JP3330282B2 - Hot-dip galvanized steel sheet with excellent phosphate treatment and blackening resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet which can remarkably enhance phosphatability and has good blackening resistance. Such a coated steel sheet provided with oil is useful in that these properties can be maintained for a long period of time.

[0002]

2. Description of the Related Art Hot-dip galvanized steel sheets are widely used for home electric appliances, building materials, etc. because of their excellent corrosion resistance. In that case, the plating may be finally used as it is, but in practice, it is almost always used after painting.

[0003] When applying a coating, a base treatment such as a phosphate treatment is performed before the coating for the purpose of improving the adhesion of the coating film. At the time of painting, in addition to hot-dip galvanized steel sheets, many steel sheets such as cold-rolled steel sheets, electro-galvanized steel sheets, and galvannealed steel sheets are often subjected to the same process.
At that time, the problem is the phosphatability between each material,
That is, the reactivity is different. Among them, hot-dip galvanized steel sheet is less reactive than electro-galvanized steel sheet,
Insufficient formation of phosphate crystals or partial formation of non-formed portions may adversely affect coating adhesion after coating. Such a tendency is particularly remarkable when the phosphating solution deteriorates with use. Although the reason why the phosphatability decreases in the case of hot-dip galvanized steel sheet is not clear in this way, the oxide film formed on the surface of the hot-dip galvanized steel sheet has some adverse effects during the manufacturing process. It is presumed to have given.

For the purpose of improving the phosphatability of the above-mentioned hot-dip galvanized steel sheet, for example, Japanese Patent Application Laid-Open No. 3-287785 discloses that a predetermined coating amount of Zn oxide, Mn oxide, boric acid, etc. A method of improving chemical conversion treatment properties by coating a zinc-coated steel sheet with such a coating is disclosed. However, according to this method,
In order to form a desired coating, the plated steel sheet is immersed in an aqueous solution to which a predetermined amount of element is added, or the electrolytic treatment is performed using the plated steel sheet as a cathode in the aqueous solution, or the aqueous solution is sprayed on the plated steel sheet. Such a new process is required, and the equipment for this is separately installed, so that the manufacturing conditions are complicated and the manufacturing cost is further increased.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to significantly improve phosphatability by using an ordinary plating line. Another object of the present invention is to provide a hot-dip galvanized steel sheet having excellent blackening resistance.

[0006]

The hot-dip galvanized steel sheet of the present invention, which is excellent in phosphatability and blackening resistance and which can solve the above problems, is formed by mechanically processing a hot-dip galvanized surface. The point is that the area ratio (R) of the newly formed surface and the Pb concentration (A) in the plating layer satisfy the relationship of the following expressions (1) and (2). 0.02 ≦ A ≦ 0.2 (1) −250 A + 20 ≦ R ≦ −250 A + 70 (2) where A: Pb concentration in the plating layer (% by weight) R: Area ratio of new surface (%) (R ≧ 0)

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted intensive studies in order to achieve the above object, and found that in order to enhance the phosphatability of a hot-dip galvanized steel sheet, it is necessary to increase the activity of the surface of the galvanized steel sheet. It is necessary to improve the Pb concentration (A) in the plating layer and the area ratio (R) of the new surface formed by mechanically treating the hot-dip galvanized surface.
However, it was found that when the activity of the plating surface was too high, blackening resistance was adversely affected. Thus, the inventors have found that the Pb concentration and the area ratio of the newly formed surface should be well controlled within a certain range in order to provide both of the above functions, and the present invention has been completed.

[0008] Of these, Pb, which is added to a normal plating bath, is an element that does not form a solid solution in Zn and has a smaller ionization tendency than Zn. Therefore, Pb is microscopically scattered on the plating surface, and this serves as a starting point of the reaction with phosphoric acid to promote the reaction. As a result, the Pb is not adversely affected by oxides such as ZnO that inhibit the reactivity.
It is considered that the phosphatability is improved.

In the present invention, the term “new surface” refers to a stable high-temperature oxide film formed on the hot-dip galvanized surface when solidifying and cooling after hot-dip galvanizing,
Reduction (destruction) by mechanical treatment of leveler, brush, etc.
It means the surface that is exposed when made, and refers to the fine concave portion made of pure metal. In this way, by destroying a part of the stable oxide film present on the plating surface and exposing active sites capable of reacting with phosphoric acid on the plating surface, it is possible to significantly improve the reactivity with phosphate. it can.

As described above, in the present invention, when improving the phosphatability, the Pb concentration and the area ratio of the newly formed surface are controlled by focusing on the oxide film on the plating surface which is considered to inhibit the reaction with phosphoric acid. To promote the reaction as a starting point of the reaction with phosphoric acid, or to destroy the formed oxide film to increase the activity with phosphoric acid. As a result, it was found that when the activity became too high, the blackening resistance was adversely affected.

