JPS6213590A - Surface-treated steel sheet having excellent coating property, adhesion after coating and corrosion resistance and its production - Google Patents

Abstract

PURPOSE:To develop a steel sheet for automobiles having excellent coating property, rust preventive property and corrosion resistance after coating by plating a Zn-Fe alloy having a specified composition on the surface of a steel sheet and further forming an Fe-P plated layer thereon. CONSTITUTION:A chloride bath contg. >=0.5mol/l Zn<2+> and Fe<2+> in total, wherein the ratio of Fe<2+>/(Fe<2+>+Zn<2+>) is regulated to 0.03-0.12, >=250g/l of >=1 kinds among KCl, NH4Cl, NaCl, CaCl2 and MgCl2 in total as a conductivity promoter, >=0.05-10g/l polyethers such as polyethylene glycol as a eta-phase deposition inhibitor is used as the plating soln., and the steel sheet is electroplated at 50-200A/dm<2> current density to form the plated layer of a Zn-Fe alloy contg. 90-97% Zn on the surface of a steel sheet for automobiles. Then a plating soln. contg. >=0.3mol/l Fe<2+> ion and 0.001-25g/l hypophosphite is used and plated at 20-200A/dm<2> current density on the Zn-Fe alloy plated layer to form a plated layer of the Fe-P alloy contg. 0.003-15% P at >=0.5g/m<2> deposition quantity.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention has excellent plating adhesion during press processing and after painting, has excellent red rust resistance, and is suitable as a base for painting, including painting. The present invention relates to a corrosion-resistant steel sheet with excellent comprehensive rust prevention properties, 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 by 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 improvements are essential. Ta. As a result, in recent years, alloyed hot-dip galvanized steel sheets alloyed with Fe do not contain the η phase, and therefore have particularly excellent corrosion resistance (i.e., blister resistance, water perforation resistance) after coating. came to be used for.

However, since this is manufactured by heating and diffusion, the adhesion of the plating is insufficient, powdering occurs during press processing, resulting in press defects, there are restrictions on the material of the original plate, and the paint clarity is poor. It had problems such as being inferior.

Therefore, Zn-Fe alloy plating was applied using electroplating method.
Since it is expected that the above-mentioned drawbacks will be solved if it can be used, various developments have been made in the past.
JP-A-54-107838, JP-A No. 57-19393,
It is disclosed in No. 57-60087, No. 57-200589, etc.

As a result, some sheets have corrosion resistance after coating that is close to that of alloyed hot-dip Zn-plated steel sheets, and have excellent adhesion during processing.

However, the performance required for automatic Ichikawa surface-treated steel sheets is not only corrosion resistance after unpacking and adhesion during processing, but also essential performance in addition to these. That is, the alloying melt 2n described above
The plated steel sheet and the conventional Zn-Fe alloy electroplated steel sheet had a serious defect in plating adhesion after painting. That is, the actual environment in which an automobile is used is not static, and the automobile may receive strong impacts from pebbles, sand particles, etc. while driving. In such a case, the paint film of the car body is
Since the entire n-Fe alloy plating layer is peeled off from the base, its anticorrosion ability is not exhibited, and the base steel plate is exposed, resulting in accelerated corrosion. 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 alloy plating mainly composed of Zn, there is a drawback that the coating performance is inferior to that of cold-rolled steel sheets. For example, phosphate treatment is essential as a base treatment for painting, but the object to be treated (in this case, a plated steel sheet)
Because the reaction mechanism utilizes metals eluted from
Hoepi Le (Zn3(PO4) 2 ・41
120) is generated. On the other hand, in the case of cold-rolled steel sheets, phosphophyllite (Zn2Fe(PO
4) 2-4820) is generated. Hoep i L
Since e is inferior to Phosphophvl 1ite in alkali resistance and dehydration performance, it is inferior in water resistant secondary adhesion. Furthermore, due to the synergistic effect that crystals are difficult to form densely, deterioration of corrosion resistance such as blistering is likely to occur. Therefore, the P ratio (Phosphophyl l
ite/Phosphophyllite + 1lo
However, 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 plated steel sheet is used for the outer surface of a car body outer panel, there is a drawback that coating film defects called so-called craters are likely to occur due to 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 and may impair the functionality of the paint film, and it will also have a negative impact on the uniformity and sharpness of the paint film after applying the middle coat or top coat. Significant loss of product value.

