JPH0447037B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Fields> The present invention has excellent plating adhesion during press processing and after painting, has excellent red rust resistance, is suitable as a base for painting, and has excellent overall rust prevention including painting. The present invention relates to a corrosion-resistant steel sheet, particularly a surface-treated steel sheet for automobiles, and a method for manufacturing the same. <Prior art and its problems> Zn-plated steel sheets are the most commonly used surface-treated steel sheets for corrosion resistance, and protect the base steel sheet through the sacrificial anti-corrosion action of Zn. Therefore, it is widely used in large quantities in home appliances, automobile bodies, small parts, building materials, etc. However, in recent years, pitting corrosion and cosmetic corrosion of automobile bodies have become a problem, and corrosion prevention has become necessary.As a result, it has been pointed out that Zn-plated steel sheets lack corrosion resistance and poor paintability, and it is essential to improve them. It became. As a result, in recent years, alloyed hot-dip Zn-plated steel sheets alloyed with Fe do not contain the η phase, and therefore have particularly excellent corrosion resistance (i.e., blister resistance and puncture resistance) after coating. , has come to be used in large quantities. However, since this is manufactured by heating and diffusion, the plating adhesion is not sufficient, powdering occurs during press processing, resulting in press defects, there are restrictions on the material of the base plate, and the paint quality is poor. It had problems such as poor image quality. Therefore, if Zn-Fe alloy plating can be produced by electroplating, it is expected that the above-mentioned drawbacks will be overcome, and various developments have been made in the past. No. 57-
It is disclosed in No. 19393, No. 57-60087, No. 57-200589, etc. As a result, some products have corrosion resistance after coating that is close to that of alloyed hot-dip Zn-plated steel sheets, and also have excellent adhesion during processing. However, the performance required of surface-treated steel sheets for automobiles is not only corrosion resistance after painting and adhesion during processing, but also other essential performances. In other words, the above-mentioned alloyed hot-dip Zn plated steel sheet and conventional Zn
-Fe alloy electroplated steel sheets had a serious defect in plating adhesion after painting. That is, the environment in which a car is actually used is not static, and the car may receive strong impacts from pebbles, sand, etc. while driving. In such cases, the car body paint film peels off from the base together with the Zn-Fe alloy plating layer, so its anti-corrosion ability is not demonstrated, and the base steel sheet is exposed, which accelerates corrosion. This results in On the other hand, in the case of Zn-plated steel sheets, there is no phenomenon in which the coating film peels off along with the plating. Furthermore, in the case of Zn plating or Zn-based alloy plating, there was a drawback in that the coating performance was inferior to that of cold-rolled steel sheets. For example, phosphate treatment is essential as a base treatment for painting, but since the reaction mechanism uses metal leached from the object to be treated (in this case, a plated steel plate), Hoepite (Zn ₃
(PO ₄ ) ₂・4H ₂ O) is generated. On the other hand, in the case of cold-rolled steel sheets, Phosphophyllite (Zn ₂ Fe (PO ₄ ) ₂
4H ₂ O) is produced. Hoepite is Phosphophyllite
Since it is inferior in alkali resistance and dehydration performance compared to that of , it is inferior in water resistant secondary adhesion. moreover,
Combined with the fact that crystals are difficult to form densely, corrosion resistance deterioration such as blistering is likely to occur.
Therefore, the P ratio (Phosphophyllite/Phosphophyllite
+Hoepite) is considered to be the main evaluation criterion,
For the reasons mentioned above, it has been impossible to increase the P ratio with alloy plating mainly composed of Zn. Furthermore, when a Zn-based alloy coated steel sheet is used for the outer surface of a vehicle body, there is a drawback that it is susceptible to coating film defects called so-called craters caused by energization in cationic electrodeposition coating. This phenomenon cannot be avoided as long as Zn-based alloy plating is used, and under severe current conditions where craters are likely to occur, craters may be observed. If a crater occurs, it is a paint film defect that may impair the functionality of the paint film, and it may also have a negative impact on the uniformity and sharpness of the paint film after applying the middle or top coat. Significant loss of value. Therefore, a two-layer plated steel sheet has been developed so that cationic electrodeposition coating can be applied safely to Zn-based alloy coated steel sheet even under severe current conditions. In other words, Fe is deposited on a Zn alloy-plated steel sheet.
