JPH042783A - Surface-treated steel sheet having high corrosion resistance - Google Patents

Abstract

PURPOSE:To improve pitting and surface rusting resistances by successively forming two Zn-Ni alloy plating layers having different Ni contents, a chromated layer and a protective resin layer on a steel sheet. CONSTITUTION:A Zn-Ni alloy plating layer having <=13wt.% Ni content is formed as the lowest layer on a steel sheet by 10-50g/m<2> and a Zn-Ni alloy plating layer having 10-40wt.% Ni content is formed on the lowest layer by 0.5-20g/m<2>. A chromated layer is then formed on at least one side (inside) of the plated steel sheet by 30-300mg/m<2> (expressed in terms of Cr) and a protective resin layer is further formed in 0.2-2.0mum thickness. A surface-treated steel sheet having superior corrosion resistance such as pitting and surface rusting resistances, superior press workability, spot weldability and suitability to coating by electrodeposition is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a highly corrosion-resistant surface-treated steel sheet suitable for use in automobiles, home appliances, and the like.

<Conventional technology and its challenges) In recent years, corrosion resistance (porous corrosion resistance) of automotive steel sheets has been improved.

The requirements for exterior rust resistance (external rust resistance) have become more sophisticated over the years, and the conventionally used "simple cold-rolled steel sheets" have been replaced by "galvanized steel sheets" and "zinc-based alloy plated steel sheets."
The trend of using .

However, in environments where there is a tendency to come into contact with corrosive substances, such as areas where rock salt is sprayed to prevent roads from freezing in winter, even if plated steel sheets such as those mentioned above are used, the amount of coating may be excessive. It was pointed out that sufficient corrosion resistance could not be obtained unless the However, when the amount of plating deposited is increased, the plating layer becomes more likely to peel off in powder form (powdering) and peel off (flaking) during press working, so there is a problem that press workability is significantly inhibited.

Therefore, in order to deal with such problems, a metal-organic multilayer coated plate has been devised, in which a plated steel plate is subjected to chromate treatment and anti-rust coating.

However, the proposal in the initial process was
The issue concerns a "rust-proof steel plate coated with a zinc-rich coating" as seen in Publication No. 30, and it is said that the level of improvement in corrosion resistance is not yet sufficient, and that the Zn powder contained in the coating peels off during press working. There were problems and the powdering resistance could not be improved to the desired level.

Therefore, a multi-layer coated steel sheet was subsequently proposed in which two layers of a chromate film and an organic composite silicate film were applied on a zinc-based plated steel sheet (Japanese Patent Laid-Open Nos. 57-108212, 58-224174, Japanese Patent Publication No. 1748/1986
No. 79, etc.). However, although these multi-layer coated steel sheets do not contain metal powder such as Zn powder in the coating film, and their powdering resistance is greatly improved, they still do not have the corrosion resistance required for current automotive steel sheets. It wasn't up to the level.

For this reason, recently, in addition to research on improving the properties of chromate films and organic films, studies have been conducted from various perspectives to further improve the properties of the zinc-plated steel sheet itself, which is the bottom layer. It has become. and,
As a result, for example, a surface-treated steel sheet (Japanese Unexamined Patent Application Publication No. 1983-1982) has been developed, in which a chromate treatment layer and a conductive paint layer are layered on a N1-Zn (7-layer) alloy plating with a Ni content of 9 to 20% by weight.
No. 210192), a surface-treated steel sheet (Special No. 210192) in which a Fe-Zn alloy plating layer with an Fe content of 10 to 40% by weight, a chromate treatment layer, and a conductive pigment layer are multilayered on a γ-phase Ni-Zn alloy plating layer. 1973-210190), Ni
A surface-treated steel sheet (JP-A-61-84381) has been proposed in which a chromate treatment layer and a polymer coating layer are multilayered on a Ni--Zn alloy plating layer having a content of 1 to 3% by weight.

Furthermore, for example, as JP-A No. 63-203778, etc., Z
By dispersing microparticles of 5 or less particles of oxides, carbides, nitrides, etc. of Si, AI, etc. in n or Zn alloy plating, we aim to improve the characteristics of the plating film itself, and also form a chromate treatment layer on the plating surface. Surface-treated steel sheets with organic coatings have also been proposed.