Therefore, a two-layer plated steel sheet has been developed so that cationic electrodeposition coating can be applied safely to Zn-based alloy plated steel sheet even under severe current conditions. That is,
It is a product in which a layer consisting mainly of Fe or Fe is applied on a Zn-based alloy plated steel sheet, and is disclosed in JP-A No. 56-133488,
It is disclosed in No. 56-142885.

As can be easily inferred from the fact that cold-rolled steel sheets (mainly composed of Fe) have excellent paintability including crater resistance, the above two
Layered plated steel sheets have excellent paintability, including crater resistance. However, since the surface layer contains Fe, although it has excellent crater resistance, it has poor red rust resistance. In other words, although Fe in the surface layer is eluted by phosphate treatment, etc., in order to ensure crater resistance, a relatively large amount of Fe is required to remain, which causes deterioration of red rust resistance. . Craters are not improved if the amount of upper layer Fe deposited is small enough not to deteriorate red rust resistance, but if a relatively large amount of Fe is deposited to improve craters, red rust resistance deteriorates.

In order to resolve the above trade-off, it is possible to increase the sacrificial anticorrosion ability of the underlying Zn-based alloy plating.
The n-Fe alloy plating did not have sufficient sacrificial corrosion protection ability to even sacrificially protect Fe in the plating film, and was unable to prevent the occurrence of red rust. In addition, when the lower layer is Zn plating or Zn-Fe alloy plating containing η phase, it is possible to prevent the occurrence of red rust, but even with an Fe layer applied to the upper layer, the corrosion resistance after painting is poor. .

Therefore, Zn90wt of the present invention! When a Zn-Fe alloy plating of 97wt96 or less and containing no η phase was applied to the lower layer and Fe-P plating was applied to the upper layer, the coating properties including crater resistance, red rust resistance, and corrosion resistance after painting were improved. It was found that both showed excellent performance.

<Objective of the Invention> The present inventor has made the invention in view of the above-mentioned actual situation, and the object thereof is to provide excellent plating adhesion after painting, corrosion resistance after painting, red rust resistance, and phosphate treatment property. The present invention aims to provide a Zn-Fe alloy electroplated steel sheet that also has excellent crater resistance.

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, since Zn-plated steel sheets have good plating adhesion after painting, and most conventional Zn-Fe alloy-plated steel sheets have poor adhesion after painting, we investigated the influence of Zn content and found the following: I found out facts like this.

That is, as shown in FIG. 1, <Zn content is 90 wt.
! In the following, it was found that the plating adhesion after painting was poor and became better when the coating strength exceeded 90 tX.

As shown in Figure 2, the shrinkage stress due to plating is
90wL! It increases rapidly below Ii, and this is considered to be the cause of poor plating adhesion after painting.

Next, the corrosion resistance after painting (blister resistance, water perforation resistance J
As is well known in alloyed hot-dip Zn plating, the presence of the η phase was poor, and the absence of the η phase was good. However, JP-A-57-19393
No. 57-200589, Zn- with a Zn content of more than 90wL*
Fe alloy electroplating always contained η phase.

That is, not only is the performance of Zn-Fe alloy electroplating with a Zn content of 90 wL% Jfl 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 obtaining a Zn-Fe alloy electroplated steel sheet with a Znn content exceeding 90 wt and containing no η phase.

The plating bath for Zn-Fe alloy plating is salt
.

Mainly chemical baths. 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. The metal ions are mainly Zn2+ and Fe2+, and their total concentration is 0.5 teal/It or more,
It is within the solubility limit. The reason for this is 0.5I! l0It/
This is because Qfuman tends to cause discoloration, and on the other hand, if the solubility limit is exceeded, only solids are formed, which is a disadvantage.

