JP2000144452A - Coated steel sheet excellent in drawability and moisture resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coated steel sheet applied to members to be drawn, free from reduction in moisture resistance in a worked part even if subjected to drawing. SOLUTION: A coated steel sheet is constituted by applying application-type chromate treatment using a coating solution containing three-dimensionally- bridged particulate resin or water-soluble resin to the surface of a steel sheet and then successively forming a thermosetting resin undercoat layer and a thermosetting resin topcoat layer on the resultant chromate film. In this coated steel sheet, the value of C/Cr by mole and the value of Si/Cr by mole, in the chromate film, are made to 0.5-4.0 and <2.5, respectively, and also the energy to rupture at 20 deg.C and elongation of the thermosetting resin topcoat layer are made to >=10 N.mm and >=30%, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coated steel sheet suitable for products which require drawing, such as home appliances, outdoor units, and articles.

[0002]

2. Description of the Related Art A pre-coated steel sheet has been conventionally used as a processed product member such as a home appliance, an outdoor unit, and a container. Pre-coated steel sheet has higher productivity than post-coating, which is applied after processing cold-rolled steel sheet or plated steel sheet, and has been the mainstream in recent years from the viewpoint of environmental issues such as recyclability, fire resistance, safety, etc. It is being reviewed, such as replacing plastic members with pre-coated steel sheets. The plastic member is formed by injection molding, so that a rounded shape can be freely processed, but it is often difficult to form a precoated steel sheet. In order to shift to a precoated steel sheet, it is essential to give a rounded shape by drawing, and the quality must not be deteriorated for a long time after the working.

[0003]

The conventional pre-coated steel sheet is mainly bent, and the evaluation of the bent portion has been performed in order to evaluate the durability. Was hardly evaluated. Draw the product in a wet box for 5
When inserted in a humid environment at 0 ° C for 240 hours, blisters are generated in the corners, and the gloss of the distorted parts received during the drawing process of the corners and side parts is remarkably reduced, and the appearance is harmonized with other parts. Can not be removes. An object of the present invention is to provide a coated steel sheet which has sufficient moisture resistance even when subjected to drawing and does not cause a decrease in gloss of a coating film.

[0004]

In order to achieve the object, the surface of a steel sheet is subjected to a coating type chromate treatment containing a three-dimensionally crosslinked fine particle resin or a water-soluble resin. In a coated steel sheet in which a thermosetting resin undercoat layer and a thermosetting resin topcoat layer having a dry film thickness of 10 to 25 μm are sequentially formed, the molar ratio of C / Cr in the chromate film is 0.5 to 4.0. , Si / Cr molar ratio is 2.
The breaking energy at 20 ° C. of the thermosetting resin overcoat layer was 10 N · mm or more, and the elongation was 30% or more.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A precoated steel sheet is generally subjected to a coating type chromate treatment on the surface of an underlying steel sheet in order to obtain a coating film adhesion. In order to clarify the relationship between the paint blisters in the wet environment after drawing and the composition of the chromate film, the relationship between each element concentration and the paint blisters in the wet environment was investigated. As shown in the figure, it was found that the ratio of Cr, C, and Si, which are the main components of the chromate film, affects the moisture resistance. FIG. 1 shows the relationship between the concentrations of C and Cr and the coating blisters.
When it is in the range of 5 to 4.0, it was recognized that the coating film swelling hardly occurred in a humid environment, and when it was beyond the range, the moisture resistance was lowered. FIG. 2 shows that C / Cr is 0.5-4.
0 shows the relationship between the concentration of Si and Cr in the chromate film in the range of 0 and the coating blister. If the slope Si / Cr of Si and Cr is less than 2.5, the coating film in a wet environment It was recognized that blistering hardly occurred. As described above, it has been found that the situation is different from the case of the flat plate or the bending process in the coating type chromate containing the three-dimensional crosslinked fine particle resin or the water-soluble resin. That is, Cr in the chromate film
The molar ratio of C to Si and the molar ratio of Si to Cr have an effect on blisters in a humid environment, and have a normal amount of chromium deposited (10 to 100 mg / m2) and a film of a thermosetting resin undercoat layer thereon. As shown in FIG. 3, in a coated steel sheet having a thickness of 4 to 15 μm and a thickness of the thermosetting resin overcoat layer of 10 to 25 μm sequentially, as shown in FIG.
It has been found that when the molar ratio of r is 0.5 to 4.0 and the molar ratio of Si / Cr is less than 2.5, the coating blister can be suppressed. Here, the molar ratio indicates a value obtained by X-ray photoelectron spectroscopy (XPS).

