JP2003286556A - Hot-dip galvannealed steel sheet superior in powder coating property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent microprojections (with diameters around 1 mm and heights around 0.1-0.5 mm) which easily tend to occur on the surface of a formed paint film by power coating subsequent to press forming, on a hot-dip galvannealed steel sheet, and to improve qualities of the coated film. <P>SOLUTION: The microprojection is caused by bumping of trapped water in the surface of the plated layer, during a baking stage after powder coating. Accordingly, the surface blemish of the microprojection can be controlled and prevented by means of avoiding the trap of moisture, namely by controlling an alloying index I of the hot-dip galvannealed layer and its surface roughness Ra so as to satisfy the followings: alloying index I[=(I<SB>ζ</SB>-IB)/I<SB>ζ</SB>]≤0.1, and surface roughness Ra=0.3-1.0 μm. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot dip galvanized steel sheet having excellent powder coatability, which is useful as an exterior plate for automobile bodies, home electric appliances, building materials, sheet metal furniture, and other articles.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets are
Applications that require paintability, spot weldability, etc., such as automobile inner and outer panels, home appliances (heating equipment, lighting equipment, refrigerators, etc.), building materials (fences, garages, etc.), kitchen equipment, sheet metal furniture, It is widely used as an exterior plate for other products. The alloyed hot-dip galvanized steel sheet is manufactured through the steps of depositing a hot-dip galvanized layer on the surface of the steel sheet in a predetermined layer thickness, and then heating and holding it at a predetermined temperature in a heat treatment furnace for alloying treatment. The obtained alloyed hot-dip galvanized steel sheet is generally formed into a required shape by pressing or the like and then coated for the purpose of decoration, surface protection, etc. according to the usage.

Since powder coating does not use a solvent (paint medium), it does not have the problem of environmental pollution like organic solvent type paints.
In addition, it is possible to use polymer resin paints that are difficult to dissolve in organic solvents, and it is possible to apply thick coating with one coat, which has the advantages of reducing the number of painting processes and improving coating performance.
Recently, it has been widely applied to the field of general industrial coating that requires cosmetic finish. The powder coating method is classified into several methods (spraying method, fluidized-bed coating method, electrostatic coating method, etc.).
In terms of efficiency and finish quality, the electrostatic coating method (electrostatic flow dipping method, electrostatic spraying method) is often used industrially.

[0004]

A coated product obtained by forming an alloyed hot-dip galvanized steel sheet (hereinafter sometimes simply referred to as "plated steel sheet") into a predetermined shape by press working or the like and subjecting it to powder coating is Fine protrusions (diameter about 1 mm,
Height around 0.1-0.5 mm) may occur locally or dispersed over a certain surface area. When such a surface abnormality occurs, the quality and commercial value of the product are significantly impaired.

As a result of detailed investigations and experiments conducted on the above phenomenon by the present inventors, the fine protrusions appearing on the coating film surface are traces of rupture caused by the formation of bubbles at the interface between the coating layer of the plated steel sheet and the coating film. I found that. If water locally adheres to and remains on the surface of the plated steel sheet before coating, it vaporizes and bumps in the baking process after coating the coating material to form the aforementioned minute projections. FIG. 1 shows a cross section in which bubbles (4) are generated at the interface between the coating layer (2) and the coating film (3) of a coated plated steel sheet (after baking). A fine piece (5) is observed in the generation part of the bubble (4). Figure 2 [1] shows the Zn characteristics X of the bubble.
The line image, [2] in the same figure, is a Fe characteristic X-ray image. In this characteristic X-ray image, the minute pieces (5) observed in the bubble portion of FIG. Suggests that.