Alternatively, a layer mainly composed of Fe is applied, which is disclosed in JP-A-56-133488 and JP-A-56-142885. As can be easily inferred from the fact that cold-rolled steel sheets (mainly made of Fe) have excellent paintability including crater resistance, the two-layer galvanized steel sheet described above has excellent paintability including crater resistance. It's summery. However, because the surface layer contains Fe, although it has excellent crater resistance, it has poor red rust resistance. In other words, Fe in the surface layer is
Although it is eluted by phosphate treatment, etc., a relatively large amount of Fe must remain in order to ensure crater resistance, which causes deterioration of red rust resistance. Craters will not be improved if the amount of upper layer Fe is deposited in a small amount that does not deteriorate red rust resistance.
If a relatively large amount of Fe is deposited to improve craters, the red rust resistance will deteriorate. In order to resolve the above trade-off, it is necessary to
It may be possible to increase the sacrificial corrosion protection ability of Zn-based alloy plating, but with conventional Zn-Fe alloy plating with a Zn content of 90wt% or less and containing no η phase, it is difficult to sacrificially prevent Fe in the plating film. However, it did not have sufficient sacrificial anti-corrosion ability and was unable to prevent the occurrence of red rust. In addition, if the lower layer is Zn plating or Zn-Fe alloy plating containing η phase, it may be possible to prevent red rust, but even if an Fe layer is applied to the upper layer, the corrosion resistance after painting will be poor. Ta. Therefore, when the Zn-Fe alloy plating of the present invention containing more than 90 wt% Zn and less than 97 wt% and no η phase was applied to the lower layer, and Fe-P plating was applied to the upper layer,
It has been found that it exhibits excellent performance in both paintability including crater resistance, red rust resistance, and corrosion resistance after painting. <Object of the Invention> The present inventor has made the invention in view of the above-mentioned actual situation, and aims to provide excellent plating adhesion after painting, corrosion resistance after painting, and red rust resistance, and to improve phosphate treatment. Excellent durability and crater resistance
The purpose is to provide a Zn-Fe alloy electroplated steel sheet. The present invention will be explained in more detail below. The present inventors first began by investigating the factors that govern plating adhesion after painting. In other words, pure Zn-plated steel sheets have good plating adhesion after painting, and most conventional Zn-Fe alloy-plated steel sheets have poor plating adhesion after painting. When investigating the effects of this, we found the following facts: That is, as shown in FIG. 1, it has been found that when the Zn content is 90 wt% or less, the plating adhesion after coating is poor, and when it exceeds 90 wt%, it becomes good. As shown in Figure 2, the shrinkage stress due to plating increases rapidly below 90wt% Zn.
This is thought to be the cause of poor plating adhesion after painting. Next, as is well known in alloyed hot-dip Zn plating, the corrosion resistance (blister resistance, puncture resistance) after painting is poor if the η phase is present.
Good results were obtained when the η phase did not exist. However, using known techniques such as those disclosed in JP-A-57-19393 and JP-A-57-200589, electroplating of Zn-Fe alloys with a Zn content of more than 90 wt% is always η
It contained a phase. That is, not only is the performance of electroplating a Zn-Fe alloy with a Zn content of more than 90 wt % and no η phase unknown, but it has not even been possible to manufacture it. Therefore, we conducted repeated research on a manufacturing method for producing a Zn-Fe alloy electroplated steel sheet with a Zn content of more than 90 wt% and no η phase. The plating bath for Zn-Fe alloy plating is mainly a chloride bath. This is because it has been found that the effects of additives and current density, which will be described later, are most likely to be exhibited. Zn ²⁺ and Fe ²⁺ as metal ions
The main concentration is 0.5mol/
The above is within the solubility limit. The reason for this is
This is because if it is less than 0.5 mol/mol, it tends to cause discoloration, whereas if it exceeds the solubility limit, solids will only be produced, which is only a disadvantage. The Fe ²⁺ /Fe ²⁺ +Zn ²⁺ ratio (molar ratio) is preferably 0.03 to 0.12. This is to control the Zn content to more than 90 wt% and less than 97 wt%. In the plating bath, KCl,
Contains 250 g or more of one or more selected from NH ₄ Cl, NaCl, CaCl ₂ and MgCl ₂ . This is to improve conductivity, reduce power, and suppress preferential precipitation of Zn by adding a large amount.
That is, by adding a large amount of Cl ^- ions,
Preferential precipitation of Zn tends to be suppressed. Current density is 50~200A/ ^dm2 , preferably 70~
150A/ ^dm2 is suitable. Current density is 50A/d
This is because the η phase tends to precipitate below ^m2 .