The Fe 2+ / Fe 2+ + Z n 2+ ratio (molar ratio) is preferably 0.03 to 0.12. This is Z
This is to control the n content to more than 90% and less than 97%.

The plating bath contains KCu and NH4Clt as conductivity aids.
, NaCIL, CaCl2, and MgCl2 in an amount of 250 g/l or more. This is to improve conductivity, reduce power, and suppress preferential precipitation of Zn by adding a large amount. That is, C2
- By adding a large amount of ions, preferential precipitation of Zn tends to be suppressed.

Current density is 50-200A/drn”, preferably 70
~150A/drn' is suitable. This is because when the current density is less than 508/drn2, the η phase tends to precipitate, and when it exceeds 200 A/drn', it tends to cause discoloration and the adhesion may be poor.

The bath temperature is preferably 25 to 70°C. If the temperature is lower than 25°C, the adhesion deteriorates, and if the temperature exceeds 70°C, the appearance tends to be black.

The pH is preferably 1.0 to 4.5. When l is less than 0, not only the cathodic deposition efficiency decreases, but also the equipment is severely corroded. On the other hand, if it exceeds 4.5, the oxidation of Fe2+ becomes extremely rapid.

In addition to the above-mentioned salts, it is preferable to add polyethers that are specifically effective in 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, precipitation of the η phase is suppressed, so Zn90wt! does not contain the η phase! I: A super Zn-Fe alloy electroplated steel sheet can be obtained.

The total amount added is 0.05 to 10 g/4, preferably 0.05 to 10 g/4.
1 to 5 g/IL is suitable. Less than 0.05 g/fi is insufficient to suppress precipitation of the η phase, and 10 g/fi
Even if it is added in excess of /u, the effect is saturated and is meaningless.

The plating obtained by the above manufacturing method has a Zn content of 9
It is more than 0wt'J and less than or equal to 97wt, and does not contain an η phase. Moreover, it exhibits a uniform color tone of white to white-gray, and also has good plating adhesion. Zn- produced by the above method
The upper layer of the Fe alloy electroplated steel sheet 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 composed 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, 0.0% Fe2+ ion was used. :
Contains more than 1naJZ/It in the melted air. The reason for this is that if it is less than 0.3 man/1, it tends to cause burns and is unsuitable for high current density plating, and if it exceeds the solubility limit, only a solid will be formed and there will be no benefit. do not have.

Furthermore, in order to contain P in the plating, 0.001 to 25 g/42 of hypophosphite is contained. Hypophosphite IQNaH2PO2.H20 or H3PO2 may be added in the form of a drug. Add amount 0.001-25g
/It is because if it is less than 0.001g/2, sufficient P will not be contained during plating, and 25g/J! This is because if it exceeds this, the P content in the plating becomes too high, resulting in plating that tends to be amorphous, and reactivity in phosphate treatment etc. deteriorates.

The current density is limited to 20-200 8/dba, because if it is less than 208/drn, the plating tends to be amorphous. In other words, if the plating is amorphous, the acid resistance This is because it has excellent properties and is difficult to be etched, so it has low reactivity with the phosphate treatment solution, so it will not produce a sufficient phosphate film.
If drn' is exceeded, it is unsuitable because it tends to cause discoloration and the voltage becomes high.

Now, the two-layer type Fe-P/Zn-F of the present invention
In order to produce e-plated steel sheets, Zn is applied under the above conditions.
- Immediately after applying Fe plating and washing with water
- It is necessary to apply P plating. That is, the surface of Zn--Fe plating becomes dirty or forms an oxide film when left alone or oiled. In such a case, if Fe--P plating is applied directly above the plating, although a normal plating is produced, microscopic defects and unplated parts are produced. This results in a product that is inferior in 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 itself will be damaged. is more active than cold-rolled steel, so it is easily attacked. Particularly in electrolytic degreasing, Zn-Fe plating is dissolved by etching with alkaline solution or anodic dissolution.

