JPH0256437B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Fields> This invention not only has excellent rust prevention performance, but also has excellent plating adhesion, paintability, and film adhesion after painting, and is suitable for use not only as inner panels of automobile bodies but also as outer panels. The present invention also relates to a suitable Zn-based alloy plated steel sheet 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 proposed one after another proposals to raise the rust prevention target in order to prolong 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 is becoming. By the way, currently the mainstream of rust-proof steel sheets for automobiles is
Zn is known for its excellent paintability and corrosion resistance.
-There is a shift to Zn-based alloy plated steel sheets, represented by Fe alloy plated steel sheets and Zn-Ni alloy plated steel sheets. However, these Zn-based alloy plated steel sheets are
Until now, Zn-Fe alloy plating has attracted attention for its excellent puncture and corrosion resistance, and has been used mainly for the interior panels of automobiles. Alternatively, studies have begun to consider applying Zn--Ni alloy plating to the exterior surfaces of vehicle bodies, and attempts have also been made to put some Zn--Fe alloy plating into practical use. However, Zn-Ni alloy plated steel sheets, which are said to have better corrosion resistance, have the following problems with the performance required for automobile outer panels, and improvements are desired. It was the current situation. That is, (a) In normal Zn-Ni alloy plating, Ni in the plating layer
Content: Good corrosion resistance can be obtained in the range of 10 to 16% by weight, but the alloy in this range is a hard single-phase γ intermetallic compound, so if the steel plate is used as an outer plate for an automobile, it will not run smoothly. Due to the chipping impact caused by flying stones, etc., which tends to occur at times, it is easy to cause peeling of the paint film and even peeling of the plating film at the same time. Red rust is likely to occur in areas 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 highly discouraged. (b) 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 paint adhesion, it is still difficult to form a stable and satisfactory chemical conversion film on Zn-Ni alloy plated surfaces. (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
Some alloy plated surfaces cannot be said to have good cratering resistance. However, in order to improve this cratering resistance, Fe
Although methods for applying plating or Fe-Zn plating have been proposed (Japanese Patent Publication No. 15554/1983, Japanese Patent Publication No. 57518/1983), it is difficult to apply such Fe-based plating as an upper layer of Zn-Ni alloy plating. Although the cratering resistance is improved, red rust is likely to occur due to the presence of Fe-based plating that naturally remains even after painting, making it extremely disadvantageous as an automobile exterior panel. Note that the chipping resistance pointed out in section (a) above is related to the adhesion of plating, and in the past, multilayer plating of Zn-Ni alloys with different Ni contents was used to improve the adhesion of plating. There have been proposals to do so (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 Application Laid-Open Nos. 58-204196 and 1982-141894). However, in order to perform multi-layer plating of these Zn-Ni alloys, it is necessary to control the concentration ratio of Zn ²⁺ and Ni ²⁺ in the plating bath by varying the ratio, which requires a huge investment in equipment, and requires operational efficiency. Management was also 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 steel sheet that is fully satisfactory as both the inner and outer panels of automobile bodies, the following knowledge was obtained. That is, first, a Zn--Ni alloy plating intermediate layer with a high Ni content and a specific thickness is formed on a steel plate base material, and then a Zn--Ni alloy outer layer with an adjusted Ni content is plated on top of that, and then a Zn--Ni alloy plating layer with an adjusted Ni content is formed. When the outer surface of the Zn-Ni alloy plating outer layer is treated with a plating bath or other acid solution, the Zn-Ni alloy plating middle layer with a high Ni content improves the adhesion between the steel sheet base and the Zn-Ni alloy plating outer layer. In addition, by treating the outer surface of the Zn-Ni alloy plating outer layer with a plating bath or other acid solution, there is a slight preferential dissolution of Zn, and the chemical conversion treatment properties are significantly improved. It has a remarkable improvement effect on secondary adhesion (paint film adhesion) after painting and cratering resistance during cationic electrodeposition coating, and also has excellent rust prevention ability of the Zn-Ni alloy plating outer layer. The objective of the present invention is to realize a surface-treated steel sheet that is fully satisfactory as an outer panel of an automobile body. Various theories have been reported regarding the mechanism of the cratering phenomenon described above, and the general theory is that ``abnormal electrical discharge during electrodeposition coating causes a partially thermally hardened coating film, and that area causes a leveling effect after baking.'' The cause of the above abnormal discharge has not been fully elucidated, although the theory that the electrodeposition coating film becomes pock-shaped craters due to the lack of This is caused by and depends 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 above-mentioned surface-treated steel sheet is due to the fact that the chemical conversion treatment properties are significantly improved by preferential dissolution treatment of Zn using a plating bath or other acid solution on the outer surface of the Zn-Ni alloy plating outer layer. What is brought about. However, Zn--Ni with a certain high Ni content in the steel sheet
Alloy plating usually requires work in a plating bath and current density that is strictly controlled, but plating equipment with multiple treatment tanks is used, and the initial treatment tank (preferably the first tank, Or the first ~
2 tanks), Zn was added to the material steel plate (including steel strip).
