JPH0790676A - Zinc-chromium dispersed plated steel sheet excellent in phosphating property and production thereof - Google Patents

Abstract

PURPOSE:To produce a zinc-chromium dispersed plated steel sheet excellent in phosphating properties and having excellent performance in which deterioration in corrosion resistance is not shown by dispersing a zinc phase and a chromium phase therein in a state in which detection is made possible by X-ray diffraction. CONSTITUTION:A zinc-chromium alloy plating layer in which a zinc phase and a chromium phase are dispersed into a steel sheet is formed; where the plating layer is constituted of a one contg. 5 to 40wt.% Cr, and the balance Zn. Then, it has an alloy phase in which the face distance (d) found by X-ray diffraction or electron beam diffraction at least has 2.14 to 2.16 and 2.19 to 2.21, and the zinc phase and chromium phase are dispersed in a state in which they are detectied by X-ray diffraction. Thus, the plated steel sheet suitable for an automobile steel sheet for a place subjected to salt damage in a severe corrosive environment can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rust-preventive zinc-chromium alloy-plated steel sheet used for automobiles, home appliances, building materials and the like, and a method for producing the same.

[0002]

2. Description of the Related Art As a plated steel sheet in which chromium is contained in a zinc or zinc alloy plated layer, for example, Japanese Patent Laid-Open No.
No. 215998, JP-A-1-162795,
Those described in JP-A-63-243295 and the like are known. These plated steel sheets have a chromium content of 1 to
It has a plating layer with a relatively wide range of chromium content of 70% by weight, and it is seen that the corrosion resistance is improved as compared with the conventional zinc or zinc-iron-nickel alloy plating. All of these plated steel sheets are obtained by electroplating from an aqueous solution containing zinc ions and chromium ions. However, these alloy layers are not thermodynamically stable phases,
When the plating conditions were different, the structure of the alloy layer was changed even though the chromium content was the same, which affected the corrosion resistance performance. Therefore, in Japanese Patent Laid-Open No. 4-009459, heat treatment is performed at 200 ° C. to 400 ° C. after plating to change to a thermodynamically stable plating layer and to improve corrosion resistance.

However, the zinc-chromium alloy plating of these zinc-chromium alloy-plated steel sheets is poor in the phosphating property which is a pre-coating treatment, and adversely affects the corrosion resistance after coating.
For example, when used for automobile bodies, a phosphate film as a pretreatment for coating is generally formed by immersing in a zinc phosphate solution having a pH of about 3, but in the case of zinc-chromium alloy plating, No film is formed and so-called scaling occurs. It is considered that this is because the corrosion resistance of the plating layer is extremely high, so that the dissolution reaction (anode reaction) in the treatment liquid does not occur and the formation reaction of the phosphate film does not proceed. Therefore, in Japanese Patent Laid-Open No. 4-009459, a chemically active zinc plating layer is formed as an upper layer of a zinc-chromium alloy layer to form a two-layer plating to improve the phosphate treatment property.

However, in order to perform the two-layer plating, it is necessary to prepare two kinds of plating solutions at the time of plating and perform the continuous plating, which complicates the process and causes operational problems such as mixing of the plating solution. There are also many. Further, since the zinc plating layer having poor corrosion resistance is applied, there is a problem that the anticorrosion effect of the zinc-chromium plating layer is reduced and the corrosion resistance of the entire plating layer is deteriorated. Therefore, it is strongly desired to improve the phosphate treatment property without impairing the corrosion resistance.

[0005]

Therefore, it is obvious that the technique of applying the upper layer plating impairs the corrosion resistance of the zinc-chromium alloy plating layer as the lower layer even if the metal plating other than zinc is applied, and a new technique is proposed. Development is essential. The present invention has been made in view of the above problems, and an object thereof is to provide a plated steel sheet excellent in phosphatability and corrosion resistance, and a method for producing the same.

[0006]

The gist of the present invention is as follows. (1) A plated steel sheet having a zinc-chromium alloy plating layer in which a zinc phase and a chromium phase are dispersed, the plating layer containing 5 to 40% by weight of Cr and the balance being Zn.
And the interplanar spacing d determined by X-ray diffraction or electron beam diffraction is 2.14 to 2.16 and 2.19 to 2.21.
A zinc-chromium dispersion-plated steel sheet excellent in phosphatability, which has an alloy phase containing at least, and wherein the zinc phase and the chromium phase are dispersed in a state detectable by X-ray diffraction. (2) In the method for producing a zinc-chromium dispersion-plated steel sheet according to claim 1, the steel sheet is plated with zinc-chromium to form a zinc-chromium alloy plating layer, and then the alloy plating layer is heated to its melting point or higher. It is a method for producing a zinc-chromium dispersion-plated steel sheet having excellent phosphating property, which is characterized in that it is melted and then cooled.

