JPS62294198A - Rustproof steel sheet for automobile and its production - Google Patents

Abstract

PURPOSE:To produce the titled rustproof steel sheet having superior plating adhesion and paintability by successively forming an intermediate layer of a Zn-Ni alloy having a high Ni content and a specified thickness and an outer layer of a Zn-Ni alloy having a regulated Ni content on a steel sheet as a base material by plating and by activating the outer layer. CONSTITUTION:In a plating equipment provided with plural treatment tanks, a steel sheet to be treated is first plated with a Zn-Ni alloy by <=3g/m<2>. The plated steel sheet is the immersed in a plating soln. or an acid soln. filled into the second tank to preferentially dissolve Zn in the resulting Zn-Ni alloy layer and to leave Zn-Ni alloy layer contg. >=30wt% Ni by 10mg/m<2>-1g/m<2>. The layer is further plated with a Zn-Ni alloy contg. 8-16wt% Ni to form an outer layer, and the surface of the outer layer is activated by treatment in a plating bath or an acid soln. Thus, a rustproof steel sheet for an automobile having superior plating adhesion, paintability and adhesion to a paint film after painting is stably produced with high work efficiency.

Description

[Detailed description of the invention] 3. Detailed description of the invention <Industrial application field> This invention not only has excellent rust prevention performance, but also has excellent plating adhesion, paintability, and coating film adhesion after painting. The present invention relates to a Zn-based alloy plated n-sheet suitable for use not only as an inner plate of an automobile body but also as an outer plate, and a method for manufacturing the same.

<Background Art> In recent years, requirements for anti-corrosion performance of automobile bodies have become increasingly strict, and countermeasures have also become more serious. In particular, various automobile manufacturers have been proposing one after another to raise the rust prevention targets by prolonging the period of suppressing holes and external rust in automobile bodies, and measures to improve the rust prevention performance of automobile rust-preventive steel sheets are an urgent issue. It becomes.

By the way, currently, the mainstream of rust-proof steel sheets for automobiles is Zn-based alloy metal sheeting, represented by Zn-Fe alloy plated FI sheet and Zn-Ni alloy plated steel sheet, because of its paintability and corrosion resistance. It is tentatively moving. However, these Zn-based alloy-plated steel sheets have been used mainly for the interior of automobiles due to their particularly excellent large-scale corrosion resistance, but they have been used mainly for the interior of automobiles. Zn-Fe alloy plating or Zn-
Studies have begun on applying Ni alloy plating to the exterior of vehicle bodies, and attempts have also been made to put some Zn-Fe alloy plating into practical use.

However, Z is said to be superior in terms of corrosion resistance.
N-Ni alloy-plated steel sheets have the following problems with respect to the performance required as outer panels for automobiles, and improvements are currently desired. That is, (a) In normal Zn-Ni alloy plating, good corrosion resistance can be obtained when the Ni content in the plating layer is in the range of 10 to 16% by weight, but alloys in this range are hard intermetallic compounds with a single γ phase. Therefore, when the steel sheet is used as an outer panel for an automobile, the chipping impact caused by flying stones and the like that tends to occur during driving tends to cause peeling of the coating film and even peeling of the plating film.

Furthermore, red rust is likely to occur at the locations where the plating has peeled off due to the chipping phenomenon, which is a fatal problem when used as an automobile exterior panel, where the occurrence of exterior rust is to be avoided as much as possible.

(bl) One of the major characteristics required for the external surface performance of automobile exterior panels is the water-resistant adhesion of the paint film. Although it has become possible to form a zinc phosphate treated film with good film adhesion, it is still difficult to form a stable and satisfactory chemical conversion film on a Zn-Ni alloy plated surface.

(C) Cationic electrodeposition coating is indispensable for automobile exterior panels, but cratering resistance during painting is a major problem with cationic electrodeposition coating, and Zn-Ni alloy plated surfaces have good cratering resistance. There are some things you can never say. However, in order to improve this cratering resistance, methods of applying Fe plating or Fe-Zn plating have been proposed (Japanese Patent Publication No. 58-15554,
(Japanese Patent Publication No. 60-57518), when such Fe-based plating is applied as an upper layer of Zn-Ni alloy plating, the cratering resistance improves, but red rust occurs due to the presence of Fe-based plating that naturally remains even after painting. It becomes easier to
This is extremely disadvantageous as an outer panel for automobiles.

Note that the chipping resistance pointed out in Section 81 above is related to the adhesion of plating, and in the past Zn-Ni with different Ni contents was used to improve the adhesion of plating.
A proposal has been made to apply multilayer plating of an alloy (Japanese Unexamined Patent Publication No. 58-204196).