- After applying a large amount of Ni alloy plating,
When this is immersed in a plating solution or an acid solution such as sulfuric acid stored in a subsequent treatment tank, Zn preferentially dissolves from the plating layer attached to the material steel plate due to the Zn-Ni alloy plating treatment and becomes Ni-rich.
Since the Zn-Ni alloy plating layer remains extremely stable and easily as the desired alloy layer with high Ni content, it is possible to form an intermediate Zn-Ni alloy with high Ni content on the raw steel plate by this method. After forming the plating layer, a predetermined amount of Zn is further applied in the subsequent electroplating bath.
-Ni alloy plating is carried out, and the plated steel sheet is then subjected to anode treatment, cathode treatment, or simple immersion treatment in a subsequent plating bath or other treatment tank filled with an acid solution to achieve a light preferential dissolution of Zn. When this process is carried out, the plating surface becomes a Ni-rich surface, although it is slightly, and the chemical conversion reaction becomes uniform and a very dense chemical conversion film is formed, making it suitable for surface treatment for automobiles as mentioned above. Ability to stably manufacture steel plates with good workability. This invention was 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 plating adhesion amount of 10 mg/m ² to 1 on a base steel plate.
A Zn-Ni alloy plating outer layer having a Ni content of 8 to 16% by weight is provided via an intermediate Zn-Ni plating layer of g/m ² , and the Zn-Ni alloy plating outer layer is exposed to a plating bath or an acid solution. It is characterized by having a structure with a Ni-enriched surface, which provides excellent plating adhesion, paintability, and paint film adhesion after painting.Furthermore, it has multiple treatment tanks. In plating equipment equipped with
After applying Zn-Ni alloy plating of less than m ² , 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.
Ni content is 30% by weight or more and adhesion amount is 10mg/m ² ~
A Zn-Ni alloy plating layer of 1 g/m ² remains, followed by a Zn-Ni alloy plating layer with a Ni content of 8-16% by weight.
By applying alloy plating and then treating in a plating bath or acid solution to preferentially dissolve the Zn on the surface of the Zn--Ni alloy plating layer, the plating adhesion, paintability, and post-painting properties are improved. Automotive rust-proof steel sheets with excellent paint film adhesion are easy to work with.
