JPS6342394A - Production of zinc-nickel alloy electroplated steel sheet having superior adhesion under shock - Google Patents

Abstract

PURPOSE:To improve the adhesion under shock after coating and the corrosion resistance by forming a Zn-Ni alloy layer as a lower layer on a steel sheet by electroplating, making the layer discontinuous to relieve the internal stress and forming a prescribed Zn-Ni alloy layer on the lower layer by electroplating. CONSTITUTION:A steel sheet is continuously passed through plural plating cells. A Zn-Ni alloy layer contg. 8-16wt% Ni is first formed as a lower layer on the steel sheet by electroplating by 200-1,000mg/m<2>. The plated steel sheet is immersed in a plating bath having the same compsn. as the first plating bath or in an acidic soln. for 1-30sec without supplying electric current to make the lower layer discontinuous. A Zn-Ni alloy layer contg. 8-16wt% Ni is then formed on the discontinuous layer by electroplating by 10-40g/m<2>.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for manufacturing a zinc-nickel alloy electroplated steel sheet having excellent quality characteristics after coating.

[Prior art and its problems]

In recent years, various zinc-based alloy coated steel sheets have been developed as corrosion-resistant materials for automobiles, which are often used in harsh corrosive environments. Among these, zinc-nickel alloy electroplated steel sheets are attracting particular attention because of the excellent corrosion resistance of the plating film.

However, zinc-nickel alloy electroplated steel sheets have the following problems. That is, since the plating layer is hard and brittle, and the internal stress of the plating film is large, numerous fine cracks are generated by light processing, and red rust from the 'EX' surface is generated in a short period of time due to these cracks. In this way, zinc-nickel alloy electroplated steel sheets have excellent corrosion resistance in an unprocessed flat plate state, but when processed, the corrosion resistance of the plating film on the processed parts deteriorates significantly, and the flat plate state deteriorates. It has a corrosion resistance that is less than half that of the case of .

A lot of research has been done to improve the deterioration of corrosion resistance due to processing, and various patents (for example, JP-A No. 56-35,790, JP-A No. 61-439,
JP-A-59-200,789, etc.) have also been filed.

Zn-Ni alloy plated steel sheets made by these techniques are
There is no problem when used without coating, but when used with coating, the following problems arise. That is, when an impact is applied to a painted Zn--Ni alloy plated steel sheet, the coating film peels off together with the plating film, and the copper base is exposed. This problem is particularly noticeable at low temperatures, even if there is no obvious deformation of the steel plate. It has been revealed that fJj ``H alone causes the coating film to peel off together with the plating film.

Therefore, when such Zn-Ni alloy plated steel sheets are used as exterior panels for automobiles, the exterior panels of automobiles may be exposed to shocks such as flying pebbles in regions such as North America, which are cold in the winter. In some cases, the paint film peels off along with the plating film, resulting in a significant loss of corrosion resistance and appearance.

As a Zn--Ni alloy-plated steel sheet that solves the above-mentioned problems, a Zn--Ni alloy-plated steel sheet consisting of the following is disclosed in JP-A-59-107,095.

[52. on at least one side of the copper plate. Zn-Ni higher than hawk
It has an alloy plating layer, and the Ni content of the plating layer is 7 wt.% or less at the steel plate interface and 10 wt.% at the plating surface.
A zinc-nickel alloy plated steel sheet with excellent workability and impact resistance, characterized in that the nickel content is 16 wt.% and increases continuously from the steel sheet interface toward the plated surface. (hereinafter referred to as the prior art) According to the above-mentioned prior art, processing such as pressing, or
It is possible to prevent cracks in the plating film due to impacts such as stone splashes.

However, this conventional technique has the following problems.

■Operational problems.

In order to continuously change the Ni content of the plating layer from the copper plate interface toward the plating surface, the following means are required.

(a) Plating bath composition (Ni content in the plating bath)
changes.

(b) Changes in plating conditions, such as plating current density or stirring conditions.

Steel sheet electroplating equipment usually consists of multiple plating tanks, so it is possible to change the plating conditions for each plating tank, but in order to do this industrially, the following steps are required. There is an opening of the country.

In other words, as shown in (a), in order to change the composition of the plating bath for each plating tank, an independent plating bath circulation system is required for each plating tank, and the plating bath composition is changed for each plating tank. Concentration must be controlled.

In addition, as shown in (b), changing the plating conditions for each plating tank causes the same problems as in (a), as well as the following problems in terms of equipment usage efficiency.

That is, in the prior art, the content of Ni in the plating layer
In order to make the Jfi1 ratio 12 to 14 wt%, the current density should be 100 A/drr? In order to set the Ni content to 7wt%, the current density was set to 25 A/dm.
Therefore, according to the prior art,
The average current density will be approximately 60 A/dm'. However, if the Ni content ratio k of a normal Zn-Ni alloy plating layer is 12-14 wt9%, then 100 k/d
rr? Therefore, the production efficiency of the conventional technology is extremely low.