Exceeding 200A/ ^dm2 will easily cause discoloration.
This is also because the adhesion may be poor. The bath temperature is preferably 25-70℃. At temperatures below 25°C, adhesion deteriorates, and at temperatures above 70°C, a black appearance tends to occur. PH is preferably 1.0 to 4.5. If it is less than 1.0, not only will the cathodic deposition efficiency decrease, but the equipment will be severely corroded. On the other hand, if it exceeds 4.5, the oxidation of Fe ²⁺ becomes extremely rapid. In addition to the above-mentioned salts, it is preferable to add polyethers that are specifically effective for suppressing precipitation of the η phase. Particularly preferred are polyethylene glycol, polypropylene glycol, polyethylene glycol, polypropylene glycol copolymers and their derivatives. By adding one or more of these compounds, the precipitation of η phase is suppressed, so Zn-Fe containing more than 90 wt% Zn that does not contain η phase
An alloy electroplated steel sheet can be obtained. The total amount added is 0.05-10g/, preferably 0.1-5
g/ is appropriate. If it is less than 0.05g/, it is insufficient to suppress the precipitation of η phase, and if it is less than 10g/
Even if it is added in excess of this amount, the effect will be saturated and it will be meaningless. The plating obtained by the above manufacturing method is Zn.
The content is more than 90 wt% and 97 wt% or less, and does not contain η phase. Furthermore, it exhibits a uniform color tone of white to white-gray, and also has good adhesion. The upper layer of the Zn--Fe alloy electroplated steel sheet produced by the method described above was coated with Fe--P plating, which was developed by the present inventors and has excellent phosphate treatment properties and crater resistance. For Fe-P plating, a bath mainly consisting of a chloride bath or a sulfate bath is used. This is because high speed, high current density plating is possible. As a bath component
Contains Fe ²⁺ ions of 0.3 mol/or more within the solubility limit. The reason for this is that if it is less than 0.3 mol/mol, it tends to cause discoloration and is unsuitable for high current density plating, and if it exceeds the solubility limit, only a solid is formed and there is no merit. Furthermore, in order to contain P during plating, 0.001 to 25 g/hypophosphite is contained. Hypophosphite may be added in the form of a drug such as NaH ₂ PO ₂ .H ₂ O or H ₃ PO ₂ . Add amount 0.001~25g/
The reason for limiting this is that if it is less than 0.001 g/l, sufficient P will not be contained in the plating, and if it exceeds 25 g/l, the P content in the plating will become too high, resulting in plating that tends to be amorphous. This is because reactivity such as salt treatment deteriorates. Although the current density is limited to 20-200A/ ^dm2 ,
The reason is that less than 20 A/dm ² tends to result in amorphous plating. That is, when the plating becomes amorphous, it has excellent acid resistance and is difficult to be etched, so that the reactivity with the phosphate treatment solution is low, so that a sufficient phosphate film cannot be formed. On the other hand, if it exceeds 200 A/dm ² , it is unsuitable because it tends to cause discoloration and the voltage becomes too high. Now, in order to produce the two-layer type Fe-P/Zn-Fe plated steel sheet of the present invention, Zn-Fe
Immediately after plating and washing with water, Fe
- It is necessary to apply P plating. That is, Zn−
This is because the Fe-plated surface becomes dirty or forms an oxide film when left unattended or oiled. In such a case, if Fe--P plating is performed directly on the surface, a seemingly normal plating will be produced, but microscopic defects and unmet parts will be produced. This results in a product that is inferior to the expected crater resistance. In addition, if you do not perform Fe-P plating continuously after Zn-Fe plating and use normal general plating pre-treatments such as electrolytic degreasing and pickling for Zn-Fe plating, the plating Because it is more active than cold-rolled steel, it can be attacked. In particular, in electrolytic degreasing, Zn is removed by etching with alkaline solution or anodic dissolution.
