JPS63235494A - Iron-zinc plated steel material and production thereof - Google Patents

Abstract

PURPOSE:To obtain a corrosion resistant iron-zinc plated steel material having a lower layer with remarkably improved adhesion to the base steel material by inhibiting the formation of gamma1 phase made of an iron-zinc intermetallic compd. in a lower layer formed by plating and by regulating the zinc content in the lower layer. CONSTITUTION:The deterioration of the adhesion of a lower layer formed on a steel sheet by plating to the steel sheet is caused by the formation of gamma1 phase made of an intermetallic compd. represented by a chemical formula Fe3Zn21 in the lower layer, so the adhesion can be considerably improved by restricting the amt. of the gamma1 phase formed to <=1vol.%. By regulating the zinc content in the lower layer to 60-90atom.%, satisfactory corrosion resistance can be maintained. Since it is supposed that the formation of the gamma1 phase depends on the flowing conditions of a plating bath used to form the lower layer, the flowing conditions in this invention are decided according to the molar ratio between Fe<2+> and Zn<2+> (Zn<2+>+Fe<2+>) and Reynolds number Re. By plating under the decided flowing conditions, the formation of the gamma1 phase in the lower layer is inhibited and the adhesion of the layer to the base steel sheet can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to iron galvanized steel materials used in automobiles, home appliances, etc.

[Conventional technology]

Conventionally, an iron-galvanized steel sheet for use as a base for painting is known, which consists of two layers on a steel sheet. This steel sheet is an electrogalvanized steel sheet for use as a paint base that has excellent corrosion resistance (corrosion resistance as plated), corrosion resistance after painting, and workability.

Of these two layers, the upper layer is a layer provided to improve paint film adhesion and chemical conversion treatment properties, so it has a high iron content, and the lower layer has a high zinc content to improve corrosion resistance and workability. .

This corrosion-resistant steel material having a two-layered plating layer is widely used as a corrosion-resistant steel material for automobile bodies and the like because of its excellent comprehensive rust prevention properties.

[Problem that the invention seeks to solve]

However, the conventional iron-zinc double-layer plated steel sheet has a drawback in the adhesion between the base steel sheet and the lower plating layer. In the case of this poor adhesion, it causes powdering during press processing and chipping during actual vehicle driving, which is a major problem.

Therefore, the present invention aims to provide a corrosion-resistant steel material having a lower plating layer that has corrosion resistance and dramatically improved adhesion to the steel material, and a method for manufacturing the same.

[Means for solving problems]

The present invention as a means for solving the above-mentioned problems is to provide Fe5Zn, which is an intermetallic compound of iron and zinc, in an iron-galvanized steel material having two iron-zinc plating layers on the surface of the steel material.
The second invention is characterized by having a lower plating layer containing 1v10 or less of the Γ1 phase called zt, and the amount of zinc contained in the plating film is 60 to 90A10. p e 2 + ion and Zn t
When the molar ratio of + ions is Fe"/ZnZn"> 0.4, the flow state of the plating bath relative to the steel plate is the Raynozzle number (Re), and plating is performed at 2XIQ" above Re, and the same molar ratio in the plating bath is , 0.2〈Zn''/Fe''≦0.
In case No. 4, the lower plating layer is formed by plating with Re≦I x 104.

[For production]

According to the research conducted by the present inventors, the cause of the deterioration of the adhesion between the lower plating layer and the steel sheet is that the lower plating layer contains an intermetallic compound r represented by the chemical formula Fe1ZnH.
The I-phase is generated. Therefore, this phase is 1v70
Adhesion can be significantly improved by keeping the amount below. In addition, the zinc content in the lower plating film was increased to 60%.
By setting it as 90A10, good corrosion resistance can be maintained.

Furthermore, based on the knowledge that the formation of the intermetallic compound r1 depends on the flow conditions of the plating bath when applying the lower layer plating, in the present invention, the flow conditions are adjusted by changing the molar ratio of Fe'' and Zn''+ (Zn2+ /Fe2) and Reynolds number (Re), and if the lower layer plating is performed under such flow conditions, the formation of the Γ1 phase is suppressed and the above-mentioned adhesion is improved.

[Specific structure of the invention]

Hereinafter, the present invention will be specifically explained in detail.

First, to explain the process leading to the completion of the present invention, as mentioned above, the iron-galvanized steel sheet according to the present invention has two layers on the steel sheet: an upper layer plating with a high iron content and a lower layer plating with a high zinc content. The composition of the lower layer plating on the high zinc side is such that the zinc content in the coating is almost the same, considering its bare corrosion resistance and post-painting corrosion resistance. It is produced in a range of 60 to 90 at%. Therefore, the present inventors decided to study the adhesion between the lower layer plating and the steel base in this composition range.

Conventionally, the crystal structure of iron-zinc alloy electroplating having the same composition range as this lower layer plating has been investigated by X-ray diffraction, and it has been found that there is an η phase in the above composition range.

The existence of r-phase has been confirmed. However, the inventors' investigation revealed that neither the η phase nor the r phase had any effect on the adhesion. Furthermore, it appears in the above diffraction results. The diffraction lines existing at 2θ-48, 5±0, 2° (based on Co- and α-rays, hereinafter simply referred to as Co-1cr) cannot be explained by the l and r phases. Therefore, when we performed electron beam diffraction on the film in which diffraction lines appeared at this position, we found that rl
Many diffraction rings that can be identified as phases appear, and in the end, 2θ=
It became clear that the diffraction line existing at 48.5±0.2° (α in Co-) was attributed to the Γ1 phase.

Here, the r1 phase is an intermetallic compound having the chemical formula pe@Znt+.