(2) Quality Issues In general, in Zn--Ni alloy plating, the plating film composition with the best corrosion resistance is a γ single phase with a Ni content of at least 8 wt.% or more.

However, since the Ni content at the steel plate interface in the prior art is 7wt% or less, a plated layer with a Ni content of less than 8wt0% exists. In such a plating layer with a low Ni 2 content, the η phase (pure Zn) is precipitated together with the γ phase, and the corrosion resistance is significantly reduced.

In addition, if the Ni content ratio between the steel plate interface and the plated surface is different, a potential difference will occur between the two, and compared to the plated surface,
The plating film on the steel plate interface becomes base. As a result, for example, when a scratch occurs on the plating film of a painted plate, the base plating film at the interface of the steel plate is leached out, making it easy to form blisters, which significantly impairs corrosion resistance and deteriorates quality. .

[Purpose of the invention]

Therefore, an object of the present invention is to provide a method for manufacturing a zinc-nickel alloy electroplated steel sheet that has excellent impact adhesion and corrosion resistance after coating at low temperatures and can be produced with high efficiency.

[Summary of the invention]

The present inventors have conducted extensive research in order to resolve the phenomenon in which the coating film peels off along with the plating film when a painted zinc-nickel alloy electroplated steel sheet is subjected to an impact at low temperatures. As a result, peeling of the plating film is caused by an increase in the nickel content relative to the plating amount of the Zn-Ni alloy plating layer as the lower layer formed in the first plating tank in the electroplating process performed in multiple plating tanks. and its plating amount, and Zn-N as the upper layer in the primary plating bath.
It has been found that the condition of the underlying plating film before the i-alloy plating layer is formed has a large effect. Therefore, Zn as the lower layer in the first plating bath
-Content of nickel in the plating amount of the Ni alloy plating layer,
A predetermined range is set for the proportion and the amount of plating, and before the Zn-Ni alloy plating layer as the upper layer is formed in the next plating bath, the Zn-N as the lower layer is
The plating film of the i-alloy plating layer is brought into a discontinuous state, its internal stress is relaxed, and a predetermined amount of Zn is added to the upper surface of the lower layer.
It has been found that plating peeling can be prevented by applying -Ni alloy electroplating. This invention has been made based on the above-mentioned findings, and consists of continuously moving a steel plate through a plurality of plating baths containing zinc-nickel alloy electroplating baths, and removing at least one of the steel plates. Electroplating the surface to form a zinc-nickel alloy plating layer as a lower layer on at least one surface of the steel sheet, and then depositing zinc as an upper layer on the zinc-nickel alloy plating layer as the lower layer. - Since a nickel alloy plating layer is formed, in the first plating tank among the plurality of plating tanks, the content ratio of nickel to the plating amount is 8 to 16 wt 0% as a preliminary plating step, and the plating Form a zinc-nickel alloy plating layer as the lower layer with an amount of more than 200 to 1000 μ/R, and then in the next plating bath, as a dipping step, the same plating bath as the first plating bath. The steel plate on which the zinc-nickel alloy plating layer as the lower layer is formed in a plating bath or acidic liquid is electrolessly immersed for 1 to 30 seconds to bring the zinc-nickel alloy plating layer as the lower layer into a discontinuous state. In addition, the internal stress is relaxed, and then in the third plating bath and thereafter, as a main plating process, the content of nickel relative to the plating amount is added to the zinc-nickel alloy plating layer as the lower layer formed in a discontinuous state. The ratio is 8 to 16 wt-%, and the plating amount is 10 to 40 t/rr? The present invention is characterized in that a zinc-nickel alloy plating layer is formed as the upper layer.

[Structure of the invention]

In the preliminary plating process, the plating amount of the zinc-nickel alloy plating layer as the lower layer should be more than 200% to 1000%.

Is the plating amount of the zinc-nickel alloy plating layer as the lower layer 1' 000 mrrr? If it exceeds this, the low-temperature impact adhesion after coating will deteriorate. On the other hand, Zn-Ni as the lower layer
If the plating amount of the alloy plating layer is less than 200η, when forming the Zn-Ni alloy plating layer as the upper layer, the plating film as the lower layer will dissolve in the plating bath and become thin, so that it will be difficult to form it afterwards. It is not effective in reducing the internal stress or changing the internal stress distribution of the plating layer as the upper layer, and furthermore, the plating layer as the lower layer cannot withstand the impact received after painting, and the low-temperature technique-g adhesion after painting is not effective. deteriorates.