[0007]

[Function] The mechanism of formation of the phosphate treatment film is that a local battery is formed on the surface of the object to be treated, a dissolution reaction (anode reaction) site and a hydrogen generation reaction (cathode reaction) site by the treatment liquid occur, and a cathode reaction occurs. It is believed that the increase in the interfacial pH of the site causes a precipitation reaction of zinc phosphate to form a film. That is, it is essential that the anode and cathode sites are formed on the surface of the object to be treated. However, the zinc-chromium alloy has a significantly high corrosion resistance, and under the condition of the treatment liquid having a pH of 3, the dissolution reaction (anode reaction) hardly occurs, so that no film is formed. In addition, when a zinc plating layer is applied to the plating surface, a phosphate film is formed, but since the zinc plating layer accelerates the progress of corrosion,
It significantly deteriorates the corrosion resistance of the entire plated layer.

Therefore, as a result of intensive studies on the method for modifying the plating layer on the basis of the mechanism of formation of the phosphate-treated coating, the present inventor found that a zinc dispersion as an anode site was formed in the zinc-chromium alloy plating layer. It was found that the presence of the chromium dispersion as the cathode site markedly improved the phosphating property, and that the zinc and chromium dispersions were finely and uniformly dispersed so that the corrosion resistance was hardly impaired. The size and content of both dispersions can be clarified by clarifying the distribution of the dispersion with an analytical instrument. When the dispersion state is extremely fine, for example, an electron probe microanalyzer ( Even if the EPMA) is used, the resolution becomes lower than that, which is difficult with the current analysis technique. Therefore, quantitative values are not specified in the present invention. Anyway, in the present invention, it is important that zinc and chromium do not form a solid solution in the alloy layer but exist alone as a phase. Therefore, it is specified that they exist in a state of being detected as a zinc-η phase and a chromium phase by X-ray diffraction measurement.

The dispersion plated steel sheet having the above structure cannot be produced by the conventionally known dispersion plating method. That is, the zinc metal fine particles are dissolved in the plating solution by the electrodeposition method from the electrolytic solution in which the dispersion material particles are suspended in the plating solution containing zinc and chromium ions. Furthermore, even if the metal particles are the finest, the limit is about 1 μm, and it is considered that even if the fine particles of this size are co-deposited, the chemical conversion treatability is not improved. Further, fine metal particles have a risk of ignition and are not industrially available.

The zinc-chromium dispersed plating layer of the present invention is Z
It is formed by heating the n-Cr alloy plating layer to its melting point or higher and cooling it. That is, when the Zn—Cr alloy plating is heated above its melting point to melt the plating layer and then cooled, the Zn—Cr alloy phase, Zn—η, is solidified during the solidification process.
Phase and Cr phase are separated into three phases. Furthermore, the Zn-Cr alloy phase that precipitates after cooling is a thermodynamically stable intermetallic compound and is effective for corrosion resistance. In the obtained alloy phase, the crystal plane spacing determined by X-ray diffractometry or the like is d = 2.14 to 2.
Measurement of the interplanar spacing d by X-ray diffraction with at least 16 and d = 2.19 to 2.21 involves some error. Furthermore, d slightly changes due to the internal stress of the film. In addition, a diffraction peak with a minute intensity may not be observed depending on the measurement conditions. There d
The value is defined as a value having the above width.

Note that these facts were discovered by the present inventors for the first time, and even when the binary alloy phase diagram of Zn--Cr is found, there is only data for a region where Cr is 5% by weight or less, and It is impossible to estimate that such a phenomenon will occur. The size of each phase in the plating layer separated into these three phases can be controlled by the length of the melting time and the cooling time. The tendency is that Zn--
The particle diameters of the η and Cr phases are reduced. The heating method for Zr—Cr alloy plating is not particularly specified, and for example, a resistance heating method, an induction heating method, a gas heating method, a burner heating method or the like can be applied. The cooling method is not particularly specified, and natural cooling, gas cooling, water cooling, oil cooling or the like can be applied.