The peeled crushed pieces (5) are generated by sliding frictional resistance between the surface of the plating layer and the surface of the molding jig during the molding process such as press working performed before powder coating, and adhere to the surface of the plating layer. It is presumed to have accumulated. If such an adhered deposit is present, water (cleaning water or the like in the surface cleaning treatment of the pre-painting treatment) is likely to be trapped therein, and it is difficult to reliably remove it even if a drying treatment is performed. If the minute protrusions are caused by the water trapped in the deposits on the surface of the plating layer, prevent the peeling and crushing of the plating layer during the molding process to prevent the residual water content and the generation of the minute protrusions. It becomes possible to do. As a result of further studies by the present inventors, by adjusting the alloying degree of the plating layer and the surface roughness thereof, peeling and crushing of the plating layer during the forming process and its adhesion and deposition are prevented, and powder coating is applied. The inventors have found that it is possible to effectively suppress and prevent the generation of microprotrusions in the film, and have completed the present invention.

[0007]

The alloyed hot-dip galvanized steel sheet excellent in powder coatability of the present invention has an X-ray diffraction intensity index I of the plating layer calculated by the formula [1] of 0.1 or less, The surface roughness Ra of the plating layer is 0.3 to 1.0 μm.

[Number 1] _{I = (I ζ -I B)} / I ζ ... [1] wherein, I _zeta: diffracted X-ray intensity of the zeta phase compounds _{I B:} background diffraction intensity]

In order to prevent minute protrusions in the coating film of a powder coated product of a galvannealed steel sheet, the moisture (water for cleaning the surface before coating), which is the direct generation factor, is the surface of the plating layer. It is necessary to prevent peeling and crushing of the plating layer in the molding process prior to the coating process so as not to be trapped in As described above, the peeling and crushing of the plating layer is prevented by adjusting the alloying degree of the hot-dip galvanized layer (alloying index I: 0.1 or less) and adjusting the surface roughness of the alloyed plated layer. (Surface roughness Ra: 0.3 to 1.0 μ)
m),

Adjustment of the alloying degree of the ζ plating layer (alloying index I: 0.1 or less) means that the production of the ζ phase is suppressed to a relatively small amount. In the alloying treatment of hot-dip galvanized steel sheet, due to the alloying reaction (generation of Fe-Zn intermetallic compound by diffusion of Fe atoms from the base iron to the plating layer),
A phase (FeZn ₁₃ ), a δ phase (FeZn ₇ ), and a Γ phase (Fe ₃ Zn ₁₀ ) are generated in this order, and the growth increases toward the surface of the plating layer as the reaction progresses. The reason why the amount of ζ phase generated was kept to a comparatively small amount was that it was viscous and increased the sliding resistance with the forming jig during forming, resulting in damage to the plating layer surface (occurrence of peeling fracture). ) Is easy to invite. The degree of alloying (progress of the alloying reaction) is controlled by the treatment temperature, time, etc. in the alloying treatment.

The alloying index I is defined to be 0.1 or less, because if it exceeds this, the surface of the plating layer during press forming is likely to be damaged and minute projections during coating due to it are likely to occur. The plating layer whose degree of alloying is adjusted in this way has a structure in which the δ phase is the main component and a small amount of the ζ phase is mixed therein, so the paintability, spot weldability, etc. are also good,
In addition, Γ phase (because it is a compound that is extremely hard and brittle, if the amount of formation is large, the powdering phenomenon occurs at the interface of the plating layer during powder forming such as pressing, resulting in a powdering phenomenon, which significantly impairs the formability of the plated steel sheet). The thickness can be set to the extent that there is no actual harm.

The alloyed plated steel sheet, as it is, has a large surface roughness and is apt to cause damage to the plated layer (formation of separated crushed pieces) due to sliding resistance with the forming jig during press working. In order to surely prevent this, it is necessary to limit the surface roughness Ra of the plating layer to 1.0 μm or less together with the adjustment of the alloying degree. If the surface of the plating layer is excessively smoothed, it becomes difficult to retain the press lubricating oil on the surface of the plating layer and the lubricity with the forming jig is reduced. There is a risk of inducing shaving. Therefore, the lower limit of the surface roughness Ra is set to 0.3 μm. The surface roughness of the plating layer can be adjusted by skin pass (temper rolling) as described later.
Can be done by.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hot-dip galvanizing is carried out in a continuous galvanizing line by using a cold-rolled steel sheet or a hot-rolled steel sheet as a plating base sheet, and in accordance with a conventional method, a hot-dip galvanizing bath such as a pure Zn bath or Z.
It is carried out by continuously passing through an n-Al alloy bath (for example, Al content of 0.13 to 0.16% by weight), etc., and the deposit amount is adjusted by a plating deposit control device just above the plating bath. Next, the hot-dip galvanized steel sheet is introduced into an alloying treatment furnace provided in the line, and a predetermined alloying treatment is performed while passing through the alloying treatment furnace.