The nickel content of the Zn-Ni alloy plating layer as the lower layer should be 8 to 16 wt.%. If the nickel content of the zinc-nickel alloy plating layer as the lower layer is less than 8 wt%, the η phase will precipitate, resulting in a significant deterioration in the corrosion resistance of the plating film. On the other hand, if it exceeds 16 wt.%, the corrosion resistance of the plating film deteriorates, internal stress increases, and workability also deteriorates.

Further, the above-mentioned nickel content is the same as the Ni content of the zinc-nickel alloy plating layer as an upper layer in the main plating step to be described later. Therefore, there is no need to change the composition of the plating bath between the preliminary plating step and the main plating step, which is effective in terms of production efficiency.

The steel sheet on which the lower zinc-nickel alloy plating layer has been formed in the first plating tank is then immersed in the same plating bath as the first plating bath or in an acidic solution in the next plating tank. , should be electroless immersed for 1 to 30 seconds. The reason why electroless dipping is performed is to make the zinc-nickel alloy plating layer as the lower layer sharply discontinuous and to relieve its internal stress.

By making the lower zinc-nickel alloy plating layer discontinuous, the upper zinc-nickel alloy plating layer formed on the lower zinc-nickel alloy plating layer is prevented from peeling off due to impact. be able to.

In other words, even if an external impact is applied to the steel sheet, the impact stress generated by the impact and applied to the upper zinc-nickel alloy plating layer is dispersed because the lower zinc-nickel alloy plating layer is discontinuous. and alleviated.

Therefore, the zinc-nickel alloy plating layer as the upper layer does not peel off from the zinc-nickel alloy plating layer as the lower layer.

If the electroless immersion time of the steel plate on which the zinc-nickel alloy plating layer as the lower layer is formed is less than 1 second, the zinc-nickel alloy plating layer as the lower layer will not be in a discontinuous state and the internal stress will not be sufficiently relaxed.

On the other hand, if the electroless immersion time exceeds 30 seconds, the zinc-nickel alloy plating layer as the lower layer will dissolve.

In addition, in order to shorten the immersion time, reverse electrolysis may be performed.

As the immersion liquid for electroless immersion, it is preferable to use the same plating bath as the plating bath in the preliminary plating step, but for example, an acidic solution such as a dilute sulfuric acid solution or a dilute hydrochloric acid solution may be used.

The plating amount of the zinc-nickel alloy electroplating as the upper layer in the main plating process performed in the third tank and after is the same as the plating amount of the zinc-nickel alloy electroplated copper plate that is usually performed. It should be ~40 foot. 10
fAr? Below 40 g/m, corrosion resistance deteriorates;
If it exceeds 2, the number of plating tanks will need to be increased, resulting in decreased production efficiency and uneconomical performance.

The content ratio of nickel to the plating amount of the zinc-nickel alloy plating layer as the upper layer should be 8% to 16% by weight. If the nickel content of the zinc-nickel alloy plating layer as the upper layer is less than 8 wt.%, the corrosion resistance of the plating film will significantly deteriorate.

On the other hand, when it exceeds 16 wt-%, the corrosion resistance of the plating film deteriorates, internal stress increases, and workability also deteriorates.

〔Example〕

A Zn-Ni alloy layer having a Zn-Ni alloy layer within the scope of the present invention shown in Table 1 is obtained by electroplating a steel plate according to the plating bath composition shown in (1) below and the plating conditions shown below (2).
- Specimens of Ni alloy electroplated steel sheets (hereinafter referred to as specimens of the present invention) Nα1 to 18 and specimens outside the scope of this invention shown in Table 1 (hereinafter referred to as comparative specimens)
Nα1-7 were prepared.

(1) Plating bath composition ZnSO4.7H,,O: 150? /IN i S
04 ・7H20: 3 5 0 ta/1NaS
O4: 55? /L (2) Plating conditions (a) pH of the plating bath: 1.3 (b) Temperature of the plating bath: 50'C (c) Immersion time in the immersion process After plating in the first plating tank, in the second plating tank It was immersed for 1 to 30 seconds without electricity.

) Plating current density Specimens N111 to 18 of the present invention: Pre-plating process: 15 A/dyn'.

Main plating process: 50 A/dttl.

Comparative specimen N [Ll, 2: Main plating process: 50 A/drr? .

Comparative specimen N [13 to 7: Pre-plating process: 15 A/dm' 1 Main plating process: 50 A/dtr? .

Next, for each of the specimens Nα1 to 18 of the present invention and the comparison specimens N11 to 7, a deep-type phosphate treatment solution commonly used by automobile manufacturers (a commercially available product manufactured by Nippon Barker Lysong Co., Ltd.) was applied. A phosphate film was formed on the surface of the specimen by phosphate treatment using the same product as the standard product), and then a cationic electrodeposition coating was applied using ED paint manufactured by Nippon Paint Co., Ltd. A coating with a thickness of 20 μm was formed on the phosphate film. Next, a 35 μm intermediate coat was applied to the coating film using an intermediate coat made by Kansai Paint Co., Ltd., and a 35 μm top coat was applied using the same intermediate coat made by Kansai Paint Co., Ltd.