The same is true for acid toys, 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.

When dissolution of Zn-Fe plating occurs, black smut is extremely likely to be generated because it contains Fe. That is, this is because Zn is selectively dissolved and Fe-rich smut is generated, a phenomenon that is not observed in the case of Zn plating. Due to this generated smut, when Fe--P plating is applied to the upper layer, although the plating appears to be normal, microscopic defects and unplated areas are generated.

Therefore, regardless of whether or not the pretreatment described above is performed, excellent crater resistance cannot be obtained unless Zn-Fe plating is followed by Fe-P plating.

It was discovered for the first time that the Zn-Fe alloy electroplated steel sheet having a Znn content of more than 90 wt and less than 97L9I and containing no η phase, which was obtained in this way, exhibited excellent corrosion resistance after painting. In other words, as shown in Figure 4, η
Zn-Fe alloy plating containing a phase exhibits corrosion resistance after coating close to that of Zn plating, whereas Zn-Fe alloy plating that does not contain an
It has become clear that Fe alloy plating exhibits excellent post-painting corrosion resistance comparable to alloyed hot-dip Zn plating.

Furthermore, as shown in Fig. 3, Zn-Fe alloy plating with a Znn content of more than 0wt51i and 97wt% or less and containing no η phase is a Zn-Fe alloy plating with a Zn content of less than 9Qwt% and containing no η phase. It is clear that the red rust resistance is significantly better than that of

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 essential basic performances required of surface-treated steel sheets for use in automobile body exterior panels.
This problem was solved for the first time by the present invention.

In addition, Zn90wt of the present invention! Super 9f wt! The unique effect of Zn-Fe alloy plating that is 4 or less and does not contain the η phase is that it has a high Zn content that prevents the occurrence of red rust, and at the same time has a sacrificial corrosion prevention ability of +4 and does not contain the η phase. This is thought to be due to its excellent corrosion resistance after painting.

The coating weight of Fe-P plating is preferably 0.5 to 5 g/m". This is because if it is less than 0.5 g/rn", there is almost no effect of improving crater resistance, and if it is less than 5 g/r
This is because red rust tends to occur when n' is exceeded.

Also, the P content is 0.003 to 15wt! k is preferred.

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.

<Example> Next, the present invention will be specifically explained with reference to Examples.

Table 1 shows Zn90wt! A method for manufacturing a Zn-Fe alloy electroplated steel sheet containing no η phase and a comparative example is shown below. Note that 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 contains impurities in the chemicals and Fe3+ produced by oxidation of Fe2+ that occurs during operation. Also, some baths have chemicals such as citric acid added for other purposes. For example, citric acid suppresses Fe3+ hydroxide precipitation, so acetic acid and H3ao
3 is to provide pHwL neutrality.

In the comparative examples, the presence of η phase is recognized, but in the present invention examples, Zn90wt,! The presence of η phase is not recognized even when the temperature exceeds k.

The Fe-P plating conditions are as follows.
After e-plating, the plate was washed with water and immediately thereafter, the amount of adhesion was changed by changing the amount of electricity applied, and the P content was changed by changing the amount of NaH2po2/H20 added.

Bath composition Fp'Cff12 ・Cfi to nH20200g/u
150g/uNaH2PO
2.H200, 01-1.0 gel pH 2-0, bath temperature 50°C.

The current density of 40 to 100 A/d is shown in Table 2 for Zn90wL with Fe-P plating on the upper layer! I
i Ji 97 wt! Zn containing no η phase in
- Various performances of Fe alloy electroplated steel sheets and comparative examples are shown.

In the comparative example, the Zn content is 90wt! Those with less than 10% have excellent corrosion resistance after painting, but red rust is likely to occur and plating adhesion after painting is poor. Further, although those containing η phase with a Zn content of 90 wt% or more are excellent in plating adhesion and prevention of red rust after painting, they are significantly inferior in corrosion resistance after painting.