The evaluation of the degree of alloying of the hot-dip galvanized layer is made by applying the X-ray diffraction technique. This method is based on the difference in the lattice spacing of each compound phase such as ζ phase, δ phase, and Γ phase (each has its own crystal lattice spacing) generated in the alloying reaction. Is used to determine the amount of compound produced from the detected diffracted X-ray intensity, and is used as a method for nondestructively and quantitatively determining the alloying degree in a continuous plating line. .

The X-ray diffraction intensity is measured by using, for example, a Cr tube (Cr-kα, wavelength λ: 2.28962Å) as an X-ray source, and a ζ-phase compound (crystal lattice spacing d: 0.126 nm, diffraction angle 2).
θ: 130.3 °) diffraction intensity and background intensity I
_B (Diffraction intensity on the crystal lattice plane where no diffraction peak exists)
To detect the alloying index I according to the above formula [1].
Ask for. In the alloying treatment, the alloying degree of the plating layer is expressed by the formula [1].
The treatment temperature and time are adjusted so as to satisfy the above condition.

The galvannealed steel sheet subjected to the alloying treatment is
The plating layer has a predetermined surface roughness (Ra: 0.3 to 1.0 μ).
m) is applied to a skin pass (temper rolling). This skin pass is performed by using a dull roll as a work roll and transferring the dull surface to the plating layer surface. The surface roughness Ra of the dull surface is, for example, 1.3 to 1.5.
μm. Dull roll, shot dull, discharge dull,
A roll that has been subjected to various dulling processes such as laser dull and electron beam dull can be appropriately used.

The rolling reduction in the above-mentioned skin pass is adjusted so as to obtain an appropriate transfer rate, but it is preferable to control the elongation (δ) of the steel sheet to be in the range of about 1% or less. This is because if a high-pressure skin pass with an elongation exceeding this is performed, the material quality of the steel sheet may deteriorate (especially, the formability may deteriorate). A skin pass with dull roll is applied and the plating layer has a predetermined surface roughness (Ra: 0.3 to 1.
0 μm) to obtain the galvannealed steel sheet of the present invention.

The alloyed hot-dip galvanized steel sheet of the present invention is formed into a desired shape by a forming process such as pressing and then subjected to a powder coating step. In the painting process, degreasing treatment (cleaning with water-degreasing-washing with water) for surface purification is performed as a pretreatment in accordance with the usual method
-Washing-drying), and chemical conversion treatment (typically phosphate treatment) to improve paint adhesion, followed by powder coating. As a powder coating method, an electrostatic coating method (electrostatic flow dipping method, electrostatic spraying method) is preferably used. Electrostatic coating consists of a coating process for electrically adsorbing powder coating on the surface of the article to be coated and a baking process for baking the coating layer in a heating furnace to complete the coating. It does not require any special conditions or restrictions.

[0018]

[Example] [1] Manufacture of hot dip galvanized steel sheet In a continuous hot dip galvanizing line, hot dip galvanizing and alloying treatment are performed, and then a plate is passed through a skin pass mill installed in the line to obtain a test material. . (1) Hot-dip galvanized base plate: Cold rolled steel plate (aluminum killed steel) 2.5m thick
Plating bath composition: Zn-0.14wt% Al Plating weight: 40g / m ² (both sides) or more

(2) Alloying treatment: temperature 480-520 ° C (3) Skin pass Dull roll roughness: Ra 1.3 to 1.5 μm Steel sheet elongation (δ): 0.3-1.0%

[2] Forming processing The cut plate (a) of the plated steel sheet is subjected to square tube drawing processing,
A rectangular tubular molded body (length 70 x width 70 x depth 25, mm) (b) is molded (see Fig. 3).

