0 to 2.5 mP · s. A hot-dip Sn-Zn plated steel sheet excellent in corrosion resistance and weldability as described in 1) and a method for producing the same.

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