It is characterized by the following features: and it can be manufactured stably. In addition, as a preferential dissolution treatment for the light Zn on the surface of the Zn-Ni alloy plating layer, anode treatment is performed in a Zn-Ni alloy plating bath or in an acid solution such as sulfuric acid contained in a treatment tank following the Zn-Ni alloy plating tank. It is preferable to carry out a cathodic treatment, but a method of simply immersing the substrate in the plating bath or acid solution may also be used. With this surface treatment, Zn
is preferentially dissolved and a Ni-rich surface layer is formed, although only slightly, so that chemical conversion reactions (especially phosphate treatment reactions) occur uniformly, and the chemical conversion coating that is formed is extremely dense. becomes. Therefore, the secondary adhesion (coating film adhesion) during coating and the cratering resistance during cationic electrodeposition coating are significantly improved. Here, even if the degree of Ni concentration is extremely small, a corresponding effect can be ensured, so there is no need to specify it, and a treatment time of several seconds or less is sufficient. Next, in this invention, "Ni content and adhesion amount (thickness) of intermediate Zn--Ni alloy plating", "Zn--
The reason why the Ni content of the Ni alloy plating outer layer and the amount of Zn--Ni alloy plating initially deposited to form the intermediate alloy plating layer are numerically limited as described above will be explained. A Ni content and adhesion amount of intermediate Zn-Ni alloy plating Ni content of intermediate Zn-Ni alloy plating is 30% by weight
less than 10mg/m ² and its adhesion amount (thickness)
If it is less than
If the amount of plating exceeds 1 g/ ^m2 , a galvanic cell will be formed between the Fe of the base and the Ni of the intermediate Zn-Ni alloy plating layer if a scratch occurs that reaches the base (steel plate) during use. corrosion is accelerated, which has an adverse effect on corrosion resistance. Therefore, intermediate
The Ni content of the Zn-Ni alloy plating is 30% by weight or more, and the amount (thickness) of the coating is 10mg/m ² to 1g/m2.
It was limited to ^m2 . B Ni content of outer layer of Zn--Ni alloy plating In general, the corrosion resistance of Zn--Ni alloy plating varies greatly depending on the Ni content. And in theory
When the Ni content is 10 to 16% by weight, it becomes a single phase of γ, which has good corrosion resistance, and when the Ni content is less than 10% by weight, (γ + η)
When the Ni content exceeds 16% by weight (γ
+β), so it is thought that a galvanic cell is formed between the phases in the plating layer, which has an adverse effect on corrosion resistance (here, the γ phase is Ni ₅ Zn ₂₁ or
Ni ₃ Nn ₂₂ and the β phase is an intermetallic compound of NiZn). However, experimental confirmation showed that good corrosion resistance was obtained with a Ni content of 8 to 16% by weight (especially
Zn-Ni
The Ni content of the alloy plating outer layer was determined to be 8 to 16% by weight. C Amount of Zn-Ni alloy plating deposited first to form intermediate alloy plating layer Intermediate Zn- with Ni content: 30% by weight or more on steel plate
When forming the Ni alloy plating layer, the Zn-Ni alloy plating that has been deposited is immersed in acid to remove the Ni in the plating layer.
When using a method of concentrating Zn-Ni alloy plating, if the initial coating weight of Zn-Ni alloy plating is less than 3 g/ ^m2 , the desired Ni
Even if concentration is achieved, it is extremely difficult to secure a residual plating amount of 10 mg/m ² , so the initial deposit amount of Zn-Ni alloy plating should be 3 g/m ² or more. Established. Next, this invention will be concretely explained using Examples while comparing with Comparative Examples, but these are merely illustrative, and this invention is not limited by the Examples. Of course. <Example> Using an electric plating line equipped with 11 horizontal plating tanks, a cold-rolled steel plate with a thickness of 0.8 mm and a width of 1117 mm was pretreated with alkaline degreasing and pickling as usual. After cleaning, Zn--Ni alloy plating was applied under the conditions shown in Table 1. Each treatment tank at this time contained the following plating baths, except for a part of the 11th tank (final tank). Plating liquid composition: Zn ²⁺ ...30g/, Ni2 ⁺ ...60g/, Sodium sulfate...50g/, Sodium acetate...20g/, Plating liquid PH: 1.5-2.5,

【table】

[Table] Plating bath temperature: 50 to 60°C. Also, during plating, the current density is adjusted by controlling the current for each cell (bath) (the "low current density" indicated in Table 1 is several A). /dm ² , the "normal current density" display was adjusted to 20 to 150 A/dm ² , and in the 2nd tank and 11th tank (with some exceptions, no current was applied for Ni concentration), the line Speed is 40~
The speed was adjusted to 100m/min. In the final 11th tank, the preferential dissolution treatment for the surface Zn was performed by immersion in the plating bath without electricity.