It is true that these technologies have made it possible to secure steel sheets that exhibit even better corrosion resistance, but the "corrosion resistance" improved by the above-mentioned technologies is primarily "porosity resistance." According to the studies conducted by the present inventors, it has become clear that each of the above-mentioned surface-treated steel sheets does not necessarily have sufficient performance as a steel sheet for automobiles in terms of "external surface rust resistance."

Here, "external rust resistance" refers to the performance that indicates the "resistance of the paint film to blistering, etc." that occurs when the paint film on the outer surface of an automobile is damaged by stone chips, scratches, etc. Needless to say.

Of course, various proposals have been made for plated copper plates that aim to improve exterior rust resistance, but even in these cases, there is still no surface-treated steel sheet that fully satisfies both exterior rust resistance and pitting resistance. The current situation was that it had not been discovered.

Therefore, an object of the present invention is to provide a highly corrosion-resistant surface-treated steel sheet that is excellent in both pitting resistance and external rust resistance and is fully satisfactory as a steel sheet for automobiles.

(Means for Solving the Problems) The present invention has been completed based on the research results of the present inventors through repeated numerous experiments in order to achieve the above object. a) Zn-Ni alloy plating layer with a Ni content of 13% or less (hereinafter, % representing the component ratio is expressed as weight %) = 1
0-50g/rd.

b) Zn-Ni alloy plating layer with Ni content of 10-40%, 0.5-20 g/rd.

C) Chromate treatment layer = C weight 30”300mg/
nr.

d) Protective resin film layer: 0.2 to 2.0 mark.

By providing multiple layers in this order, corrosion resistance including pitting resistance and external rust resistance is significantly improved.''

Note that FIG. 1 is a conceptual diagram illustrating the structure of a highly corrosion-resistant surface-treated steel sheet according to the present invention, and FIG. As an example of a steel plate, FIG. 1(b) shows a JGI plate provided with a multi-layer surface treatment according to the present invention on both sides.

Here, the above rZn-Ni alloy (Ni content is 13%
"Zn
-Ni alloy, of course, but also C, which is an element that improves corrosion resistance.
It also means those containing O in a range of 0.3% or less,
Furthermore, a Zn-Ni alloy with an rNi content of 13% or less.”
Also includes Zn metal whose Ni content is substantially zero.

Moreover, the above-mentioned "chromate treatment layer" may be formed by any of the known electrolytic chromate treatment, coating type chromate treatment, or immersion type chromate treatment.

The resin composition of the above-mentioned "protective resin film layer" is as follows:
BaCr is added to epoxy resins, polyester resins, melamine resins, vinyl resins, styrene resins, acrylic resins, polyurethane resins, phthalic acid resins, etc. alone or in modified form.
Known pigments containing anticorrosive pigments such as O, colored pigments such as Fetos, or pigments such as SiO□ may be used.

By the way, the highly corrosion-resistant surface-treated steel sheet according to the present invention is produced by electroplating the first layer (lowermost layer) and second layer by the same electrolytic treatment operation as in the production of ordinary Zn-Ni alloy plated steel sheets, and then Stable production can be achieved simply by carrying out conventional chromate treatment and coating of a protective resin film, and an outline of an example of the process is shown in FIG.

Next, the reason why the composition of the plating layer, the plating weight, etc. in the highly corrosion-resistant surface-treated steel sheet of the present invention is numerically limited as described above will be explained in detail together with the effect thereof.

<Function> (a) Bottom layer (first layer) plating layer The bottom layer (first layer) of the multilayer surface-treated steel sheet according to the present invention should have sufficient external rust resistance and hole resistance. If the Ni content is 13
Zn-Ni alloy plating containing 0.3% or less of Co or alloy plating containing 0.3% or less of Co is applied.
If the Ni content of the n-Ni alloy exceeds 13%, the external rust resistance will deteriorate, and the N generated as corrosion progresses.
The local battery action of the i residue results in accelerated corrosion of the steel plate, which is the base material.

