JPS58210192A - Weldable painted steel plate - Google Patents

Abstract

PURPOSE:To obtain the titled steel plate having superior adhesive strength to the paint film, superior blister and corrosion resistances by plating a steel plate with an Ni-Zn alloy having a single gamma phase, chromating the plated steel plate, and coating it with paint contg. an electrically conductive substance. CONSTITUTION:A steel plate plated with an Ni-Zn alloy layer having a single gamma phase is chromated and coated with paint contg. an electrically conductive substance to obtain a weldable coated steel plate. The plate has superior adhesive strength to the paint film and does not cause the stripping of the film even when worked to a high degree. The plate has blister resistance comparable to that of a coated plate using a cold rolled plate, and it is superior to a conventional plate using a Zn plated steel plate in corrosion resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a coated steel sheet that has excellent weldability, and has good coating film adhesion and corrosion resistance.

The term "paint film adhesion" as used herein refers to the difficulty of peeling off the paint film during the forming process of a steel plate, and hereinafter all references to paint film adhesion will refer to this property.

2. Description of the Related Art Generally, various types of equipment, parts, etc. that are painted and put into actual use are painted entirely after being processed and assembled. However, during such processing and painting after assembly, a sufficient coating film may not be obtained depending on the area. In order to overcome this disadvantage, so-called painted steel sheets, which are pre-painted, have been widely used. By using painted steel plate,
This is to compensate for the lack of paint film on the entire surface after processing and assembly. However, resistance welding is usually not possible with this type of painted steel plate, and the assembly requires methods other than welding.
For example, fixing means such as bolts and nuts must be used, which is disadvantageous in terms of workability and assembly cost.

However, it can be said that this inferiority of coated steel sheets has been largely eliminated through recent research and development. In other words, a countermeasure has been proposed in which various conductive substances such as ZN and Az are included in the coating film of painted steel sheets to impart weldability. It is a ruru thing.

By the way, the main uses of painted steel sheets include automobile bodies, electrical equipment, etc., but especially the painted steel sheets used in these applications have other properties in addition to the above-mentioned weldability.
Many properties are required, such as corrosion resistance and coating adhesion.

Here, the following types of coated steel plates made weldable by taking the above-mentioned measures (hereinafter referred to as weldable coated steel plates) have been known. For general cold rolled steel plate base material,
There are those coated with hexavalent chromium and coated with paint containing electrically conductive substances such as Zn and At (Special Publication No. 1987-6882), and those coated with a Zn-plated steel plate as the base material. , coated with paint containing various conductive substances (
Special Publication No. 54-11381). However, none of these conventional weldable coated steel sheets can be said to be sufficient for the above-mentioned applications. The former method, which uses cold-rolled sheets, has a problem in paint film adhesion.For example, the molding process for automobile fenders and the like requires very high processing, so there is a high risk that the paint film will peel off. This is caused by the insufficient adhesion between the cold-rolled sheet base material and the chromate 9 film as a paint base, but this is not a problem, and the paint film itself is due to the presence of conductive substances. It also tends to be prone to cracking and peeling, and these properties are also added to the factors that increase the damage to the coating film during the above-mentioned molding process. Furthermore, with this coated steel sheet, if the coating film is damaged during processing or use, red rust will develop on the base material at an early stage, resulting in a loss of commercial value, which is a problem in terms of corrosion resistance.

The latter type, which uses galvanized steel sheets, is intended to solve the problem of corrosion resistance of the above-mentioned painted steel sheets, and it certainly has an advantage in terms of red rust resistance. however,
The corrosion resistance of galvanized steel sheets originally means that zinc preferentially dissolves in corrosive environments and prevents corrosion of the steel sheet base.
This is dependent on so-called sacrificial corrosion protection, and because the dissolution rate of zinc in a corrosive environment is high, zinc dissolution under the paint film progresses near the eaves or end faces, leading to early paint film blistering ( There is a tendency for blisters (hereinafter referred to as blisters) to occur.