C / Cr in the chromate film indicates the molar ratio of the resin of the film to chromium. When C / Cr is less than 0.5, the interface between the fine particle resin and other film components is preferentially formed during bending. By generating cracks, it is not possible to obtain stress relaxation and uniform coating properties that suppress the destruction of the entire film. C / Cr
Is 4.0 or more, coating blisters tend to occur in a wet test after drawing. The flat part that does not receive processing distortion is
Even if C / Cr becomes 4.0 or more, the moisture resistance does not decrease.
In addition, since the area subjected to the processing strain is small even in the bent portion, sufficient adhesion is maintained, and there is no occurrence of coating blisters in a wet environment. However, when the drawing process is performed, the compressive / elongation strain covers a larger area than the bending process.
It may exceed 0%. Therefore, it is considered that stress is concentrated on the interface between the three-dimensional crosslinked fine particle resin and other coating film components such as chromium and silica, and cracks are generated. In a humid environment, moisture penetrates through the cracks, and the adhesion decreases according to the amount of the fine particle resin in a wide area subjected to drawing deformation. In the drawing process, the decrease in adhesion proceeds in a wide area, the distortion at the corner portion is large, and blistering of the coating film occurs near the portion where the adhesion is significantly reduced.

[0007] Si / Cr in the chromate film indicates the molar ratio of chromium to silica in the film. When Si / Cr is 2.5 or more, the film is formed in a wet environment even when the C / Cr is in an appropriate range. Swelling is likely to occur. Silica used for the coating type chromate treatment is fumed silica or colloidal silica having a particle size of 10 to 50 nm. These have many silanol groups on the surface and are effective in improving the adhesion by hydrogen bonding with the coating and the rigidity of the coating by self-condensation of silica. Is reduced.

The molar ratio of C / Cr and Si / Cr uses the composition ratio by X-ray photoelectron spectroscopy (XPS).
This is because the coating type chromate film is a thin film, and a suitable analysis depth is obtained. Measurements were taken by Perkin Elmer
Using a PHI5500 manufactured by Co., Ltd. at an incident angle of 45 degrees with a 1253.6 eV magnesium X-ray source, the calculation of the molar ratio of the element concentration was performed using the C1S peak, the Si2P peak, and the Cr2 peak.
Each was calculated from the peak area of the P peak. The sensitivity correction coefficient is C1S: 0.296, Si2P: 0.339, C
r2P: 2.427 was used.

C is caused by the resin component of the chromate film, and is a three-dimensional cross-linked acrylic resin microgel emulsion having a particle size of about 0.3 to 0.6 μm, which is emulsion-polymerized at the time of producing a coating type chromate treatment liquid, or water-soluble. This is because a water-soluble polyacrylhydroxyamide or the like is added, and improves uniform coatability of the coating film, adhesion of the coating film, and the like. The acrylic resin microgel emulsion improves the adhesion between the coating film and the substrate by hydrogen bonding due to its own carboxylic acid group or hydroxyl group, and has a particle size of 0.3 to 0.6 μm, which is larger than the coating type chromate film. In addition, there is a function of improving the scratch resistance of the coating film by roughening (anchor effect). In addition, there are advantages such as uniform application by volume effect and improvement in adhesion due to stress relaxation during bending.

[0010] For the same reason as acrylic resin, polyacrylhydroxyamide improves coating film adhesion, coats phosphoric acid and silica, and balances each dispersing component and polar component to form a coating. Improves the balance between film adhesion and moisture resistance. Chromium is the main component of the chromate treatment.It improves the adhesion and corrosion resistance of the coating by covalent bonds and hydrogen bonds, such as a cross-linking effect by forming a complex with the coating and a continuous film by self-condensation. Stronger than agent.

When the breaking energy at 20 ° C. of the thermosetting resin overcoat layer is 10 N · mm or more and the elongation is 30% or more, the occurrence of coating film cracks and fine cracks on the coating film surface is suppressed, Gloss reduction can be suppressed. One of the reductions in the gloss of the coating film is to roughen the surface of the top coating film. Roughening is due to fine cracks on the coating film surface due to processing distortion. In a coating film that has been strained by drawing, fine cracks are generated from the surface in accordance with the amount of distortion and the elongation and breaking energy of the coating film, and eventually the coating film is broken. If the coating film has a high film elongation with respect to the strain stress, the coating film can be processed without breaking, but if the coating film has a low breaking energy, fine cracks on the coating film surface cannot be suppressed. The breaking energy here is 5 mm in width and 30 m between chucks.
m, a free coating film having a thickness of 20 μm at 20 ° C. and a pulling speed of 10
It is a value obtained from the result of a test under the condition of mm / min.

In order to make the elongation and the breaking energy of the thermosetting resin overcoat layer as described above, a resin having a relatively high molecular weight, for example, a resin having a number average molecular weight of 12,000 to 1,200 is used as the coating resin.
Using about 30,000 polyester-based resins or high-molecular-weight polyester-based resins, the type and amount of the curing agent to be mixed may be adjusted. Further, the thermosetting resin undercoat layer has a number average molecular weight of 1 as used in pre-coated steel sheets for home appliances.
What is necessary is just to use the coating film using 0000-27000 polyester resin and epoxy resin. A coloring pigment or extender may be added to the thermosetting resin undercoat layer as needed.

