JPH0852420A - Black precoated steel sheet with excellent pressure mark resistance - Google Patents

Abstract

(57) [Abstract] [Purpose] Achieve both guard film-free pre-coated steel sheets and pressure-mark resistant properties required for home appliances such as audio products and cooking products. [Structure] The front side of the steel sheet subjected to chemical conversion treatment is coated with a primer, and a melamine-curable polymer polyester resin having a glass transition point of the coating film of 40 to 70 ° C. and an average molecular weight of 15,000 to 50,000 is added to the upper layer as a solid content weight. 40 to 40
A black coating film containing 90% of aggregate silica having a primary average particle size of 0.5 to 10 μm in a solid content weight ratio of 1 to 15% and polyethylene powder as a lubricant of 1 to 5% in a solid content weight ratio is formed. A black precoated steel sheet having excellent pressure mark resistance in which the surface glossiness of the back side 1C1B coating film is 50 to 100% of the surface glossiness of the front side coating film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a black precoated steel sheet obtained by making a guard film free of a precoated steel sheet required for home electric appliances such as audio products and cooking products.

[0002]

2. Description of the Related Art In recent years, there is a great need for cost reduction of coated steel sheets in the fields of home appliances and automobiles, and as precoating of post coats progresses further, market demand for high-performance pre-coated steel sheets with different functions is increasing. .

Particularly for pre-coated steel sheets (hereinafter simply referred to as "pre-coat") having a high designing property, which are often used as outer panels for audio products and cooking products, the appearance of the top coating film that occurs in the product forming process at the customer's side is not easy to handle. As a preventive measure, on the side that manufactures the precoat, a guard film having a film thickness of several tens to 100 μm is separately attached to the top coated surface having high designability, and it is provided on the market. For this reason, it is quite difficult to provide an inexpensive precoat on the market, and in addition to this, there is a need for precoats to be free of guard film due to the complexity of work such as peeling off the film and post-treatment of the peeled film at the final product stage in the customer. It is increasing more and more.

[0004]

On the other hand, when a guard film-free precoat is manufactured on a highly productive line, it may be temporarily placed on the floor and stored as a coil product until it becomes a cut plate product. In many cases, indentation flaws (pressure marks) are generated on the coated surface due to the weight of the coil during this period, and this improvement countermeasure becomes a problem including the floor mounting method of the coil product.

In the guard film-free, a considerable amount of line or strip-shaped indentation marks are formed in the plate width direction on the coating film in contact with the floor due to the unit weight of the coil, which are generated at a constant pitch in the longitudinal direction of the coil to improve productivity. Since it causes a large decrease, the greatest problem to be solved is a countermeasure against pressure-induced scratches on the coating film (hereinafter, simply referred to as PM resistance). A similar problem regarding PM resistance is also found in a cut plate product that is hooped on a skid and placed on the floor, and there is a strong demand for improvement. In this sense, there is almost no conventional guard film-free precoat having PM resistance or a manufacturing method thereof.

[0006]

According to the present invention, the surface side of a steel sheet subjected to chemical conversion treatment is coated with a primer, and the glass transition point of the coating film is 40 to 70 ° C and the average molecular weight is 150 on the upper layer.
Melting curable high molecular weight polyester resin of 0 to 50000 is 40 to 90% in solid content weight ratio, and aggregate silica having a primary average particle diameter of 0.5 to 10 μm is 1 to 1 in solid content weight ratio.
A black coating film containing 5% of polyethylene powder as a lubricant in a solid content weight ratio of 1 to 5% is formed, and the back side 1C1B
The surface gloss of the coating film is 50 to 10 of the surface gloss of the front coating film.
It is a black precoated steel sheet excellent in pressure mark resistance, which is characterized by 0%. The chemical conversion treatment may be any of the known chromate treatment or phosphate treatment. Further, the precoat coating film may be a known coating system, and the same resin system as the top black coating film is preferable. Also, the treatment or coating method of these coating bases may be known.

[0007]

The present invention is intended to supply a guard film-free black precoat with stable PM resistance to the market at a low cost, and for that purpose, from production to final products for consumers, It must be a precoat that does not cause flaws in processing and handling, including PM resistance. To impart PM resistance, the pressure resistance must be increased, and the coating film itself must be appropriately hard and stable. In order to impart PM resistance, it is necessary to control the surface gloss of the back surface coating film, and other coating film performance should not be impaired. Thus, in order to secure stable PM resistance, it is basically necessary to use a hard coating film and control the gloss of the back surface coating film.

Therefore, the present invention is based on the following
Top black coating film with high design, that is, PM resistance on the front side is stably imparted.