This is a major problem because the blisters significantly reduce the commercial value of this type of coated steel sheet. Moreover, even if this galvanized steel sheet is used, it may be hoped that the adhesion of the paint film will improve; on the contrary, there are many cases where it actually deteriorates.

In this way, all weldable painted steel sheets have insufficient soil, and in particular, they suffer from the drawback of insufficient paint film adhesion, which is common to conventional products, and also have poor corrosion resistance and blister resistance (to prevent blistering). However, there was still room for improvement in these areas.

The present invention has effectively solved the above two problems, that is, it has excellent paint film adhesion without peeling even when subjected to high processing, and has blister resistance comparable to that of cold-rolled sheets. The object of the present invention is to provide a weldable coated steel sheet that has superior corrosion resistance to conventional products using galvanized steel sheets.

In other words, the gist of the present invention is to apply chromate treatment to a plated steel sheet having an N'h-Zn alloy plating layer consisting of a single γ phase, and coat the coated steel sheet with a coating containing a conductive substance. It is a weldable painted copper plate, which is characterized by becoming.

It is clear that one of the important factors that influences the performance of weldable painted steel sheets is the properties of the base material itself, without referring to the two examples listed above. As mentioned above, the problem of blistering occurring in the base material of galvanized steel sheets is a problem, and the occurrence of blisters becomes more noticeable as the plating thickness increases. This is clear from the fact that the galling phenomenon is caused by the dissolution of the plating layer under the coating film, but on the other hand, the corrosion resistance of zinc plating is due to the sacrificial corrosion protection of the plating layer, as mentioned above. Therefore, its quality depends entirely on the amount of zinc, and if the basis weight is insufficient, high corrosion resistance cannot be guaranteed. In other words, in galvanizing, when trying to obtain sufficient blister properties,
The amount of date is naturally limited, and the original corrosion resistance cannot be expected at all.

By the way, a wide variety of corrosion-resistant platings are already known, but some of these include sacrificial corrosion protection as well as anti-corrosion properties by electrochemically forming a skin on the surface in a corrosive environment. Many also have the functions of

The inventors of the present invention have combined these two points, and from the point of view of avoiding the blister problem while ensuring high corrosion resistance, even if the same plated steel sheet is used, the electrochemical Wouldn't it be better to adopt a type that focuses on the anti-corrosion function?
I guessed. This is because it is thought that the stronger the tendency for sacrificial corrosion protection, the more likely blisters will appear.

The inventors first conducted various experiments and investigations and confirmed that this idea is valid.

Therefore, we conducted experiments and studies regarding the possibility of applying various types of plated steel sheets with strong electrochemical anticorrosion function to the base material of weldable painted steel sheets from the viewpoint of coating adhesion, and as a result, we found the above-mentioned Among these types of plated steel sheets, electroplated steel sheets (hereinafter simply referred to as N11-Zn alloy plated steel sheets) have a Ni-Zn alloy plating layer consisting of a single γ phase. We discovered a new fact that this method is extremely effective in ensuring membrane adhesion. N'x-Zn
The mechanism by which the coating adhesion of this type of coated steel sheet improves due to the presence of an alloy plating layer has not been sufficiently elucidated, but compared to a single zinc plating layer,
It is certain that the N'L-Zn alloy plating layer has a high reactivity with the chromate wave pattern used as the coating base, and this seems to be probably involved.

N1--Zn alloy plated steel sheet also has thermal sacrificial corrosion resistance, but it is thin in nature and its electrochemical corrosion protection function is much stronger, and its bare corrosion resistance surpasses that of general galvanized steel sheet. Everything will turn out the same.

If this Ni--Zn plated steel sheet is used, it is possible to obtain the same corrosion resistance as when a cold-rolled sheet is used, and the corrosion resistance is also greatly improved compared to the case where a galvanized steel sheet is used. This was confirmed through experiments by the present inventors, and regarding this point, the above-mentioned Ni-Z
This can be understood as a result of the inherent characteristics and performance of n-plated steel sheets.