Therefore, as a result of further research, the present inventors found that this Γ
It was found that the presence of one phase was the biggest cause of the adhesive deterioration mentioned above.

Therefore, in order to improve adhesion, it is necessary to suppress the formation of the rI phase as much as possible, and specifically, the content must be 1% or less by volume. If the content is 1% or less, there will be no problem with adhesion, but if it exceeds 1%, the lower plating layer will peel off.

Next, various plating conditions for suppressing the content of the Γ1 phase within the above range were investigated, and it was found that the formation of the Γ1 phase is closely related to the flow state of the plating solution during plating. Therefore, the plating conditions were defined by the Raynozzle number (Re), which is a dimensionless parameter that corresponds to the flow state of the plating solution, depending on the molar ratio of Pe"+ and Zn" in the plating bath. (Conditions will be described later.) In this respect, the flow state in the electrolytic cell for producing the conventional plating layer for the iron-zinc underlayer is similar to the dendrite growth of the deposits during high current density operation for high-speed electrodeposition operation. Or, it was brought about by forced injection of liquid to prevent powder growth, so control was not performed reliably. That is, the flow conditions were not constant due to local fluctuations in the flow velocity within the electrolytic cell, and the generation of the Γ1 phase was unavoidable.

〔Example〕

Next, the effects of the present invention will be clarified through examples.

First, the test methods employed in the examples will be explained. In the electrodeposition experiment, rotating disk electrodes (R, D, E) were used in order to improve the reproducibility of flow conditions. In the case of a rotating disk electrode, the Re number is expressed by the following formula.

In addition, the volume content of r1 phase in the plating film can be determined by X-ray diffraction (
Co4α) was defined using the following formula.

820.644 X r 1 (7) Integrated intensity (2θ=4
8.5+ 0.2°') X 10010, I Xη
(integrated strength of 2θ = 50.7 ± 0.3) Furthermore, as a test method to examine adhesion, plating was applied to the tensile test piece (30 x 60 m parallel part) shown in Figure 1, and ε1
-10% deformation was applied, and after the tape was peeled off, the peeled area ratio was enlarged and measured using an optical microscope photograph. Here, the peeled area ratio is defined by the following formula.

〔Example〕

This example shows that Zn''/Fe'' in the plating bath > 0.4
This is the case. The bath composition and electrodeposition conditions are shown below.

Zn2+/Fe2=0.5. 0.6. 0.8. 1
．． 0 'Na1SOn -1,2sol/IF
e'' = 50.0 to 1500 (ppm) pH - 1.5 Temperature - 50°C Current density - 80 A/da'' Electrodeposition time - 10 seconds The results are shown in Table 1.

As is clear from the above table, when the Re number exceeds 2×10 3 , the volume content of the Γ1 phase becomes 1% or less, and at the same time, the peeled area ratio of the plating film also sharply decreases. Therefore, it can be seen that the scope of the present invention is extremely effective in improving the adhesion of the coating. Note that since the Zn:a degree in the film tends to increase as the Re number increases, it also contributes to improving the corrosion resistance, which is the purpose of applying the lower layer plating.

[Example 2] In this example, Zn2+/Fe2 in the plating bath was 0.2<
This is the case when Zn''/Fe''≦0.4. The bath composition is zn
t*/Fe" was set to 0.2.0.3.0.4.

Other conditions are the same as in Example 1.

The results are shown in Table 2.

From Table 2, it is clear that the range of the present invention (CRe number≦lXl0') has a large effect on improving adhesion.

Next, to show an example of the peeling state of the coating in Examples 1 and 2 described above using photographs, FIGS. 2 and 3 show Γ1
# after tensile test when phase volume ratio is 0.8% and 25%
This is a close-up of the layer condition.

At first glance, it is clear that the peeled area is much larger in the film shown in FIG. 3, that is, with the rl volume ratio of 25%.

The following two points can be considered as reasons why the peeled area becomes large outside the scope of the present invention.

■Hardness of intermetallic compound of r1 phase (500kg/Tsuru 2)
Since it has a high weight and is extremely brittle compared to the r-phase (300 kg/m" or less), it cannot fully follow the deformation of the steel plate, resulting in increased peeling.

■Γ1 phase with a larger lattice constant a than the η and r phases (a =
17.963), the electrodeposition stress was large, which caused peeling.

〔Effect of the invention〕

As described above, in the present invention, by suppressing the generation of the rl phase, the adhesion of the lower plating layer to the steel substrate is significantly improved.

[Brief explanation of drawings]

Figure 1 is a diagram showing a tensile test piece, Figures 2 and 3 are respectively,
FIG. 2 is a photograph of a metal U-woven fabric showing the peeled area after a tensile test of a film with an rl volume ratio of 0.8 Vlo and 25 V/O (7). FIG.

Claims (2)

[Claims] (1) In an iron-galvanized steel material having two iron-zinc plating layers on the surface of the steel material, the steel material contains Γ_1 phase consisting of Fe_5Zn_2_1, which is an intermetallic compound of iron and zinc, at 1 V/O (volume ratio) or less, and is plated. The amount of zinc contained in the coating is 60
An iron-galvanized steel material characterized by having a lower plating layer having an atomic ratio of ~90 A/O. (2) Fe^2^+ ions and Zn^2^+ in the plating bath
The molar ratio of ions is Zn^2^+/Fe^2^+>0.4
In this case, plating is performed when the flow state of the plating bath relative to the steel plate is the Reynolds number (Re), and Re≧2×10^3,
The same molar ratio in the plating bath is 0.2<Zn^2^+/Fe
A method for producing iron-galvanized steel material, characterized in that when ^2^+≦0.4, plating is performed with Re≦1×10^4 to form a lower plating layer.