Hardening of the top coating film for imparting PM resistance is based on increasing the glass transition point (Tg) of the coating film and optimizing the molecular weight and blending amount of the binder resin.

In order to stably impart the PM resistance of the top coating film, the surface gloss of the back coating film is optimized.

In order to improve the scratch resistance of the top coating film, the blending of the lubricant is optimized.

The coating composition of the present invention will be described below.

According to the present invention, a steel sheet having a front surface and a back surface appropriately subjected to chemical conversion treatment is used as a substrate, a suitable primer coating film is formed as a lower layer on the front side, and a PM coating is added as a top coating film on the upper layer to form a 2C2B coating film. And the back side has a 1C1B service coating with a specified surface gloss. Alternatively, 2C2B coating may be applied to the front and back sides according to the same coating specifications. The top coating has a Tg of 40 to 70 ° C as a coating.
The average molecular weight of the melamine resin is 15,000 to 50,000, and a high-molecular polyester resin of a type for coating film hardening is contained in a solid content weight ratio of 40 to 90%, and a bone having a primary average particle diameter of 0.5 to 10 μm. The silica content is 1 to 15% by weight of solid content, and the polyethylene powder is used as a lubricant in 1 to 5% by weight of solid content. In addition, the surface glossiness of the back coating is 50 to 50 times that of the top coating.
100%

The present invention is based on the fact that the coating strength is increased by increasing the Tg of the coating, and the synergistic effect of making the surface glossiness of the back coating equal to or less than that of the top coating makes the top coating Stable PM resistance is obtained.

The steel plate used in the present invention may be either a bare steel plate or a zinc-based plated steel plate. The zinc-based plated steel sheet may be any one obtained by a known plating method. For example, in the electroplating system, Zn plating, Z containing an alloying element of at least one of Ni, Cr, and Fe.
An n-based alloy plated steel sheet can be used. In the electrodispersive plating system, Zn-Ni and Zn-Fe are used as bases for S
A Zn-based dispersion alloy plated steel sheet in which a metal oxide such as iO ₂ , TiO ₂ , ZrO ₂ , or BaCrO ₄ is uniformly dispersed and precipitated can be used. Furthermore, in the hot dip system,
A hot dip galvanized steel sheet, a Zn-Al alloy plated steel sheet, and a galvanized steel sheet obtained by alloying them can be used.

The requirements for each coating film of the steel sheet of the present invention will be described below.

(1) Top coating film: Molecular weight of high molecular polyester resin as a binder: In order to impart PM resistance to the top coating film, it is premised that the coating film is appropriately hardened so that it can withstand rolling. Therefore, for that purpose, it is necessary to specify the molecular weight range of the polymer polyester resin as the binder. Molecular weight is 15,000
If the amount is less than the above, the cross-linking density is too high, so that work cracks are likely to occur in the coating film, and if it is 50,000 or more, the elongation workability of the coating film is deteriorated and a coating film defect such as a wrinkled skin as a product appearance occurs. Therefore, the molecular weight is set to 15,000 to 50,000 and 2
0000-30000 is preferable.

Blending amount of high molecular polyester resin: The blending amount of binder resin is 40 to 90% as a solid content weight ratio.
And If it is less than 40%, the strength of the coating film is lowered, the scratch resistance and workability of the coating film are deteriorated, and it is difficult to obtain a uniform coating appearance. On the other hand, if it exceeds 90%, it becomes difficult to control the desired uniform coating film appearance such as coloring and glossiness, and the cost also increases. Therefore, the blending amount of the resin is 40 to 90%, preferably 50 to 80%.

Coating film Tg: The coating film Tg is a factor to be controlled especially in order to obtain the PM resistance of the top coating film more stably. If Tg is less than 40 ° C, sufficient PM resistance cannot be obtained, and if it exceeds 70 ° C, the PM resistance of the coating film is saturated, but on the contrary, the elongation workability of the coating film becomes poor and cracking occurs. It becomes easy and costly. Therefore, the coating film Tg is 4
The temperature is set to 0 to 70 ° C, preferably 50 to 65 ° C.

Particle Size and Amount of Aggregate Silica: Aggregate silica is incorporated to impart scratch resistance to the coating film. Hard silica, which has relatively high crystallinity, is preferable, but sufficient consideration must be given to its particle size and blending amount.

If the primary average particle size is less than 0.5 μm, the secondary agglomeration of silica in the coating composition impairs the uniform dispersibility in the coating film, and the scratch resistance of the coating film deteriorates, and the appearance is deteriorated. Lacks uniformity. On the other hand, if it exceeds 10 μm, the workability of continuous drilling (punching resistance) in the steel sheet in machining such as punching is lacking, die wear becomes large, and burrs or flaws frequently occur on the steel sheet, resulting in high cost. Therefore, the particle size is 0.5-10μ
m, preferably 1 to 3 μm.