By the way, in the past, regarding Nj-Zn alloy plated copper plate,
There is no view that it is particularly effective in improving paint film adhesion of weldable painted steel sheets, and it goes without saying that this Nj-Z
There are no practical examples of the above-mentioned coated steel sheets using n-alloy plated steel sheets.

In the present invention, N1'-Z of the plated steel plate as the base material
This is because the n-alloy plating layer is made of a single γ phase, but other types, such as η+γ phase, γ+α phase, etc., are inferior in corrosion resistance and blister resistance, as is clear from the examples described later. be. The N'1-Zn alloy plating layer consisting of a single γ phase has both moderate sacrificial corrosion protection and the above-mentioned electrochemical corrosion protection function, and has extremely excellent corrosion resistance. Since the speed is sufficiently low, the problem of bristle formation after painting can be solved. In the case of a single γ phase, the N1 content of the Nj-Zn' alloy plating layer is 9 to 2
It becomes 0%. The Nj-Zn alloy plating N containing 9-20% Ni is different from the generally known Zn electroplating, although there is no particular limitation on the basis weight, but from the viewpoint of corrosion resistance, it is preferable to have a weight of 1 g7m* or more. Shikatana Shinaka 6
Q: It is not necessary for thick gauges exceeding 9/m' and is undesirable from the viewpoint of weldability since it is unnecessary and uneconomical.

The chromate treatment performed prior to paint application is a common treatment for this type of coated steel sheet as a base treatment for painting. For this chromate treatment, any method such as electrolytic chromate or coating chromate may be used, but the Cr content of the chromate film should be 1 to 500 mg7゜2.
degree is suitable.

Although not limited to this, it is recommended from the viewpoint of weldability that a conductive substance such as Zn, which will be described later, be appropriately included in this chromate film. To prevent the above-mentioned substances from entering the chromate film, use chromate-1 treatment solution (chromic acid aqueous solution).
The chromate film, which can be formed by adding the substance to be mixed in the form of powder, can be heated and reduced under appropriate conditions, so-called baking, to promote the reaction with the base material and obtain adhesion. This thermal reduction is achieved by drying at 160''Ct for about 1 minute after the chromate treatment, followed by baking with the paint at 180-280°C for 30-120 seconds.

When painting the chromate film as a base, a paint containing a conductive substance must be used, and the conductive substance may include Zn, A, 4. Sh.

Any of C, Fe, Ni, C○, Cr, etc. can be used. In either case, these conductive substances are contained in the paint in the form of powder. The content is not particularly limited, and may be determined as appropriate depending on the type of conductive material used, etc., so as to ensure the required weldability. In the case of Zn, extremely good electrical conductivity can be obtained with a content of 85 wt% or more. There are no particular restrictions on paint vehicles, such as acrylic,
Synthetic resins such as epoxy, urethane, vinyl, polyester, butadiene, alkyd, styrene, and phthalic acid.

Other natural resins, oil vehicles, etc. may also be used. For painting, paint is applied by a known method such as a roll coater or a curtain flow coater method, and then 180%
Bake at ~280°C for approximately 30-120 seconds. The film thickness is usually about 5 to 15 microns.

Next, the effects of implementing the present invention will be explained in detail by giving specific examples.

Using the various base materials shown in Table 1, chromate treatment (coating type) was performed using a solution of chromic acid mixed with Zn powder (Dacromet 200 manufactured by Nippon Dacro Shamrock Co., Ltd.), and this was heated at 160°C x 1. After heating and drying for several minutes, a paint (Zinchromet made by Nippon Dacro Shamrock Co., Ltd.) made of epoxy resin mixed with Zn powder was applied on top using a roll coater.
The film was painted using a 100% polyester resin, and then baked at 280°C for 1 minute.

The obtained painted steel sheet has corrosion resistance, sharpening resistance,
Weldability and paint film adhesion were evaluated by the following tests.