On the other hand, there are also proposals to apply multilayer plating of Zn-Ni alloy as a measure to improve corrosion resistance and powdering resistance (Japanese Patent Laid-Open No. 8-204196, Japanese Patent Laid-open No. 60-14189).
Publication No. 4). However, in order to perform multilayer plating with these Zn-Ni alloys, it is necessary to control the concentration ratio of Zn'' and N12'' in the plating bath by varying the ratio, which requires serious investment in equipment, and also requires operational management. It was extremely difficult.

<Means for Solving the Problems> From the above-mentioned viewpoints, the present inventors have developed a product that not only has excellent rust prevention performance but also has good plating adhesion, paintability, and paint film adhesion. As a result of repeated research in order to stably provide a surface-treated fence board that is fully satisfactory both as an inner panel and an outer panel of an automobile body, the following findings were obtained. That is, i) first deposit Z with a high Ni content and a specific thickness on the E plate substrate;
An n-Ni alloy plating intermediate layer is formed, and a ZnNi alloy outer layer with an adjusted Ni content is plated thereon, and the outer surface of the Zn-Ni alloy plating outer layer is placed in a plating bath or other acid solution. When activated, the Zn-Ni alloy plating intermediate layer with a high Ni content improves the adhesion between the steel sheet base and the Zn-Ni alloy plating outer layer, and the chipping resistance is significantly improved. By activating the outer surface of the Zn-Ni alloy plating outer layer using another acid solution, the chemical conversion treatment property is significantly improved, and the secondary enrichment (coating film adhesion) after painting and the crater resistance during cationic electrodeposition painting are improved. Zn-
To realize a surface-treated board that is fully satisfactory as an outer panel of an automobile body, taking into account the excellent rust prevention ability of the Ni alloy plating outer layer.

ii) Various theories regarding the mechanism of the cratering phenomenon described above have been reported, and the general theory is that ``abnormal electrical discharge during electrodeposition coating causes a partially thermally hardened coating, and that area is baked off later.'' Despite the supported theory that the electrodeposition coating film forms pock-shaped craters because it does not have a leveling effect, it has not been fully clarified what causes the above abnormal discharge. However, this abnormal discharge is caused by and is dependent on the non-uniformity of the chemical conversion coating, and if the chemical conversion coating such as zinc phosphate coating is uniform and fine, the cratering resistance will improve. The excellent cratering resistance of the surface-treated steel sheet is due to the fact that the chemical conversion treatment property is significantly improved by activation treatment of the outer surface of the Zn-Ni alloy plating outer layer with a plating bath or other acid solution. what is brought about.

1ii) Zn-N with a specific bright Ni content on the steel plate
i-alloy plating usually requires work in a plating bath and current density that is strictly controlled, but plating equipment with multiple processing tanks is used, and the initial processing tank (preferably the first tank) , or after applying a large amount of Zn-Ni alloy plating to the raw steel plate (including the @ band) in the first and second tanks), this is stored in the subsequent treatment tank using a plating solution or an acid solution such as sulfuric acid. When immersed in the Zn-Ni alloy plating process, Zn preferentially dissolves from the plating layer attached to the raw steel sheet and becomes Ni-rich, and the Zn-Ni alloy plating layer can be extremely stably and easily converted to the desired high Ni content. Therefore, after providing an intermediate Zn-Ni alloy plating layer with a high Nj content on the raw steel plate by such a method, a predetermined Zn-N alloy layer is formed in a subsequent electroplating bath.
If alloy plating is performed and the plated steel sheet is surface activated by anode treatment, cathode treatment, or simple immersion treatment in a subsequent plating bath or treatment bath filled with other acid solutions, the above-mentioned results can be obtained. To be able to stably manufacture surface-treated steel sheets suitable for use in automobiles, such as those for automobiles, with good workability.