If the blending amount is less than 1% by weight of the solid content, it is difficult to obtain stable scratch resistance of the coating film, and if it exceeds 15%, the elongation workability of the coating film decreases and the punching resistance is sufficient. Not. Therefore, the blending amount is 1 to 15%, preferably 5 to
10%.

Blending amount of polyethylene lubricant: The polyethylene lubricant is used for imparting scratch resistance of the coating film and processability in press working. If the solid content weight ratio is less than 1%, it is difficult to obtain the above-mentioned coating performance sufficiently stably, and if it exceeds 5%, it is difficult to obtain a uniform coating appearance due to the water cooling pattern during baking of the coating film. Therefore, the content of the lubricant is 1 to 5%,
It is preferably set to 1.5 to 3%.

(2) Surface gloss of back coating: The surface gloss of the back coating is a control factor for more stably obtaining PM resistance of the top coating. If the glossiness of the back surface coating is less than 50% of the glossiness of the top coating, the PM resistance of the top coating decreases, and if it exceeds 100%, the PM resistance of the top coating similarly.
Sex decreases. Therefore, in order to stably obtain the PM resistance of the top coating film, the back surface glossiness should be 50 to 50% of that of the top coating film.
It is set to 100%, preferably 60 to 90%. The gloss of the back surface coating film can be controlled by a known method using powdered silica or the like.

[0025]

EXAMPLES Examples of the present invention will be described in detail below.

A total chromium deposition amount of 50 mg was applied to the both sides of a zinc-based plated steel sheet having a plate thickness of 0.7 mm and a plate width of 914 mm at a line speed of 70 m / min as a coating base treatment by the conventional technique.
After applying a commercially available chromate treatment of / m ² , it was dried and immediately put into the coating process.

On the highly painted top coated surface (2C2B), an organic solvent-based high molecular polyester resin coating was used for both primer coating and top coating, and the coating was baked under predetermined conditions. Primer coating is 5μ with Nippon Paint's Flex Coat 600 EU primer as a solid film.
m, and a black coating film as shown in Tables 1 to 5 as a top coating film was applied as a solid coating film thereon by curtain to a thickness of 19 μm, followed by baking under standard conditions and water cooling drying.
The surface glossiness of the top coating film at this time was controlled to 30%, which has high market needs. On the other hand, for coating the back surface of the steel sheet, NP02 paint made by Nippon Paint Co., Ltd., whose gloss of the coating film was adjusted, was applied by a known roll coating method so that the thickness of the coating film became 1.5 μm as a solid film, and the predetermined plate was applied. It was baked at a temperature, washed with water and dried.

As described below, it was found that the black precoat produced in this manner can sufficiently obtain PM resistance required for a top coating film having a high design property.

(1) Binder resin: No. 1 including the examples and comparative examples of the binder resin which imparts PM resistance to the coating film. 1 to No. 22.

Examples No. 1 and No. 1 in which the resin molecular weight was changed. 1
~ No. 5 and a comparative example No. 6-No. 7 shows. In addition, in Examples in which the coating film Tg was changed, 8 to N
o. No. 13 is a comparative example. 14-No. Shown in 15.
Further, in the examples in which the compounding amount was changed, No. 16-No.
No. 20 shows a comparative example. 21-No. 22.

As is apparent from these, in order to impart PM resistance to the coating without impairing other properties of the coating, it is essential to increase the Tg of the coating, and in addition, the appearance of the coating is uniform. In order to obtain stable properties and processability, it is necessary to limit the resin molecular weight and blending amount.

(2) Aggregate silica in the coating film: Nos. 2 and No.
23-No. No. 28 is a comparative example. 29-No. 30
Shown in In addition, in Examples in which the compounding amount was changed, No. 2 and No. 31-No. 36, comparative examples N37-No. Three
8 shows.

As is clear from these, in order to impart PM resistance to the coating film without lowering the punching resistance and other coating performance, it is necessary to control the particle size and blending amount of the silica of the aggregate. is necessary.

(3) Polyethylene lubricant: Nos. In the examples in which the blending amount of the polyethylene lubricant was changed. 2 and No. 39-No. 45, comparative examples N46-No. Four
7 shows.

As is clear from these, in order to impart lubricity to the coating film and obtain stable press workability, it is necessary to properly mix the polyethylene lubricant. Especially, if the upper limit is exceeded, the appearance of the coating film will be water-cooled after baking. Occurs and the workability also deteriorates.

(4) Regarding the glossiness of the back surface coating film: Nos. 2 and No. Four
8 to No. No. 52 in the comparative example. 53-No. 54.