■ Red rust resistance test: A cross cut was made in the coating film of a painted steel plate, and a salt water spray test was conducted for 480 hours, and the occurrence of red rust in the black rust area after the test was investigated. The evaluation is ○
Based on 8 levels: : No red rust, △: Also slightly, 1, and ×: Also noticeable.

■ Blister resistance: After the salt spray test described in (1) above, the maximum peeling at the cross-cut portion was investigated.

■ Weldability test 1: Nupot welding was performed under the conditions of electrode diameter 5N, pressure force 200#, time 10~, and current 8500A, and the number of consecutive points that could be made was determined.

The evaluation was performed on an eight-point scale: ◯: 5000 points or more, △: 500 points or more but less than 5000 points, X: less than 8000 points.

■ Paint film adhesion test A bead adhesion test was conducted using a nibbles molding machine to investigate the paint film survival rate.

The above results are summarized in Table 1.

Table 1 In the above table, (1), which uses cold-rolled sheets, either has no problems at all in terms of blister resistance and weldability, or has extremely poor red rust resistance, and also has poor paint film adhesion compared to automobiles. This is a big dissatisfaction in practical use considering applications that undergo high processing such as steel sheets.Also, galvanized steel sheets were used (2)
In terms of red rust resistance, satisfactory results have been obtained that far exceed the above.

The use of galvanized steel sheets can be said to be effective in this respect. On the other hand, however, the use of galvanized steel sheets is problematic, especially in that it leads to significant deterioration in blister resistance, and is also unfavorable from the viewpoint of weldability, and furthermore, it does not lead to improvement in paint film adhesion.

When using alloyed hot-dip galvanized steel sheet, (3)
As shown in the example above, this is a custom-made product that is approximately intermediate between the above two, and there is a tendency for improvement over (1) and (2) in terms of coating film adhesion. However, this level of performance is still not satisfactory for the above-mentioned practical purposes. Comparative example (4).

(5) HlNj-Zn alloy plated steel plate was used under the same conditions as the coated steel plate of the present invention, but the plated layer exhibited a two-phase structure of η + γ (Ni: 5%) and γ + α (N 180%), respectively. (4) contains Znj in the plating layer.
The sacrificial corrosion resistance is so high that it is not much different from that of a zinc-plated layer alone, and therefore has sufficient red rust resistance.
(2) Using galvanized steel plate for blister resistance
The weldability is almost as bad as that of cold-rolled sheets (
Compared to 1), it shows inferiority. In addition, (5) also has poor red rust resistance and insufficient blister resistance due to poor balance between the sacrificial corrosion protection of the plating layer and the electrochemical anticorrosion function, and the corrosion resistance of the plating layer as a whole is poor. Not. However, in both (4) and (5), at least N
Because it uses i-Zn alloy plated steel plate,
In terms of coating film adhesion, a value that was satisfactory for use was recorded, similar to the invention example (6) described later.

Finally, present invention example (6) Nj- consisting of a single phase of td and γ phase
It uses a Zn alloy plated steel sheet, and has achieved satisfactory results in terms of all properties including red rust resistance, abrasion resistance, weldability, and paint film adhesion. In terms of blister properties and weldability, there is no inferiority at all to the cold-rolled steel sheet used in (1), and the red rust resistance is also extremely good, and in terms of paint film adhesion, it is suitable for automotive use The results showed a value that guarantees performance that can sufficiently withstand high processing conditions and prevent paint film peeling.

As is clear from the above description, the weldable painted steel sheet of the present invention has excellent corrosion resistance and blister resistance, extremely high coating film adhesion, and has the excellent weldability inherent to this type of painted copper sheet. Therefore, it can be applied to steel plates for automobiles, etc., which undergo high processing and are used in a severe corrosive environment, and can achieve satisfactory effects.

Applicant: Sumitomo Metal Niiichi Kogyo Co., Ltd. Representative Patent Attorney Motoshige

Claims (1)

[Claims] (1) A weldable coated steel sheet, which is made by subjecting a plated steel sheet having an Nj-Zn alloy plating layer consisting of a single γ phase to chromate treatment, and applying a paint containing a conductive substance thereon. .