This invention has been made based on the above knowledge, and provides a rust-proof steel plate for automobiles with a Ni content of 30% by weight or more and a coating amount of 10mg/m'' to 1g7m2 on the same base plate.
Through the intermediate Zn-Ni plating layer, the Ni content is 8~
By providing a 16% by weight Zn-Ni alloy plating outer layer and having a surface activated by a plating bath or acid solution, excellent plating adhesion and paintability are achieved. It is also characterized by the fact that it imparts coating film adhesion after painting, and furthermore, it is characterized by the fact that the steel plate to be treated (including the strip) is first plated in a plating equipment equipped with multiple treatment tanks. After applying Zn-Ni alloy plating with an adhesion amount of 3 g/m2 or less, it is immersed in a plating solution or acid solution filled in a subsequent treatment tank to preferentially dissolve the Zn in the Zn-Ni alloy plating layer and remove the Ni. Zn- with a content of 30% by weight or more and an adhesion amount of 10mg/m2 to Ig/m2
After leaving the Ni alloy plating layer, a Zn-Ni alloy plating with a Ni content of 8 to 16% by weight is applied thereon, and then the Zn-Ni alloy is plated by a treatment in a plating bath or an acid solution.
By activating the surface of the Ni alloy plating layer, it is now possible to stably manufacture rust-proof steel sheets for automobiles with excellent plating adhesion, paintability, and post-paint film adhesion with good workability. It has the following characteristics.

Note that the activation treatment for the surface of the Zn-Ni alloy plating layer includes anode treatment or cathode treatment in a Zn-Ni alloy plating bath or an acid solution such as sulfuric acid contained in a treatment tank following the Zn-Ni alloy plating bath. It is preferable to use a method of immersion in the plating bath or an acid solution. In addition, by this surface activation treatment, ZJ<1 is first dissolved on the surface of the Zn Ni alloy plating layer, and a Ni-rich surface layer is formed, albeit slightly.

Next, in this invention, "Ni content and adhesion amount (thickness) of intermediate Zn Ni alloy plating", rZn -N
The reason why the Ni content of the outer layer of i-alloy plating and the amount of Zn--Ni alloy plating deposited first to form the intermediate alloy plating layer are numerically limited as described above will be explained.

A) '+i content and adhesion amount of intermediate Zn-Ni alloy plating If the Ni content of intermediate Zn-Ni alloy plating is less than 30% by weight or the adhesion amount (thickness) is less than 10 mg/m2, it cannot be used for automobile bodies. When used as an outer panel, no improvement in low-temperature chipping was observed, and on the other hand, if the amount of plating exceeds 1 g/m, damage to the base (steel plate) may occur during use. Fe and intermediate ZnN
A galvanic cell is formed between the i-alloy plating layer and the Ni, and corrosion is accelerated, resulting in an adverse effect on corrosion resistance. Therefore, the Nt content of the intermediate Zn-Ni alloy plating is 30% by weight or more, and the deposited amount (thickness) is 10mg.
/ff12 to 1g/ff12.

B) Ni content of Zn-Ni alloy plating outer layer Zn-Ni
In alloy plating, corrosion resistance generally varies greatly depending on the Ni content. Theoretically, Ni content = 10-1
At 6% by weight, T becomes a single phase, so corrosion resistance is good.Ni content: 1
If it is less than 0% by weight, (T + η) is mixed, and the amount of Ni is 1
If it exceeds 6% by weight, (γ and β) will be mixed, which will form a galvanic cell between the phases in the plating layer, which will have a negative effect on corrosion resistance (here, the γ phase is Ni,
ZnH or Ni3Zn2□, and the β phase is! It is an intermetallic compound of JiZn). However, it has been experimentally confirmed that good corrosion resistance can be obtained with an i-containing gk of 8 to 16% by weight (12 to 13% by weight is particularly preferred), and if it is outside this range, the desired corrosion resistance will not be exhibited. From, Zn-Ni
The Ni content of the alloy plating outer layer was determined to be 8 to 16% by weight.

C) Zn deposited first to form intermediate alloy plating layer
- Adhesion amount of Ni alloy plating Intermediate Zn -N with Ni content: 30% by weight or more on the steel plate
When forming the i-alloy plating layer, the Zn-
When adopting a method of immersing Ni alloy plating in acid to concentrate Nl in the plating layer, if the initial deposited amount of Zn-Ni alloy plating is less than 3g7m2, even if concentration to the desired Ni content is achieved, Remaining plating amount: 10mg/
Since it is extremely difficult to secure Zn-Ni
The initial coating weight of the alloy plating was determined to be 3 g/m'' or more.

Next, this invention will be concretely explained using Examples while comparing with Comparative Examples, but these are merely illustrative, and it goes without saying that this invention is not limited by the Examples. .

<Example> Using an electroplating line equipped with 11 horizontal plating tanks, a cold-rolled FA board with a board thickness of 0.8 mm and a board width of 1117 mm was pretreated with alkaline degreasing and pickling as usual. After surface cleaning, Zn Ni alloy plating was performed under the conditions shown in Table 1.

At this time, each of the processing tanks contained the following plating baths, except for a part of the 11th tank (final tank).