On the other hand, it can be seen that the examples of the present invention are excellent in all of the plating adhesion after painting, the red rust resistance, and the corrosion resistance after painting.

It can be seen that the examples of the present invention are excellent in plating adhesion after painting, 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) The adhesion after the 3 coats in Table 2 is the normal phosphate treatment (Bonderite #3030, dip type made by Nippon Bariki Rising) and cationic electrodeposition coating (Power Top U- made by Nippon Paint Co., Ltd.) after plating. :10,20μm), medium color (Kansai Paint Co., Ltd. ES primer: 10μm)
, , ): Coloring (Kansai Paint T 13.30μ
m), then the dupon L impact test (punch diameter l
/2 inch, weight Ikg, 50 cm), and judgment was made based on whether the plating peeled off along with the coating film.

(3) The red rust resistance of the cross-cut area after painting is determined by the salt spray test 360 after cross-cutting after the above-mentioned 3-coat painting.
The occurrence of red rust after a period of time was observed.

(4) Post-painting corrosion resistance was evaluated by performing a salt water spray test for 840 hours after cross-cutting the cationic electrodeposition coated material and measuring the maximum peeling ('arm) on one side of the cross-cut portion.

(5) Crater resistance was determined by intentionally setting conditions that tend 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 surface M/counter electrode area=115. The sample is 50 [gos] and the numbers in table 1 are crater.
It is the number of pieces.

<Effects of the Invention> According to the method of the present invention, Zn90wL% M
i97 wL! A Zn-Fe alloy plating layer containing no η phase, with a P content of 0.003 to 15 as the outer layer.
WL%; , Fe with adhesion amount of 0.5 g/rn' or more
- A surface-treated steel sheet having a p, f-alloy plating layer 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, and is also excellent in crater resistance.

[Brief explanation of drawings]

Figure 1 shows the plating adhesion after 3 coats.
It is a graph which shows the relationship between what was evaluated by the peeling height in the L impact test, and Zn content rate. Figure 2 shows the bending stress generated by plating and the Zn-containing 4
- It is a graph showing the relationship with T rate. Figure 3 shows the red rust resistance and Zn of the cross cut part after 3 coats.
It is a graph showing the relationship with content rate. 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 (2)

[Claims] (1) A Fe-P alloy plating layer with a P content of 0.003 to 15.0 wt% and a coating amount of 0.5 g/m^2 or more is provided on at least one side of the steel sheet, and the inner layer has a Zn content. is 9
A surface-treated steel sheet having excellent paintability, post-painting adhesion, and corrosion resistance, characterized by having a Zn-Fe alloy plating layer of more than 0 wt% and 97 wt% or less and containing no η phase. (2) A Fe-P alloy plating layer with a P content of 0.003 to 15.0 wt% and a coating amount of 0.5 g/m^2 or more is provided on at least one side of the steel plate, and the inner layer has a Zn content. is 9
When manufacturing a Zn-Fe alloy plating layer that contains more than 0 wt% and 97 wt% or less and does not contain η phase, Zn
A total of 0.5 mol/l of ^2^+ and Fe^2^+
The molar ratio Fe^2^+/Fe^2^+Z
n^2^+ is 0.03 to 0.12, and KCl, NH_4Cl, NaCl, CaCl_2,
In a chloride bath containing a total of 250 g/l or more of one or more selected from MgCl_2 and 0.05 to 10 g/l of one or more substituted or unsubstituted polyethers, the current density is 50. Zn- at ~200A/dm^2
Fe alloy electroplating is applied and washed with water, and then continuously in a chloride bath or sulfate bath, containing Fe^2^+ ions up to the solubility limit of 0.3 mol/l or more, and further hypophosphite. A surface-treated steel sheet with excellent paintability, post-coating adhesion and corrosion resistance, characterized by applying Fe-P plating at a current density of 20 to 200 A/dm^2 from a bath containing 0.001 to 25 g/l of Production method.