The substrate of the coated steel sheet of the present invention may be any of a zinc-coated steel sheet, a zinc-aluminum-plated steel sheet, an aluminum-plated steel sheet, and a stainless steel sheet. The coating type chromate treatment method may be a known method such as a roll coating method.
After applying the chromate solution, drying may be performed at 50 to 200 ° C., and the formation of the thermosetting resin undercoat layer and the thermosetting resin overcoat layer may be performed by a known method.

[0014]

Example: Plate thickness 0.5 mm, adhesion amount per side 60 g /
Each coated steel sheet of m2 is subjected to a coating type chromate treatment having a coating composition shown in Table 1, and as a thermosetting resin undercoat layer, a polyester resin-based undercoat paint having a number average molecular weight of 25,000 is dried to a dry film thickness of 3 to 12 μm. And baked and dried at 215 ° C. for 40 seconds. After that, as a thermosetting resin topcoat layer, various high-molecular polyester resin topcoats having the properties shown in Table 2 are applied to a dry film thickness of 7 to 20 μm and baked at 230 ° C. for 50 seconds for drying. did. The characteristics of the obtained coated metal plate were evaluated. The evaluation results are as shown in Table 3.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

The evaluation method of each characteristic is as follows. (1) Drawability All-purpose deep drawing tester (Type 142-20 manufactured by Eriksen)
And squeeze it into a square tube type (42 mm in length and width, 20 mm in depth, corner R11 mm), observe the crack state of the coating film at the corner with a loupe (magnification: 10), and if there is no abnormality in the coating film, The film was evaluated by the symbol を when a very slight crack was observed in the film, the symbol Δ when the film was considerably cracked, and the symbol X when the film was cracked by the naked eye. (2) Moisture resistance The flat plate and the sample after drawing were subjected to a wet test at 50 ° C. for 240 hours in accordance with JIS K5400, and the state of occurrence of coating blisters in a wet environment was observed. The symbol ○ indicates no coating blisters, and no blisters were observed with the naked eye, but 40
A double loupe was used to evaluate a sample having one or two very small coating blisters with a symbol △, and a sample with visible coating blisters with the naked eye was evaluated with a symbol x.

(3) Gloss retention (A) Draw processing The gloss of the drawn square tube sample is measured, and those having a gloss retention of 80% or more as compared with the unprocessed gloss are represented by symbols ○, 60 to 79. % Or less was evaluated with the symbol △, and those with 59% or less were evaluated with the symbol x. (B) Tensile test Using a universal type tensile tester (Model GT2000 manufactured by Shimadzu Corporation), the gloss when a 15% elongation was given was measured, and the gloss retention was 80% or more compared to the gloss before the test. The samples were evaluated with the symbol 、, those with 60 to 79% or more were evaluated with the symbol Δ, and those with 59% or less were evaluated with the symbol X. As shown in the evaluation results,
The composition according to the present invention is excellent in drawing workability and moisture resistance, and there is no decrease in gloss.

[0020] As described above, the coated steel sheet of the present invention has excellent moisture resistance even when subjected to drawing, and does not decrease the gloss of the coating film.
Therefore, it is suitable for members that require stable quality for a long time after processing.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the Cr and C concentrations of a chromate film and moisture resistance.

FIG. 2 is a diagram showing a relationship between Cr and Si concentrations of a chromate film and moisture resistance.

FIG. 3 is a diagram showing a relationship between a molar ratio of a main component of a chromate film and moisture resistance.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Tetsuo Sakai 7th, Takatani-Shimmachi, Ichikawa-shi, Chiba F-term (reference) 4D075 BB74X CA38 DA03 DB02 DC10 DC18 EA06 EA07 EA19 4F100 AB03A AK01B AK01C AK01D AT00A BA04 BA07 BA10A BA10D DE01B EH46B EJ69B GB48 GB51 GB81 JB09B JB12B JB13C JB13D JD04 JK01D JK08D JL01 YY00BY02BC03 YY00BY02BC

Claims (2)

[Claims] 1. A surface of a steel sheet is subjected to a coating type chromate treatment containing a three-dimensionally crosslinked fine particle resin or a water-soluble resin, and a thermosetting resin undercoat layer having a dry film thickness of 4 to 15 μm is formed thereon. In a coated steel sheet in which a thermosetting resin overcoat layer having a thickness of 10 to 25 μm is sequentially formed, the molar ratio of C / Cr in the chromate film is 0.5 to 4.0, and Si /
A coated steel sheet having excellent drawability and moisture resistance, characterized in that the molar ratio of Cr is less than 2.5. 2. The drawing processability and moisture resistance according to claim 1, wherein the thermosetting resin overcoat layer has a breaking energy at 20 ° C. of 10 N · mm or more and an elongation of 30% or more. Excellent coated steel plate.