Plating solution composition: Zn”-30g/l, N+2°...
・60g/l.

Sodium sulfate...50g/jl! .

Sodium acetate...20g/l, pH of plating solution: 1.5-2.5, plating bath temperature: 50-60℃, and during plating, control the current for each cell (tank) to adjust the current density. Please adjust the value to "several A/dm" for "low current density" in Table 1, and 20 for "normal current density".
~150 A/dm”, and the second tank and the first
In one tank, no current was applied except for a part for N i R shrinkage and surface activation), and the line speed was 40 to 100.
It was adjusted 8 times at m/min.

The final surface activation treatment in the 11th tank includes 2. immersion in the plating bath without electricity. ON sulfuric acid solution (50℃
) in a sulfuric acid solution (1
0A/dm" for 1.5 sec) or cathode treatment (15 A/dm2 for 1.5 sec).

Next, the formability of the Zn-Ni alloy plated steel sheet obtained in this way was examined in a cylindrical drawing test, and
Nippon Parkerizing Co., Ltd.'s ' is used as a base treatment for painting.
Chemical conversion treatment using ``B T-3020 (product name)'', cationic electrodeposition coating with a thickness of 20μ using Nippon Paint Co., Ltd.'s ``Power Top U-30 (product name)'', and melamine alkyd paint. A three-coat treatment consisting of an intermediate coat and a top coat each having a thickness of 35 to 40 μm was applied, and the water-resistant adhesion, cratering resistance, chipping resistance, and external rust resistance were evaluated.

In addition, to test the water resistance adhesion, after immersing the paint film in deionized water at 50°C for 10 days, make cuts at 2N intervals on the paint film and peel it off with tape, and then investigate the paint film survival rate at that time. method was adopted.

In addition, for the cratering resistance test, a method was adopted in which the electrodeposition voltage was increased by instantaneously applying a set voltage during electrodeposition coating, and the voltage that caused cratering at that time was measured.

To test the chipping resistance, first, the painted steel plate that has been treated with three coats is cooled to -20"C and then tested using a gravelometer.
A chipping test was conducted under the conditions specified in STM D-3170-74, and evaluation was made based on the presence or absence of peeling of the plating at that time.

Furthermore, the external rust resistance was determined by applying cross force and throat to the painted board and drying it outdoors, and applying υ1 twice a week to 3%: JaC! ! Evaluation was made based on the presence or absence of red rust after half a year had elapsed while the solution was being sprayed, and the creep width on one side from the crosscut.

These results are calculated based on the amount of plating deposited on each layer and the Zn-Ni
It is also shown in Table 1 along with the Ni content of the alloy plating layer.

From the results shown in Table 1, the present invention has excellent water-resistant adhesion, cratering resistance, chipping resistance, external rust resistance, and formability, and can be used not only as the inner panel of automobile bodies but also as the outer panel. On the other hand, a plated steel sheet whose manufacturing conditions deviate from the specifications of this invention and therefore does not meet the conditions specified in this invention will not meet any of the above-mentioned performances. It is clear that it is not suitable for rust-proofing steel sheets for automobiles, which is a future goal.

<Overall Effects> As described above, according to the present invention, a rust preventive for automobiles that exhibits excellent performance regardless of whether it is used as an inner panel or an outer panel of a vehicle body. This makes it possible to mass-produce steel plates stably with good workability, and brings extremely useful industrial effects such as further improving the durability of automobiles.

Claims (2)

[Claims] (1) Intermediate Zn with a Ni content of 30% by weight or more and a coating amount of 10mg/m^2 to 1g/m^2 on the base steel plate
-Ni content is 8-16% by weight through the Ni plating layer
a Zn-Ni alloy plated outer layer of
A rust-proof steel sheet for automobiles, characterized in that the Ni alloy plating outer layer has a surface activated by a plating bath or an acid solution. (2) In a plating equipment equipped with multiple treatment tanks, first coat the steel plate with Zn-Ni with a coating weight of 3 g/m^2 or less.
After applying alloy plating, it is immersed in a plating solution or acid solution filled in a subsequent treatment tank to preferentially dissolve the Zn in the Zn-Ni alloy plating layer, so that the Ni content is 30% by weight or more and the adhesion amount is 10mg. /m^2 ~ 1g/m^2 Zn-Ni alloy plating layer remains, followed by a Ni content of 8
-16% by weight of Zn-Ni alloy plating, and then the surface of the Zn-Ni alloy plating layer is activated by treatment in a plating bath or acid solution. Production method.