JPH03274149A - Organic coated steel plate excellent in weldability - Google Patents

Abstract

PURPOSE:To obtain an org. coated copper plate keeping good weldability by bonding a paint composition compounded with a specific amount of silica and a specific amount of a metal salt of fatty acid to the surface of a chromate film. CONSTITUTION:A chromate film of 5-500mg/m<2> as Cr is provided to the surface of a zinc or zinc alloy plated copper plate and a paint composition wherein 10-150 pts.wt. of silica and 0.5-10 pts.wt. of a metal salt of fatty acid are compounded with 100 pts.wt. of an org. resin is bonded to the surface of the chromate film in an amount of 0.3-4.0g/m<2> as a solid film. As the org. resin, an epoxy resin, an acrylic resin, an alkyd resin, a phenol resin and a urethane resin are designated and, as silica, colloidal silica or fumed silica is designated. As the metal salt of fatty acid, stearate, laurate and octylate of cobalt or manganese are designated.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides cationic electrodeposition coating properties for use in automobiles;
This invention relates to organic coated steel sheets with excellent workability, weldability, corrosion resistance, etc.

<Prior Art> In recent years, increasing the corrosion resistance of automobile steel sheets has attracted attention as a social demand, and various rust-proof steel sheets have been proposed to meet this challenge, and are gradually becoming established.

These rust-preventing steel sheets include hot-dip galvanized, hot-dip zinc alloy plated, electrogalvanized, and electrolytic zinc alloy plated steel sheets. When applied to hemming parts, the surface thereof is required to have a high degree of corrosion resistance, and the above-mentioned plated steel sheet has a problem in that its corrosion resistance is not quite sufficient.

As a steel plate having a high degree of corrosion resistance, so-called synchrometal, which is coated with a zinc-rich coating, is known.

However, even in this anti-rust coated steel sheet, there is a problem in that the film peels off in the processed parts such as press forming, and the corrosion resistance deteriorates.

Furthermore, for the purpose of these improvements, recently, a steel sheet that enables electrodeposition coating by providing an organic thin film (0.3 to 3 μm) on the plating that does not use any conductive pigment has been published, for example, in JP-A No. 62-289274. It was proposed in Japanese Patent No. 63-22637 and No. 63-35798, and is now being used by many automobile manufacturers as a thin-film type organic coated steel sheet.

<Problems to be Solved by the Invention> Recently, there has been a strong demand for improved weldability, that is, longer electrode life, in order to increase work efficiency during welding in the automobile assembly process.

The present invention has been made in view of the above-mentioned points, and an object of the present invention is to provide an organic coated steel sheet that maintains good weldability.

Measures to Solve the Problems> The deterioration mechanism of the spot weldability of organic coated steel sheets is that while the electrode of the spot welder performs continuous welding, the resin adhering to the electrode generates heat (approximately 700 to 800 degrees Celsius) on the electrode surface. ) causes thermal decomposition to produce carbon dioxide gas and water, but some of the undecomposed portion is deposited on the electrode as a residue, which ultimately prevents the formation of nuggets. That is, the deterioration of the spot weldability of the organic coated steel sheet is caused by the residual resin deposited on the electrode.

In the present invention, as a result of intensive studies based on such an electrode deterioration mechanism, we have discovered that adding a fatty acid metal salt to the resin accelerates the thermal decomposition of the resin, thereby significantly extending the electrode life. This led to the invention.

That is, the present invention has a chromate film of 5 to 500111 g/m2 in terms of Cr on the surface of a zinc or zinc-based alloy plated steel sheet, and a solid content of silica based on 100 parts by weight of an organic resin is added to the top of the chromate film. as 10-150
parts by weight and 0.5 to 10 parts by weight of fatty acid metal salts, and 0.3 to 4.0 g/m2 of the paint composition as a solid film.
An object of the present invention is to provide an organic coated steel sheet with excellent weldability, which is characterized by being coated with organically coated steel sheets.

In the present invention, it has become possible to significantly improve weldability by incorporating a fatty acid metal salt into the organic resin as described above.

The present invention will be explained in more detail below.

In the present invention, the plated steel sheets used include electrolytic galvanized steel sheets, hot-dip galvanized steel sheets, electrolytic zinc-based alloy-plated steel sheets (Zn-Ni, Zn-Fe), hot-dip zinc-based alloy-plated steel sheets (Zn-AIL, Zn-Mg %Zn-F
e), a hot-dip aluminum-plated steel sheet, or a composite plated steel sheet made of multiple layers of these may be used.

A chromate treatment is performed on the plated steel sheet in order to improve the adhesion of the organic film described below and, in turn, improve its corrosion resistance. For this chromate treatment, a coating amount of 5 to 500 mg/rn2 is appropriate in terms of metallic chromium.

If it is less than 5 mg/m2, not only the corrosion resistance will be insufficient, but also the adhesion to the organic film will be poor, and if it exceeds 500 B/m2, workability and weldability will deteriorate, which is not preferable. In addition,
When the adhesion amount is 10 to 200 mg/m2, corrosion resistance,
This is preferable because weldability becomes even better.

Such lomate treatment may be performed by any known method such as reaction type, coating type, electrolytic type, etc.

Next, specific aspects of forming an organic polymer resin film on the upper layer of the chromate film thus formed will be described below.