That is, when the Pb concentration in the plating layer or the area ratio of the newly formed surface under a skin pass pressure is increased, the phosphate treatment property is improved, but the blackening resistance tends to be deteriorated.
Although the reason is not clear, if the activity becomes too high, it becomes easy to form a black oxide film, and it is considered that the effect becomes remarkable especially when the activity exceeds a predetermined range.

Therefore, in the present invention, it is necessary that the Pb concentration (weight%, abbreviated as A) in the plating layer satisfies the range of the following equation (1). 0.02 ≦ A ≦ 0.2 (1) If A is less than 0.02, the effect of improving phosphatability cannot be obtained. It is preferably at least 0.04, more preferably at least 0.07. However, if it exceeds 0.2, the deterioration of blackening resistance becomes remarkable, so the upper limit is made 0.2. Preferably it is 0.17 or less, more preferably 0.15 or less.

Further, according to the present invention, Pb in the plating layer
It is necessary that the concentration and the area ratio (%, R, where R ≧ 0) of the new surface of the plating surface satisfy the relationship of the following expression (2). −250A + 20 ≦ R ≦ −250A + 70 (2) When the value is below the left side of the expression (2), the effect of improving the phosphatability is not obtained. On the other hand, when the value exceeds the right side of the expression (2), And remarkable deterioration of blackening resistance. Preferably -250A
+ 30 ≦ R ≦ −250A + 60.

FIG. 1 is a graph showing the relationship between the above (1) and (2). As shown in the figure, those satisfying both the ranges of the formulas (1) and (2) have good phosphatability and blackening resistance, whereas those falling below the left side (lower limit) of each formula have phosphorus. It can be seen that those which are inferior in acid salt treatability and exceed the right side (upper limit) of each formula are inferior in blackening resistance.

Hereinafter, the present invention will be described in detail with reference to examples.
However, the following examples do not limit the present invention,
All modifications and alterations without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.

[0016]

EXAMPLE Using a continuous hot-dip galvanizing equipment, which is a non-oxidizing furnace type continuous plating equipment, and performing plating while varying the Pb concentration in the plating bath, skin pass rolling is performed with various rolling reductions. As a result, various hot-dip galvanized steel sheets were produced as shown in Table 1. For each of the materials thus obtained, the Pb concentration in the plating layer was analyzed and the area ratio of the new surface was measured in the following manner.

[Pb Concentration in Plating Layer] After the plating layer was dissolved in dilute hydrochloric acid, the Pb concentration was analyzed by atomic absorption spectrometry. [Area ratio of new surface] The plating surface was observed with an optical microscope, and the ratio of the new surface to the entire plating surface was measured by an image analyzer.

Next, the phosphatability and blackening resistance of each material were evaluated according to the following criteria. [Phosphating property] Using a phosphating solution (PB3140, manufactured by Nippon Parkerizing Co., Ltd.) at 55 ° C. for 120
After phosphate treatment for 2 seconds, the surface was observed with a scanning electron microscope, and the integrity of the phosphate crystals was evaluated according to the following criteria. ：: good △: slight non-uniformity of phosphate film crystals ×: large non-uniformity of phosphate film crystals

[Blackening resistance] The above material was treated at 50 ° C and 98% R
After being stored in a constant temperature and humidity chamber of H for 48 hours, the degree of blackening of the surface was visually evaluated. ：: no blackening Δ: slight blackening ×: large blackening The results are also shown in Table 1.

[0020]

[Table 1]

From the results in the table, the following can be considered. Nos. 1 to 13 are examples that satisfy the requirements of the present invention, but it can be seen that all of them are excellent in both phosphatability and blackening resistance.

On the other hand, when the A value and the R value are below the lower limit of the formula defined by the present invention (Nos. 14 and 15),
Example where A value is below the lower limit of equation (1) (No. 16 to 19)
All have poor phosphatability, and the examples in which the R value exceeds the upper limit of Formula (2) (Nos. 21 to 27) all have poor blackening resistance. No. 20 is an example in which the A value is lower than the lower limit of the present invention and the R value is higher than the upper limit of the present invention, but it is understood that both the phosphate treatment property and the blackening resistance are inferior.

[0023]

According to the present invention, since the present invention is constructed as described above, it is possible to remarkably improve the phosphatability of a plated steel sheet by using a normal plating line without special equipment. A hot-dip galvanized steel sheet having excellent blackening resistance can be obtained.

[Brief description of the drawings]

FIG. 1 shows the relationship between the Pb concentration in a plating layer and the area ratio of a new surface.
It is the graph which investigated the effect on phosphate treatment property and blackening resistance.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. The area ratio (R) of a newly formed surface formed by mechanically treating a hot-dip galvanized surface and the Pb concentration (A) in a plating layer are determined by the following formulas (1) and (2). Hot-dip galvanized steel sheet excellent in phosphating property and blackening resistance characterized by satisfying. 0.02 ≦ A ≦ 0.2 (1) −250 A + 20 ≦ R ≦ −250 A + 70 (2) where A: Pb concentration in the plating layer (% by weight) R: Area ratio of new surface (%)