As is apparent from these, if the back surface glossiness is out of the control range, it becomes difficult to stably maintain the PM resistance of the top coating film having high commercial value in the high productivity line, and the cost also increases.

(5) Application example of plating system for base steel sheet: Example N for plating system of base steel sheet used in the present invention
o. 2 and No. 55-No. 64. As is apparent from these examples, the present invention does not hinder the coating film function even if the plating system of the base steel sheet is different.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

[0042]

[Table 4]

[0043]

[Table 5]

(Note) * 1. Plating system: EZ: Electrogalvanizing. ZN: Electric Zn-Ni alloy plating (Ni; 11.5
%). EC: Electric Zn-Cr-Ni alloy plating (Cr; 10
%, Ni; 2%). EF: Electric Zn-Fe alloy plating (Fe; 15%). ZNS: electrical _{Zn-Ni-SiO 2 (Ni} : 12%, S
iO _2; 3%). ZNT: Electrical _{Zn-Ni-TiO 2 (Ni} : 12%, T
iO _2; 3%). ZNB: electrical _{Zn-Ni-BaSO 4 (Ni} : 12%,
BaSO ₄ ; 3%). ZNS: Electrical _{Zn-Fe-SiO 2 (Fe} ; 10%, S
iO _2; 3%). GZ: hot-dip Zn plating. GA: hot-dip Zn-Al plating (Al; 5%). GF: Alloyed hot-dip Zn plating (Fe; 8-11%).

(Note) * 2. Top coating film properties: Polymer polyester resin is used by curing melamine resin. Tg is measured by TMA. All the blending amounts are expressed by weight ratio with respect to coating film solid content.

(Note) * 3. Glossiness of the back side coating film: It means the glossiness ratio of the top side coating film due to specular reflection of 60 degrees.

(Note) * 4. PM resistance evaluation: a load of 1.6 kg / on a test piece in which a back coating film is overlaid on the top coating film surface.
Place the cm ^2, RH95%, in a humidified atmosphere of 40 ° C. 16
After standing still for 8 hours, the test piece was taken out and the indentation state of the coated surface was visually evaluated. ⊚: No change in initial appearance. ○: Very slight indentation transfer when transparent. Δ: A slight indentation transfer when viewed from the front. X: Clear impression transfer.

(Note) * 5. Evaluation of perforation resistance: Punching perforator: Continuous perforation with electric crank press. Mold: Uses a mold with a hardened surface. Wall thickness 1mm x
Uses a set of four 4.5 mm wide blades. Evaluation: For the 300,000th punching hole, the state of burrs around the hole was visually evaluated. ◎: No burr. ◯: Slightly flawed but no burr. Δ: Several burrs are generated. X: Frequent burr.

(Note) * 6. Workability: Square cylinder press, 50W
× 50L × 50H, the degree of occurrence of scratches on the coating film in contact with the die was visually evaluated. ⊚: No coating film flaw. ◯: Slight occurrence. Δ: Partial occurrence. × Full surface occurrence.

(Note) * 7. Appearance of coating film (visual): A: Appearance is uniform. ◯: An extremely slightly uneven pattern through the watermark. Δ: Partially uneven. X: Non-uniform over the entire surface.

[0051]

INDUSTRIAL APPLICABILITY The present invention makes it possible to provide a market for a black precoat excellent in pressure mark resistance at an industrial level without deteriorating other coating performance in response to the demand for free precoat guard film in the market. To do. That is, the black precoat of the present invention has PM resistance and punching resistance as cut plate products and coil products.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryoji Nishioka 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Co., Ltd. Technology Development Division (72) Kenji Yamamoto 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Co., Ltd. Company Kimitsu Works (72) Inventor Ryoji Kobayashi 1 Kimitsu, Kimitsu City, Chiba Shin Nippon Steel Co., Ltd. Kimitsu Works (72) Inventor Satoshi Kobori 4-1-15 Minamishinagawa, Shinagawa-ku, Tokyo Japan Stock Share Company Tokyo office

Claims (1)

[Claims] 1. A chemical conversion treated steel sheet is coated with a primer on the front side thereof, and a glass transition point of the coating film is 40 to 70 ° C. as an upper layer.
And a melamine-curable high molecular weight polyester resin having an average molecular weight of 15,000 to 50,000 is used in a solid content weight ratio of 40.
˜90%, a black coating film containing aggregate silica having a primary average particle size of 0.5 to 10 μm in a solid content weight ratio of 1 to 15%, and polyethylene powder as a lubricant in a solid content weight ratio of 1 to 5%. A black pre-coated steel sheet having excellent pressure resistance, which is formed and whose surface glossiness of the back side 1C1B coating film is 50 to 100% of the surface glossiness of the front side coating film.