First, organic resins used in the present invention include epoxy resins, acrylic resins, alkyd resins, phenolic resins, urethane resins, etc., and these may be used alone or in combination. . Furthermore, if necessary, an amino curing agent such as melamine, urea, or benzoguanamine, or a curing agent such as blocked isocyanate may be used.

Silica is added to such a resin to improve its anticorrosion effect. Silica is blended at a solid content of 10 to 150 parts by weight per 100 parts by weight of the resin. If the silica content is less than 10 parts by weight, no improvement in corrosion resistance can be expected, and 1503! If the amount of silica is larger than the amount of silica, the adhesion after two coats will decrease and the processability will decrease.

Examples of the silica to be used include colloidal silica and humid silica, and any of them may be used.

A fatty acid metal salt is added to the resin composition blended as described above to promote thermal decomposition.

The fatty acid metal salts to be added include cobalt, manganese,
copper, iron, vanadium, nickel, titanium, calcium,
Typical examples include stearates, laurates, and octylates of silver, zinc, aluminum, magnesium, cadmium, barium, strontium, lead, etc.

The amount of the metal salt to be added is suitably in the range of 0.5 to 10 parts by weight per ioo parts by weight of the organic resin, but in particular 1 to 5 parts by weight.
A range of % is preferred.

If the amount of the fatty acid metal salt added is less than 0.5 parts by weight, the effect of accelerating thermal decomposition of the resin will be small, and no improvement in weldability will be observed. 10 again! If the amount exceeds i part, the thermal decomposition effect becomes too large and the effect of improving weldability is extremely large.
Thermal decomposition progresses during baking after topcoating, which is undesirable as it reduces adhesion after coating.

In addition, the plate temperature during heat treatment for drying and curing is 1
00 to 200°C is enough, but if you select an appropriate resin and make it sufficiently harden even at temperatures below 150°C, it will have a remarkable effect without impairing the BH properties of the steel plate when applied to BH-prone steel plates. can get.

The dry coating thickness of the resin (paint) composition, that is, the amount of solid film deposited, must be 0.3 to 4.0 g/m2, and preferably 0.5 to 2.0 g/m'.

If it is less than 0.3 g/m2, sufficient corrosion resistance cannot be obtained, and if it exceeds 4.0 g/cm, workability deteriorates, which is not preferable.

<Example> Next, the present invention will be specifically explained based on examples.

(Example 1) The following characteristics were obtained for various organic coated steel sheets obtained by performing chromate treatment and resin composition treatment on the zinc or zinc-based alloy coated steel sheets shown in Table 1 under the various conditions shown in Table 1. I conducted a test. The results are also shown in Table 1.

(Corrosion resistance: CCT) Each test steel plate was subjected to the following conditions at 35℃ and 5wt% NaC11 for 4 hours! Fog ↓ Drying at 60°C for 2 hours ↓ A cycle corrosion test was conducted in which one cycle was standing at 50°C and 95% RH (humidity) for 2 hours, and the occurrence of red rust after 100 cycles was evaluated.

The evaluation criteria were as follows.

◎: No red rust ○: Less than 10% red rust △: 10% to 50% red rust ×: More than 50% red rust (workability) Each test steel plate (90mmφ) was cut into a diameter of 50mmφ and a depth of 25m.
m cylindrical drawing process (BHF=1 ton) L/, the processed part was separated from the Cellotape strip 1], and the amount of film peeled off (mg/circumference) was measured.

The evaluation criteria were as follows.

◎, Amount of film removed: less than 1 mg ○, Amount of film removed: 1 to less than 2 mg △: Amount of film removed: 2 to 5 mg ×: Weight of film removed: More than 5 mg (film adhesion after 2 coats) Test sample after electrodeposition The material was further spray-painted with 30 μm of Amilac No. 002 manufactured by Kansai Paint Co., Ltd., for two coats. In the 2-hour adhesion test, after painting, each sample material was
After being immersed in warm water (pure water) at a temperature of 0.degree. C. for 240 hours, the sample was taken out, and within 30 minutes, 100 squares were cut at 1 mm intervals, and an adhesive tape was attached to and peeled off from the squares.

The evaluation criteria were as follows.

◎No peeling O Peeling @ 1/100 or less △ Peeling 1/100 to 10/100 × Peeling @ 11/100 or more (weldability) Electrode R (40R), pressure 170 Kg, welding time 10
Under the conditions (1), two of each test steel plate was stacked alternately, and the number of continuous welding points was determined.

Judgment ◎: Continuous dots 2000 points or more O: Continuous dots 1000 to 200 points Thermal decomposition of the material is accelerated, and electrode deterioration during spot welding can be effectively prevented.
Spot weldability is greatly improved.

Claims (1)

[Claims] (1) Cr on the surface of zinc or zinc-based alloy coated steel sheet
It has a chromate film of 5 to 500 mg/m^2 in terms of conversion, and on the top of the chromate film, 10 to 150 parts by weight of silica as a solid content and 0.5 to 10 parts of fatty acid metal salt are added to 100 parts by weight of organic resin. An organic coated steel sheet having excellent weldability, characterized in that it is formed by adhering a coating composition containing parts by weight as a solid film at a rate of 0.3 to 4.0 g/m^2.