Furthermore, if the basis weight of the first layer Zn-Ni alloy plating is less than 10 g/rrr, the effect of improving the external rust resistance and pitting resistance will not be sufficient, while if the basis weight exceeds 50 g/td, the cost will increase. It becomes impossible to secure an improvement effect commensurate with the increase. Therefore, the basis weight of the first layer Zn-Ni alloy plating was limited to 10 to 50 g/rtr.

佽) Second layer plating layer In the multilayer surface-treated steel sheet according to the present invention, in order to ensure the desired hole resistance and press sliding properties, the Ni content is 10 to 10%.
40% or less Zn-Ni alloy plating" or 0.
An alloy plating containing up to 3% Co is applied as the second layer. In this case, if the Ni content of the Zn-Ni alloy that is the second layer is less than 10%, the press sliding properties will deteriorate, and if there is no resin coating, problems such as cracking and galling of the steel plate will occur, and if the Ni content is less than 10%, sufficient It also becomes impossible to ensure puncture resistance. On the other hand, if the Ni content exceeds 40%, the external surface rust resistance becomes inferior, and as the corrosion progresses, the corrosion of the copper plate that is the base material is accelerated, and the perforation resistance deteriorates. No further improvement can be seen.

Furthermore, even when the basis weight of the second layer Zn-Ni alloy plating is 0.5 g/tri, sufficient hole resistance and press sliding properties cannot be ensured.
If it exceeds d, it will not be possible to ensure an improvement effect commensurate with the cost increase, so the basis weight of the second layer Zn-Ni alloy plating was limited to 0.5 to 20 g/nr.

(C) Chromate treatment layer The amount of chromate treatment layer formed is 30V based on the amount of Cr.
If it is less than ag/rd, the desired perforation resistance cannot be secured;
On the other hand, if the amount of chromate treatment exceeds 30,011 Ig/rd in terms of Cr content, the electrodeposition coating properties and spot weldability will deteriorate, so the amount of chromate treatment layer formed was limited to 30 to 300 mg/rd in terms of Cr content. .

(d) Protective resin film layer As mentioned above, resin coatings for the protective film include epoxy resin, polyester resin, melamine resin, vinyl resin, styrene resin, and acrylic resin.

Known products such as polyurethane resins, phthalic acid resins, etc., alone or modified, containing anticorrosive pigments such as BaCr O4, coloring pigments such as FezO3, or pigments such as 5i02, etc., are applied, but this protection If the thickness of the resin film layer is less than 0.2M, sufficient porosity resistance cannot be ensured, while if the thickness exceeds 2.0M, electrodeposition coating properties and spot weldability will deteriorate. The thickness of the protective resin film layer is 0.2 to 2. It was limited to OIrm.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

<Example> A steel plate having a thickness of 0.8 m was prepared and treated according to the process shown in FIG. 2 to obtain a multilayer surface-treated steel plate.

That is, first, a steel plate is degreased and pickled, and then Zn'' and Niz
By controlling the concentration of +, the Ni content rate of the "first plating layer" and "second plating layer" can be changed, and by controlling the amount of electricity, the basis weight can be changed. Two layers of Zn-Ni alloy electroplated steel sheets with different contents were obtained.

Next, this two-layer plated steel plate was washed with water and dried, then coated with a coated chromate treatment solution and baked and dried. When applying the chromate treatment liquid, the peripheral speed ratio and touch pressure of the pick-and-roll and applicator roll of the roll coater are changed, and the concentration of the chromate treatment liquid is also changed to determine the amount of chromate treatment layer formed (Cr adhesion amount). adjusted.

Next, in order to form a protective resin film on the surface of the steel sheet after the chromate treatment, a clear paint was applied using a roll coater method. The thickness of the protective resin film was controlled by adjusting the amount of solvent in the resin and the peripheral speed ratio of the applicator roll and pickup roll.

Then, the multilayer surface-treated steel sheets produced in this way were investigated for external rust resistance, hole resistance, press sliding properties, electrodeposition coating properties, and spot weldability.

These results are shown in Table 2, and are organized into graphs and shown in Figures 5 to 17. In addition, 5th to 17th
Among the figures, Figures 5 to 9 show the perforation resistance, Figures 10 to 12 show the external rust resistance, Figure 13 shows the electrodeposition coating properties, and Figures 14 to 1 show the resistance to surface rust.
Figure 5 compares press sliding properties, and Figures 16 and 17 compare spot weldability.
The number and alpha vent) correspond to the type code of the sample material in Table 2.

Each of the above-mentioned investigations was conducted in the following manner.

of A) Preparation of painted plate test piece.

Zinc phosphate treatment (PBL-3020 (product name of Nihon Parkerizing Co., Ltd.)) → Cationic electrodeposition coating (U-600
(Nippon Paint Co., Ltd. product name): 20m) - Middle and top coat: Melamine alkyd resin 351m each.

B) Make a cross cut as shown in Figure 3 on the painted plate test piece.

C) Outdoor exposure test (conducted for 1 year with twice weekly 5χNa (J) spraying).

D) Measure the blistering convergence of the paint film (the maximum creep width on one side from the crosscut shown in Figure 3) to evaluate the external rust resistance.

The back side and edges of the unpainted test piece after degreasing with alkali were sealed with a polyester tube, and the following cycle of "accelerated perforation corrosion test (19 cycles: 24 hr)" was performed.
The maximum erosion depth of the corroded part after 00 cycles was measured and evaluated using a point micrometer.

Salt spray (6 hr) - = dry (2 he at 50 °C) - wet (95 χ, 16 hr at 50 °C).

To investigate the sliding properties between the plated surface of the press and the tool surface, we adopted a method to determine the friction coefficient of the plated surface using the "improved Bowden test method", which is an improved version of the Bowden test, as shown in Figure 4. The sliding properties were evaluated.

ll Hoso Yuoka 1 Even if it is the inside of equipment such as the trunk lid or bonnet of a car, when it is opened, the paint finish will be seen by the public and will lead to evaluation of the product, so it is used as the inside surface. The multilayer surface of the surface-treated steel sheet of the present invention, which has a large number of coatings, also requires electrodeposition coating properties. Therefore, the finish condition of the electrodeposition coating was visually observed and ranked into 5 grades (◎...excellent, ○...good).

Evaluation was made as △...fair, ×...poor, XX...unsatisfactory).

Spot weldability was tested using a CF type electrode (made of a Cu-Cr alloy), pressure crab 200 kg-f, squeeze time: 20-9, current application time: 10-9. A continuous dotting test was conducted under the conditions of a holding time of 15~ and a welding current of 11 kA, a pitch of 20 dots per minute at 71 seconds per point, and a nugget diameter of 4 Jt (= 3.6111 m, where t is the plate thickness. So 0.
8I11ml) or less was defined as the lifespan of continuous dots.

As is clear from the results shown in Table 2 and Figures 5 to 17, the surface-treated steel sheet according to the present invention has shown excellent results in all property tests, and is superior to recent rust-proof steel sheets for automobiles. It can be seen that the comparative steel sheets that do not meet the conditions specified in the present invention do not have sufficient properties, whereas the steel sheets fully satisfy the strict requirements for.

Summary of Effects> As explained above, according to the present invention, not only corrosion resistance such as hole resistance and external rust resistance, but also press workability,
We can provide surface-treated steel sheets with excellent properties such as spot weldability and electrodeposition coating properties, and can be applied to rust-proof steel sheets for automobiles, home appliances, etc. to further improve their performance. This brings about extremely useful effects industrially.

It is.

5 to 17 are graphs that organize and compare the test results in the examples, and the numbers in the figures correspond to the type codes of the sample materials in Table 2.

Claims (1)

[Claims] On at least one side of the steel plate, a) a Zn-Ni alloy plating layer with a Ni content of 13% by weight or less: 10 to 50 g/m^2, b) a Ni content of 10 to 40% by weight Zn-Ni alloy plating layer: 0.5 to 20 g/m^2, c) Chromate treatment layer: Cr amount of 30 to 300 mg/m
^2, d) A highly corrosion-resistant surface-treated steel sheet comprising multiple layers of protective resin film layer: 0.2 to 2.0 μm in this order.