The anode was immersed in a 2.0N sulfuric acid solution (50°C) without electricity, and the anode was immersed in a 2.0N sulfuric acid solution (at 10A/dm2 ^).
1.5sec) treatment or cathode treatment (at 15A/dm2 ⁾
1.5 seconds). 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 "BT-3020 (product name)" was applied as a base treatment for painting. The chemical conversion treatment used, cationic electrodeposition coating with a thickness of 20μ using Nippon Paint Co., Ltd.'s "Power Top U-30 (product name)", and 35-40μ intermediate coating and topcoat each with melamine alkyd paint A three-coat treatment consisting of painting was applied, and water resistance, adhesion, cratering resistance, chipping resistance, and external surface rust resistance were evaluated. In addition, to test the water resistance adhesion, after immersing the paint in deionized water at 50°C for 10 days, make grid cuts at 2 mm intervals on the paint film and peel it off with tape, and then investigate the paint film survival rate. It 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. Chipping resistance is determined by first cooling a 3-coat coated steel plate to -20°C and then performing a chipping test using a gravelometer under the conditions specified in ASTM D-3170-74. Evaluation was made based on the presence or absence of Furthermore, the external rust resistance was tested by putting a cross cut in the painted board and exposing it outdoors twice a week to reduce the rust by 3%.
Evaluation was made based on the presence or absence of red rust after half a year had passed while spraying NaCl solution, and the creep width on one side from the cross cut. These results are calculated based on the amount of plating on each layer and
Table 1 also shows the Ni content of the Zn--Ni 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 an inner panel of an automobile body but also as an outer panel. While a rust-proof steel plate that exhibits sufficiently excellent performance can be obtained even when used, a plated steel plate whose manufacturing conditions deviate from the provisions of this invention and therefore does not meet the conditions specified by this invention is one of the above. It is clear that it is not suitable for rust-proofing steel sheets for automobiles, which is a future target. <Overall Effects> As described above, according to the present invention, regardless of whether it is used as an inner panel or an outer panel of a vehicle body,
It has become possible to stably mass-produce rust-proof steel sheets for automobiles that exhibit excellent performance no matter where they are applied, with good workability, and it has brought extremely useful industrial effects such as further improving the durability of automobiles. It is.

Claims (1)

[Claims] 1. Intermediate Zn--Ni with a Ni content of 30% by weight or more and a plating adhesion amount of 10 mg/m ² to 1 g/m ² on a base steel plate.
Through the plating layer, the Ni content is 8 to 16% by weight.
A Zn—Ni alloy plating outer layer is provided, and the Zn—
The Ni alloy plating outer layer is coated with a plating bath or acid solution.
A rust-proof steel sheet for automobiles, characterized by having a Ni-enriched surface. 2. In a plating equipment equipped with multiple treatment tanks, Zn with a plating amount of 3 g/m ² or less is first applied to the steel plate to be treated.
―After applying Ni alloy plating, the Zn―
By preferentially dissolving Zn in the Ni alloy plating layer, the Ni content is 30% by weight or more and the adhesion amount is 10mg/ ^m2 to 1g/ ^m2 .
Leave the Zn-Ni plating layer and then apply Ni on top of it.
Apply Zn-Ni alloy plating with a content of 8 to 16% by weight, and then perform preferential dissolution of Zn on the surface of the Zn-Ni alloy plating layer by treatment in a plating bath or acid solution. A method for manufacturing a rust-proof steel plate for automobiles, characterized by: