JPH028397A - Rustproof steel sheet having superior suitability to coating by electrodeposition - Google Patents

Abstract

PURPOSE:To provide a rustproof steel sheet having superior corrosion resistance and suitability to coating by electrodeposition by forming an Al-Cr alloy layer contg. a specified wt. percentage of Cr on a steel sheet and further forming a coating film based on urethane resin and silicon dioxide. CONSTITUTION:An Al-Cr alloy plating layer contg. 5-30wt.% Cr is formed on a steel sheet and a coating film based on urethane resin and silicon dioxide is further formed on the plating layer optionally with a chromate layer in-between. In the coating film, the urethane resin and silicon dioxide are present as a compound substance formed by coupling with a silane coupling agent. A rustproof steel sheet having superior corrosion resistance and suitability to coating by electrodeposition can be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a rust-preventing steel sheet with excellent electrodeposition coating properties, which is used for automobile bodies, household electric appliances, building materials, and the like.

[Conventional technology] Various methods are known for rust prevention technology for steel plates.
Among the most common rust-proof steel sheets, Zn, Zn alloy plated steel sheets (hereinafter referred to as Zn-based plated steel sheets), AI,
A1 alloy-plated steel sheets (hereinafter referred to as A1-plated steel sheets) are used to prevent rust by utilizing the sacrificial anti-corrosion ability of the plating metal against the base steel sheet, and they can be used depending on the purpose of use by taking advantage of their respective characteristics. It is.

In other words, for Zn-based plated steel sheets, we take advantage of the property that phosphate treatment on the surface of the plating layer provides excellent paintability. It is often used as a board.

On the other hand, Al-based plated steel sheets have poor paintability and have not been given the opportunity to fully utilize their advantage of excellent corrosion resistance.

This is because AI plating has very poor phosphate treatment properties (phosphate film formation properties), which are essential as a pre-painting treatment. Therefore, when painting AI-based plated steel sheets, chromate treatment is performed as a pre-painting treatment. However, in the chromate treatment, it is necessary to use a solution containing harmful ROM ions, which poses the problem of being unfavorable in terms of the manufacturing environment, and does not necessarily provide sufficient paintability. Furthermore, when applying electrodeposition coating to Al-based steel sheets, as the voltage is increased, coating unevenness called craters occurs, which naturally limits the coating voltage and causes a decrease in production efficiency. There are also problems, and when manufacturing products using Al-based galvanized steel sheets, there are many problems due to paintability.

Therefore, as a method for improving the paintability of A1-based plated steel sheets, a zinc-rich paint is applied to the A1-plated surface. However, since the coating film made with zinc-rich paint contains a large amount of Zn or Zn alloy powder, when forming processes such as press working are performed,
There are problems such as the resin layer peeling off into powder form, which causes processing scratches. Therefore, Zn or Z in the film
It may be possible to keep the content of n powder low, which would improve processability, but the electrical conductivity of the film would deteriorate, resulting in poor electrodeposition coating properties.

[Problems to be Solved by the Invention] As described above, Al-based plated steel sheets have poor paintability, so although they have excellent corrosion resistance and heat resistance, they are not used in the non-painted field, such as automobile mufflers. It is mostly used in areas that do not require painting, such as parts, inner panels of general electrical appliances, and building materials, and its range of applications is more limited than that of Zn-based plated steel sheets.

In view of the above circumstances, in the present invention, a rust-proof steel plate having high corrosion resistance and excellent electrodeposition coating properties and coating adhesion was investigated.

[Means for Solving the Problems] The present invention that has solved the above problems is that an AlCr alloy plating layer containing 5 to 30% by weight of Cr is formed on a steel plate, with or without a chromate treatment layer. The gist is that a film containing urethane resin and silicon dioxide as main components is formed on the surface, and in particular, urethane resin and silicon dioxide form a composite material through a silane coupling agent. The coating film adhesion is even better.

[Function] The following will explain the process of the research that led to the completion of the present invention.

For some time now, the present inventors have developed A by adding various alloying elements to At for the purpose of improving the corrosion resistance of the inner body of the A1 plating layer.
1- An alloy plating layer was formed on a cold-rolled steel sheet, and it was investigated how the corrosion resistance of the At-based plated steel sheet would change.

The corrosion test for evaluating corrosion resistance consists of one cycle (1 cycle/30 minutes) from salt water spray to drying as shown below, and salt water immersion (5% NaCl solution, 35°C x 7.5 minutes) → Wet (R895% or higher, 40℃ x 15 minutes) → Dry (60℃
x 7.5 minutes) After performing the above-mentioned cyclic corrosion test 1000 times, the depth of pit holes caused by corrosion in the cold-rolled steel plate was investigated.

As a result, it was found that the Al-Cr alloy plated steel sheet using Cr as the alloying element had the smallest perforation depth and excellent corrosion resistance.

Therefore, we next investigated how the corrosion resistance (hole depth in the above cycle test) changes by variously changing the Cr content in the Al--Cr alloy plating layer. The results are shown in FIG. As is clear from Figure 1, when the Cr content in the coating layer is 5% or more, the perforation depth of the base steel sheet is significantly improved, and when it exceeds 30%, the effect of reducing the perforation depth decreases. At the beginning, when it reaches 50%, the effect becomes almost unrecognizable. That is, the amount of Cr in the plating layer is not limited to a large amount, but only within a specified range of 5 to 30% will exhibit excellent anticorrosion ability.

Based on the results of these studies, we began investigating ways to improve paintability. That is, the Cr content in the Al--Cr alloy plating layer was set to 5 to 30%, and the phosphate treatment properties used as pre-painting treatment were investigated. As mentioned above, Al-based plating has poor phosphate treatment properties, but Al-C
When I investigated r-alloy plating, No. 2 in Figure 2 was found.
As shown in 1 to 3, results were obtained that confirmed the conventional trends, and it was found that almost no phosphate film adhered to the surface of the plating layer [For comparison, Zn-based (two-layer) plating Steel plates and cold-rolled steel plates treated in the same manner are also listed].

Next, cationic electrodeposition coating was applied to various steel plates that had been subjected to the phosphate treatment, and the electrodeposition coating voltage and crater in the coating film were determined.
We investigated the relationship between occurrence conditions. The results are also shown in FIG.

As is clear from FIG. 2, craters began to occur at a coating voltage of 190 to 210 V, and the appearance after coating was significantly impaired. This is the same result as for the pure AI-plated steel sheet, and the A1-Cr alloy-plated steel sheet has poor phosphate treatment and electrodeposition coating properties as well as the conventional A1-plated steel sheet, resulting in poor coating adhesion and poor appearance. Therefore, it was found that it was not suitable as a plated steel sheet for painting.

Therefore, we conducted further intensive research to improve the electrodeposition coating properties of Al-Cr alloy plated steel sheets, and found that a film made of a mixture or composite material mainly composed of urethane resin and silicon dioxide was formed on the At-Cr plating layer. By forming a layer or a film layer in which urethane resin and silicon dioxide are a composite material through a silane coupling agent, craters are less likely to occur during electrodeposition coating and the electrodeposition coating properties are improved. That's what I found out. The inventors of Naoki and others conducted similar studies on epoxy resins other than urethane resins, acrylic resins, phenol resins, and various other synthetic resins, as well as silicon dioxide. Electrodeposition coating properties are markedly improved only when a film layer is formed of a mixture or composite material that
In addition, it was found that excellent effects such as no peeling of the coating film were exhibited even during molding such as press working.

Examples of the urethane resin in a film containing urethane resin and silicon dioxide as main components include, but are not limited to, reaction products of an organic polyisocyanate, a polyhydric hydroxy compound, and a polyvalent amino compound. Depending on the application, a small amount of one or more of epoxy resins, acrylic resins, phenolic resins, fluorine resins, vinyl resins, ester resins, etc. may be added to an extent that does not impair paintability and moldability. Also good. In addition, in order to improve the corrosion resistance of the coating, a small amount of Zn, Zn-Fe alloy, Zn-Ni alloy, Zn-Al
alloy, Zn-Mg alloy.

Zn-based powder such as Zn-Al-Mg alloy or antirust pigment may be added. However, the particle size of the Zn-based powder or anticorrosion pigment in the coating is preferably several μm or less, preferably 1 μm or less, from the viewpoint of aesthetics.

A film mainly composed of urethane resin and silicon dioxide is an essential requirement for giving At-Cr plated steel sheets excellent electrodeposition coating properties, but as the film thickness increases, the conductivity decreases and the electrodeposition becomes more difficult. Paintability deteriorates. However, since the coating layer also has a protective effect in a corrosive environment for the Al--Cr alloy plating layer that is the base of the coating, it is necessary to have a certain thickness from the viewpoint of corrosion resistance. Therefore, as a result of examining the thickness of the film layer from the two viewpoints of electrodeposition coating properties and corrosion resistance, the film thickness that satisfies both is 0.5 to 3°.
It was μm. That is, if the film thickness is less than 0.5 μm, Al-C
rThe alloy plating layer could not be sufficiently protected, and the thickness was 3.0μ.
If it exceeds m, the conductivity will decrease and the electrodeposition coating properties will decrease.

Further, a chromate layer may be formed between the Al-Cr alloy plating layer and the coating layer containing urethane resin and silicon dioxide as main components. The method for forming the chromate layer is not particularly limited, but it is possible to adopt the coating type chromate film forming method 1, the reactive chromate film forming method, and the electrolytic chromate film forming method, and of course, a combination of these is also possible. For example, Al-Cr A chromate film is formed on the alloy abutment layer by an electrolytic chromate method, and then subjected to a coating-type chromate treatment, resulting in a steel sheet with better corrosion resistance and film adhesion.

By the way, the method for forming the Al--Cr alloy plating layer on the base steel plate is not limited to hot-dip plating, electroplating, chemical plating, vapor deposition plating, etc., but vapor deposition plating is most preferred. That is, when carrying out the vapor deposition plating method, A
The I base metal layer and the Cr base metal layer are housed in separate crucibles under reduced pressure, and heated by electron beam (EB) or high frequency.

Then, the amount of evaporation is controlled by controlling the output of the heating source for each crucible, and an Al--Cr alloy plating layer with a predetermined composition and thickness is formed on the steel plate passing over the crucible. At this time, since At and Cr forming the plating layer have a high affinity with oxygen and are easily oxidized, the degree of vacuum in the vacuum container is 1O-2Torr or less, preferably 1O-3Torr.
It is preferable that it is below rr. In addition to the usual vacuum evaporation method (PVD method), a so-called ion blating method may be used in which Al and Cr vapors are ionized by high frequency waves and then deposited on a steel plate to which a bias voltage is applied. The Al-Cr alloy plating layer obtained by the ion blating method has finer crystal grains, resulting in a dense plating layer with fewer pinholes, so it has even better corrosion resistance.

The rust-proof steel sheet obtained in the above manner has excellent corrosion resistance and heat resistance, as well as excellent electrodeposition coating properties and coating adhesion, so that the coating film does not peel off during processing. Furthermore, since it can be coated with various types of coatings, it can be used as the outer panels of automobiles and household electrical appliances, expanding the range of applications.

[Example] Two crucibles were prepared in a vacuum chamber, A1 and Cr metals were charged into each crucible, and two electron beams were irradiated onto each metal to form At and Cr metals. The gold was heated and evaporated. On the other hand, a cold-rolled steel plate of 0.7 mm thickness whose surface had been activated in vacuum in advance was prepared on the crucible, and while controlling the output of the two electron beams, a predetermined film thickness and a predetermined Cr content were applied to the cold-rolled steel plate. Al-Cr alloy plating is applied to
A plated steel plate was obtained.

After the obtained Al-Cr alloy plated steel sheet was degreased with alkaline and washed with water, the Cr content in the chromate film was 50 mg.
Chromate coating treatment was carried out so that the thickness was /m2. After the chromate coating treatment, a mixture of water-soluble urethane resin emulsion colloidal silica distilled water was used as a coating solution, and it was coated with a bar coater.
Baking was carried out at 0° C. for 1 minute to form a film layer such that s i 02 /urethane resin: 0.4 film thickness: 1.2 to 2.0 μm to prepare a rust-proof steel plate.

The obtained anti-rust steel plate was cut into 70mm' x 150+nm', the untreated parts (the back side and the cut end face) were sealed with tape, and the following cycle from salt water spraying to wetting was carried out (1 cycle/8 hr). A cyclic corrosion test was conducted for 200 cycles.

Salt water spray (5% NaCl solution, 35°C x 4 hr.

→ Dry (60tx2hr) - + Wet (50tx2hr,
(RH 95% or higher) → Next, this cycle-corroded steel plate was derusted in an ammonium citrate solution, and the depth of holes due to corrosion in the base steel plate was determined using a dial gauge. The results are shown in FIG. 3 together with comparative examples. In FIG. 3, Nos. 5 to 8 have plating layer compositions outside the range of the present invention, and No. 9 has a different coating layer composition.

As is clear from Figure 3, 5 to 15 Cr of No. 1 to 4
It can be seen that the perforation depth of the base steel plate in the corrosion test of the Al-Cr alloy plated steel plate containing 5 in.

On the other hand, No. 5 (pure A1 plated steel sheet that does not contain Cr),
No. 6 (Cr content is outside the range of the present invention),
No. 7.8 (Zn-based plated steel sheet) and No.
, No. 9 (those with different coating compositions even though the plating layer composition is within the range regulated by the present invention) have poor perforation depth and poor corrosion resistance.

Next, in order to investigate the electrodeposition coating property of the rust-proof steel plate, a rust-proof steel plate (70
mmwx 150 mm'), the back surface of which was not subjected to rust prevention treatment was sealed with magnetic rubber, and cationic electrodeposition coating was performed under the conditions shown below.

Electric coating conditions Paint: Cation type paint Liquid temperature: 28℃ Current application method: 30 seconds rise control method Current application time: 2.5 minutes Voltage: 180V to 320V every 20V Baking conditions after electrodeposition coating: 165℃ x 20 min The electrodeposition coating properties were evaluated by visually observing the occurrence of craters on the coating surface. The results are shown in Figure 4.

As is clear from FIG. 4, the electrodeposition coating properties of the examples of the present invention were extremely superior to those of the comparative examples, and no craters were observed even at an electrodeposition coating voltage of 320 .mu.m.

As described above, it can be seen that the rust-proof steel sheet of the present invention has high corrosion resistance and excellent electrodeposition coating properties.

[Effects of the Invention] Since the present invention is configured as described above, the rust-proof steel sheet of the present invention has excellent corrosion resistance and also excellent electrodeposition coating properties.

[Brief explanation of the drawing]

Figure 1 shows At-Cr in Al-Cr alloy plated steel sheet.
Figure 2 is a diagram showing the relationship between the Cr content in the alloy plating layer and the perforation depth of the base steel plate, and Figure 2 is a diagram showing the relationship between the electrodeposition coating voltage and the crater generation state on the coating film surface for various steel plates. Figure 3 is a diagram showing the relationship between various rust-preventing steel plates and maximum perforation depth in a cycle corrosion test, and Figure 4 is a diagram showing the relationship between electrodeposition coating voltage and crater generation state on the coating surface for various steel plates. It is.

Claims (2)

[Claims] (1) Al-C containing 5 to 30% by weight of Cr on a steel plate
A rust-proof steel sheet with excellent electrodeposition coating properties, characterized in that an r-alloy plating layer is formed, and a film mainly composed of urethane resin and silicon dioxide is formed with or without a chromate treatment layer. (2) Claim (1) in which the urethane resin and silicon dioxide form a composite substance via a silane coupling agent.
Rust-proof steel plate with excellent electrodeposition coating properties.