-Fe plating dissolves. The same problem occurs with pickling, and the Zn-Fe plating is significantly dissolved. Therefore, a mild pretreatment may be considered, but even in this case, it is difficult to avoid a small amount of dissolution. If dissolution of Zn-Fe plating occurs, Fe
, it is extremely easy to produce black smut. In other words, Zn selectively dissolves and Fe
This is because rich smuts are generated, a phenomenon that is not observed in the case of Zn plating. Because of this generated smut, when Fe--P plating is applied to the upper layer, although the plating appears to be normal, microscopic defects and unmet parts are generated. Therefore, regardless of the pretreatment mentioned above, Zn
- Unless Fe plating is followed by Fe--P plating, excellent crater resistance cannot be obtained. Zn content of 90wt obtained for the first time in this way
It has been discovered for the first time that a Zn-Fe alloy electroplated steel sheet containing no η phase in an amount of more than 97% exhibits excellent corrosion resistance after coating. In other words, as shown in Figure 4, the Zn-Fe alloy plating containing η phase is
Zn-Fe alloy plating that does not contain η phase shows corrosion resistance after painting close to that of plating, whereas Zn-Fe alloy plating that does not contain η phase
It has become clear that it exhibits excellent corrosion resistance after painting, comparable to plating. In addition, as shown in Figure 3, the Zn content exceeds 90wt%.
Zn-Fe alloy plating that does not contain η phase at a Zn content of less than 97wt%, does not contain an η phase at a Zn content of less than 90wt%.
It is clear that the red rust resistance is significantly superior to that of Zn-Fe alloy plating. That is, by the present invention, for the first time, it was possible to obtain a Zn--Fe based plated steel sheet that combines crater resistance, red rust resistance, post-painting corrosion resistance, and post-painting plating adhesion. These properties such as crater resistance, post-painting corrosion resistance, red rust resistance, and post-painting plating adhesion are indispensable basic performances required of surface-treated steel sheets for use in automobile body exterior panels, and the present invention has achieved these properties. This is the first time it has been solved. The unique effects of the present invention's Zn-Fe alloy plating with Zn of more than 90wt% and less than or equal to 97wt% and containing no η phase are as follows:
This is thought to be because it has sufficient sacrificial corrosion prevention ability to prevent red rust due to its high Zn content, and at the same time, it has excellent corrosion resistance after painting because it does not contain the η phase. The amount of Fe-P plating applied is preferably 0.5 to 5 g/m ² . The reason for this is that at less than 0.5g/ ^m2 , there is almost no effect of improving crater resistance, and at less than 5g/m2,
This is because if it exceeds m ² , red rust tends to occur. Further, the P content is preferably 0.003 to 15 wt%.
The reason for this is that in this range, not only is the crater resistance good, but also the phosphating property is excellent, and finer crystals are produced than in cold-rolled steel sheets. <Examples> Next, the present invention will be specifically explained with reference to Examples. Table 1 shows a method for manufacturing a Zn-Fe alloy electroplated steel sheet containing more than 90 wt% Zn and 97 wt% or less and no η phase, as well as comparative examples. The presence or absence of the η phase was determined by X-ray diffraction. The plating bath uses the chemicals shown in Table 1, but it contains Fe ³⁺ produced by impurities in the chemicals and oxidation of Fe ²⁺ that occurs during operation. Also, some baths have chemicals such as citric acid added for other purposes. For example, citric acid is used to suppress Fe ³⁺ hydroxide precipitation, and acetic acid and H ₃ BO ₃ are used to provide pH buffering properties. In all of the comparative examples, the presence of η phase is observed, but in all of the present invention examples, the presence of η phase is not observed even when Zn exceeds 90 wt%. The Fe-P plating conditions are as follows, and the Zn
-After plating with Fe, rinse with water and immediately thereafter, change the amount of applied electricity to change the amount of electricity applied, and apply NaH ₂ PO ₂ .