The specimens on which the coating films were formed in this manner were subjected to bending tests, chip resistance tests, and diamond shot tests as described in (1) to (3) below, and the results are also shown in Table 1.

(1) Bending test The specimen was cooled to -20°C and heated to 50°C. The presence or absence of peeling of the coating film was examined by performing a φ bending test.

The evaluation criteria are as follows.

○ mark: No peeling of paint film.

×: Paint film peeled off.

(2) Chip resistance test The specimen was cooled to -20°C, and a chip resistance test was conducted using No. 6 crushed stone at an air pressure of 4 Kr/m to check for peeling of the paint film. The evaluation criteria are as follows.

○ mark: No peeling of paint film.

×: Paint film peeled off.

(3) The diamond shot test specimen was cooled to -20°C, and the industrial diamond was heated to 17°C.
Shotting was performed at a speed of 0 KjJUA, taping was performed, and the presence or absence of peeling on the plated surface was examined. The evaluation criteria are as follows.

○ mark: No peeling.

× mark: peeling.

As shown in Table 1, specimens N[11 to 18 of the present invention had good results in all of the above-mentioned test results (1) to (3), and had poor low-temperature impact adhesion.

On the other hand, comparative specimens No. 1 and 2, which were not electroplated with zinc-nickel alloy as the lower layer in the preliminary plating process and were not immersed in a plating bath in the dipping process,
All test results were poor.

Comparative specimens Nos. 113 to 7, in which the plating amount of the zinc-nickel alloy plating layer as the lower layer in the preliminary plating step was outside the range of the present invention, had poor test results.

As mentioned above, comparative specimens Nos. 1 to 7 are all inferior in impact adhesion.

FIG. 1 is a 20,000 times enlarged view of the surface of specimen N113 of the present invention after immersion treatment in the immersion process. As shown in FIG. 1, the zinc-nickel alloy plating layer 2 as a lower layer formed on the surface of the steel plate 10 is in an island-like discontinuous state.

FIG. 2 is a 600 times enlarged view of the surface of the specimen Nα18 of the present invention after the immersion treatment in the immersion process. Second
As shown in the figure, many cracks 3 have occurred in the zinc-nickel alloy plating layer 2 as the lower layer formed on the surface of the steel sheet 1, and the zinc-nickel alloy plating layer 2 as the lower layer has It is in a discontinuous state.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to obtain a zinc-nickel alloy electroplated steel sheet that has excellent impact adhesion and corrosion resistance after coating at low temperatures and can be produced with high efficiency. Useful effects are produced.

[Brief explanation of drawings]

Figure 1 is a 20,000 times enlarged view of the surface of specimen N (L13) of the present invention after immersion treatment in the immersion process, and Figure 2 is a 20,000 times enlarged view of the surface of specimen N (L13) of the present invention after immersion treatment in the immersion process. [This is a 600 times enlarged view of the surface of 118. In the drawing, 1... Steel plate, 2... Zinc-nickel alloy plating layer as lower layer, 3
···crack.

Claims (1)

[Scope of Claims] A steel plate is continuously moved through a plurality of plating baths containing zinc-nickel alloy electroplating baths, and at least one surface of the steel plate is electroplated. A zinc-nickel alloy plating layer is formed as a lower layer on at least one surface of the base layer, and then a zinc-nickel alloy plating layer is formed as an upper layer on the zinc-nickel alloy plating layer as the lower layer. In the method for producing a zinc-nickel alloy electroplated steel sheet having a plurality of zinc-nickel alloy electroplated layers, the method comprises: forming a pre-plating step in a first plating bath among the plurality of plating baths; , the content ratio of nickel to the plating amount is 8 to 16 wt. %, and the plating amount is 20
A zinc-nickel alloy plating layer as the lower layer having a concentration of more than 0 to 1000 mg/m^2 is formed, and then in the next plating bath, the same plating bath as the first plating bath is performed as a dipping step. The steel plate on which the zinc-nickel alloy plating layer as the lower layer has been formed is electrolessly immersed for 1 to 30 seconds in a plating bath or acidic solution to form the zinc-nickel alloy plating layer as the lower layer discontinuously. Then, from the third plating tank onwards, as the main plating process,
On the discontinuously formed zinc-nickel alloy plating layer as the lower layer, a nickel content ratio of 8 to 16 wt. % and has a plating amount of 10 to 40 g/m^2, forming a zinc-nickel alloy plating layer as the upper layer, which has excellent impact adhesion. Method for producing plated steel sheets.