[3] Powder coating construction After degreasing treatment and chemical conversion treatment, coating construction by electrostatic spraying method. (1) Degreasing treatment (washing-degreasing-washing-washing-drying) Chemical liquid: Enares EC-20 (manufactured by Nippon Parkerizing Co., Ltd.) Treatment condition: Spray (liquid temperature 35 ° C, time 120 seconds) (2) Chemical conversion Treatment (phosphate treatment) Chemical liquid: Valbond 3308 (manufactured by Nippon Parkerizing Co., Ltd.) Treatment condition: Spray (liquid temperature 35 ° C, time 60 seconds)

(3) Powder coating (electrostatic spraying method) (3.1) Paint: Polyester resin paint "Powdax" (Manufactured by Nippon Paint Co., Ltd.) (3.2) Coating thickness: 50-60 μm (3.3) Baking conditions: 180 ° C x 15 minutes (holding)

[4] Evaluation of Quality Each sample material was inspected (visual observation) for the presence of minute projections on the coating film surface, and the results shown in Table 1 were obtained. The powdering test results are also shown in the table. Comparative example No. 11 (alloying index I is satisfied, but skin pass is omitted), No. 12
(The alloying index I is satisfied, but the surface roughness in the skin pass is not satisfied) and No. 13 (the surface roughness is satisfied but the alloying index I is not satisfied), fine protrusions are generated. There is. On the other hand, in each of the invention examples, there are no minute protrusions, and even if they occur, the number of occurrence is very small (about 100 / m
² or less), and has a good coating film surface. The powdering test results were also good (no peeling of the base steel sheet-plating layer interface due to powdering), and there was no excessive formation of the Γ-phase compound, and an appropriate alloying treatment was achieved.

[0024]

[Table 1]

[0025]

EFFECTS OF THE INVENTION According to the present invention, it is possible to prevent the generation of minute protrusions which are coating film surface defects of powder coating products using alloyed hot-dip galvanized steel sheets, and to guarantee a sound coating quality. It is possible to improve the yield and performance of coated products used for home appliances, automobile bodies, building materials, and other various applications.

[Brief description of drawings]

FIG. 1 is a drawing-substitute photomicrograph (magnification: 750) showing a cross section (secondary electron image) of a coating layer and a coating film of a powder-coated galvannealed steel sheet.

2 is a drawing-substituting photograph showing a characteristic X-ray image of the cross section of FIG. 1 (FIG. 2A: characteristic X-ray image of Zn; FIG. 2B: characteristic X-ray image of Fe).

FIG. 3 is an explanatory diagram of forming of a test steel plate in an example.

[Explanation of symbols]

1: Base steel plate 2: Plating layer 3: Painted coating 4: Bubble 5: Delaminated fragment of the plating layer (adhered deposit)

Claims (2)

[Claims] 1. The X of the plating layer calculated by the formula [1]
Line diffraction intensity index I is 0.1 or less, surface roughness R of plating layer
An alloyed hot-dip galvanized steel sheet having excellent powder coatability, wherein a is 0.3 to 1.0 μm. _{I = (I ζ -I B)} / I ζ ... [1] wherein, I _zeta: diffracted X-ray intensity of the zeta phase compounds _{I B:} the background intensity] 2. A hot-dip galvanized layer is formed on the surface of a steel sheet, which is then subjected to an alloying treatment satisfying the above formula [1], and the galvanized steel sheet subjected to the alloying treatment is subjected to a skin pass by a dull roll to roughen the surface of the plated layer. The alloyed hot-dip galvanized steel sheet according to claim 1, wherein the degree Ra is adjusted to 0.3 to 1.0 μm.