The P content was changed depending on the amount of H ₂ O added. Bath composition FeCl ₂・nH ₂ O 200g/ KCl 150g/ NaH ₂ PO ₂・H ₂ O 0.01~1.0g/ PH2.0, bath temperature 50℃ Current density 40~100A/dm ² Table 2 shows Fe-P in the upper layer. Zn90wt% with plating
This figure shows various performances of Zn-Fe alloy electroplated steel sheets containing no η phase at less than 97 wt% and comparative examples. In comparative examples, those with a Zn content of less than 90wt%
Although it has excellent corrosion resistance after painting, it is prone to red rust and has poor plating adhesion after painting. Furthermore, those containing η phase with a Zn content of 90 wt% or more have excellent plating adhesion and prevention of red rust after painting, but are significantly inferior in corrosion resistance after painting. On the other hand, in the example of the present invention, plating adhesion after painting,
It can be seen that both red rust resistance and corrosion resistance after painting are excellent. It can be seen that the examples of the present invention have excellent plating adhesion, red rust resistance, and corrosion resistance after painting, and are also excellent in crater resistance. (Test conditions) (1) The presence or absence of the η phase in Table 1 was investigated by X-ray diffraction. (2) Adhesion after 3 coats in Table 2 After plating, regular phosphate treatment (Bonderite #3030, dip type made by Nippon Parkerizing), cationic electrodeposition coating (made by Nippon Paint Co., Ltd. Power Top U-
30, 20μm), medium color (Kansai Paint Co., Ltd. ES
Primer 30μm), topcoat (manufactured by Kansai Paint)
TM-13, 30 μm), a dupont impact test (punch diameter 1/2 inch, weight 1 kg, 50 cm) was performed, and the plating was judged based on whether the entire coating film peeled off. (3) Resistance to red rust on the cross-cut area after painting was determined by observing the occurrence of red rust after 360 hours of a salt spray test after cross-cutting after the above-mentioned 3-coat painting. (4) Corrosion resistance after painting is determined by performing a salt spray test for 840 hours after cross-cutting the cationic electrodeposition coating material, and measuring the maximum peeling width (mm) on one side of the cross-cut area.
It was evaluated by measuring. (5) Crater resistance was tested under conditions that were intentionally more likely to cause coating film defects during cationic electrodeposition coating. Coulomb control was performed so that the film thickness was 20 to 25 μm at a voltage of 300 V, a bath temperature of 27° C., sample area/counter electrode area = 1/5. The sample size is 50 cm ² and the numbers in the table are the number of craters.

【table】

【table】

[Table] <Effects of the invention> According to the method of the present invention, Zn90wt% is used as the inner layer.
Zn-Fe alloy plating layer containing no η phase with a P content of 0.003 to 15 wt% as the outer layer, less than 97 wt%,
A surface-treated steel sheet having an Fe--P alloy plating layer with an adhesion amount of 0.5 g/m ² or more on at least one surface is obtained. The steel sheet of the present invention thus obtained has excellent plating adhesion, red rust resistance, and corrosion resistance after painting, as well as excellent crater resistance.

[Brief explanation of the drawing]

Figure 1 shows the plating adhesion after 3 coats.
It is a graph showing the relationship between what was evaluated by the peeling height in the dupont impact test and the Zn content. Figure 2 shows the bending stress generated by plating and the Zn
It is a graph showing the relationship with content rate. Figure 3 is 3
It is a graph showing the relationship between the red rust resistance of the cross-cut portion after coating and the Zn content. FIG. 4 is an explanatory diagram showing the relationship between the blister resistance of the cross-cut portion of the cationic electrodeposition coating material and the phase and Zn content.

Claims (1)

[Claims] 1 P content is 0.003 to 15.0wt%, adhesion amount is 0.5
The steel plate has an Fe-P alloy plating layer of g/m ² or more on at least one side, and the inner layer has a Zn content.
A surface-treated steel sheet having excellent paintability, plating adhesion after painting, and corrosion resistance, characterized by having a Zn-Fe alloy plating layer of more than 90 wt% and 97 wt% or less and containing no η phase. 2 P content is 0.003-15.0wt%, adhesion amount is 0.5
The steel plate has an Fe-P alloy plating layer of g/m ² or more on at least one side, and the inner layer has a Zn content.
When producing a Zn-Fe based alloy plating layer that is more than 90wt% and less than 97wt% and does not contain η phase,
Contains a total of 0.5 mol/or more of Zn ²⁺ and Fe ²⁺ ,
The molar ratio Fe ²⁺ /Fe ²⁺ + Zn ²⁺ is 0.03 to 0.12, and KCl, NH ₄ Cl, NaCl,
A total of 250 g/or more of one or more selected from CaCl ₂ and MgCl ₂ , and 0.05 to 10 g/more of one or more substituted or unsubstituted polyethers.
Current density 50 to 200 A/dm ² in a chloride bath containing
Zn-Fe alloy electroplated at
Thereafter, the current density is 20 to 200 A from a bath mainly composed of a chloride bath or a sulfate bath, containing Fe ²⁺ ions up to the solubility limit of 0.3 mol/or more, and further containing 0.001 to 25 g/hypophosphite. Fe− at /dm ²
A method for producing a surface-treated steel sheet with excellent paintability, plating adhesion after painting, and corrosion resistance, characterized by applying P plating.