Regarding the composition of the Zn-Cr alloy plating layer, chromium has a great effect on the corrosion resistance and the corrosion resistance after coating, and the content necessary for forming a stable alloy phase is at least 5% by weight or more. And 5% by weight
Below, the corrosion resistance does not differ much from that of normal galvanizing.
When it is 5% by weight or more, for example, in a salt spray test, the generation of red rust is dramatically suppressed, and an epoch-making effect appears. Even if the chromium content exceeds 40% by weight, high corrosion resistance is exhibited, but the so-called powdering property in which the plating layer peels off during processing such as pressing deteriorates, which causes practical restrictions. Therefore, the chromium content is 40% by weight or less.

Incidentally, a small amount of Fe, Ni, Co, Sn, Cd, Pb, C was added during the Zn--Cr alloy dispersion plating of the present invention.
Zn even if it contains u, C, O, S, P, Na, B, etc.
It is acceptable if the crystal plane spacing d of -Cr and the structure of the dispersed particles do not change substantially. When the coating amount of the plating layer of the present invention is 5 to 150 g / m ² , sufficient corrosion resistance can be secured. The Zn-Cr plating method is not particularly limited, and any plating bath may be used. The base steel sheet may be any of ordinary steel sheets, stainless steel sheets, copper-containing steel and the like.

[0014]

EXAMPLES The present invention will be described with reference to examples. The Zn-Cr plated steel sheet shown in Table 1 was produced by the electroplating method. That is, as a plating solution, zinc sulfate, chromium sulfate, sulfuric acid,
Using an acidic liquid consisting of organic additives, current density 100A
/ Dm ² , and a Pt-plated Ti plate as an anode was used for electrodeposition. The Cr content in the plating layer was adjusted by changing the ratio of metal ions in the plating solution. For the heat treatment, after heating to 500 ° C. with a resistance heating device, it was immersed in water and rapidly cooled. The obtained plated steel sheets (Examples 1 to 10 and Comparative Example 1)
About 5), the chemical conversion treatability and corrosion resistance were measured, and X-ray diffraction was performed.

Chemical conversion treatability: The amount of coating formed after coating with a phosphate solution for automobile steel sheets, the coating size, and the coating property were evaluated. That is, a material having a coating amount of 1.5 g / m ² , a uniform crystal size, no enlarged crystals and a completely covered underlayer was judged to be a good material, and a material not having such a material was judged to be a bad material. Corrosion resistance: By the salt spray method according to JIS2371
Evaluation was made based on the percentage of occurrence of red rust after 1008 hours. X-ray diffraction: Cu-K rays were used as the X-ray source, and the relationship between the 2θ value and the X-ray intensity of the Cu target was obtained. Zn-C
The interplanar spacing d was calculated for the peak of the r alloy.

[0016]

[Table 1]

The evaluation results are shown in Table 1 and FIGS. C
Zn content of r-content of 5-40% by X-ray diffraction
The examples in which the existence of the phase, Zn-η phase and Cr phase are confirmed and the interplanar spacing d of the alloy phase is 2.14 and 2.21 all show good phosphatability and corrosion resistance. There is. On the other hand, the comparative material has a small amount of adhesion, and the film is enlarged,
The coverage is poor, so-called scaling occurs, the processability is poor, and the corrosion resistance is also poor.

[0018]

Industrial Applicability As described above, the present invention has excellent performance in comparison with conventional materials in terms of berylate treatment property, and has excellent performance in which corrosion resistance is not deteriorated. Therefore, it is most suitable for automobile steel sheets for salt-damaged areas where the corrosive environment is severe, and building materials for beach areas.

[Brief description of drawings]

1 is an X-ray diffraction pattern of Example 2,

FIG. 2 is an X-ray diffraction pattern of Comparative Example 1.

Claims (2)

[Claims] 1. A plated steel sheet having a zinc-chromium alloy plating layer in which a zinc phase and a chromium phase are dispersed, the plating layer containing 5 to 40% by weight of Cr, the balance being Zn, and X. The interplanar spacing d determined by line diffraction or electron beam diffraction is 2.14 to 2.16 and 2.19 to
A zinc-chromium dispersion-plated steel sheet excellent in phosphate treatment, comprising an alloy phase having at least 2.21, and the zinc phase and the chromium phase being dispersed in a state detectable by X-ray diffraction. . 2. The method for producing a zinc-chromium dispersion-plated steel sheet according to claim 1, wherein the steel sheet is plated with zinc-chromium to form a zinc-chromium alloy plating layer, and the alloy plating layer is heated to a temperature not lower than its melting point. A method for producing a zinc-chromium dispersion-plated steel sheet having excellent phosphating property, which comprises heating to room temperature, melting, and cooling.