JP2005272972A - Method of pretreatment for coating of bright annealing-treated stainless steel sheet and bright annealing-treated stainless steel sheet for coating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of pretreatment for coating for obtaining a clear-coated stainless steel sheet whose appearance is not damaged and having satisfactory adhesion to a coating film even if a bright annealing-finished stainless steel is used as a substrate, and to provide a steel sheet for coating. <P>SOLUTION: In the pretreatment method for coating of a bright annealing-treated stainless steel, for attaining the objective improvement of adhesion to a coating film, anode electrolysis treatment is performed in an alkaline aqueous solution with the pH of ≥12. Further, by performing the anode electrolysis treatment, the bright annealing-treated stainless steel sheet for coating having a surface oxide film with a composition in which the ratio of Fe is ≥0.4 can be obtained. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a pre-coating method for a bright annealed stainless steel plate capable of exhibiting excellent coating film adhesion, and a bright annealed stainless steel plate for coating.

Stainless steel sheets have been widely used in building materials, kitchen supplies, and automobile parts, taking advantage of their excellent corrosion resistance and beautiful appearance. In these applications, the conventional stainless steel sheet was usually used in so-called bare materials with various surface finishes such as pickling (+ skin pass rolling), bright annealing, and polishing, but recently, corrosion resistance, In order to improve fingerprint resistance, designability, etc. and to prevent wrinkles during processing, there are increasing cases of being used by painting.

When coating metal products, in order to ensure good coating film adhesion, a chemical treatment called pre-coating treatment is usually performed prior to coating. However, in the case of a stainless steel plate, its own corrosion resistance is good and chemically inert, and it is difficult to obtain good coating adhesion by ordinary coating pretreatment. A number of methods for pretreatment of stainless steel have been proposed.

JP-A-5-106057 JP-A-6-235100 JP-A-8-074098 JP 2002-275658 A

The bright annealed stainless steel plate is glossy in the finished state and excellent in design, so it can be used as a so-called clear painted stainless steel plate coated with a transparent (including translucent) resin film layer in addition to its use as a bare material. However, in this case, the coating pretreatment that has been used for 2D, 2B and polished stainless steel sheets has a problem in that sufficient film adhesion cannot be obtained (the finish is JISG4305). Regulations). As the stainless steel plate finally subjected to the bright annealing treatment, there is a BA finish material which has a high gloss appearance and is widely used. Products that have been polished by bright annealing after polishing equivalent to 4 finishes (hereinafter referred to as No. 4 MAT materials) and those that have a slightly lower gloss than BA finish materials are also commercially available. Here, no. The 4MAT material has a unique surface finish such as a fine balance between polishing marks and gloss and a white appearance.

In conventional coating pretreatments, hexavalent and trivalent Cr compounds, which are environmentally hazardous substances, have been used in a considerable proportion (so-called chromate treatment), but recently, so-called non-Cr treatments that do not contain Cr compounds. Is becoming mainstream. In the case of this non-Cr treatment, the reactivity of the treatment liquid is lower than that of the Cr-based treatment, so that it becomes more difficult to obtain good coating film adhesion in the bright annealing treatment material.

The bright annealed material is generally obtained by annealing a cold-rolled stainless steel plate in a hydrogen-nitrogen atmosphere with a dew point of −45 to −65 ° C., a temperature of 900 to 1000 ° C., and a time of 30 to 180 seconds. Under such processing conditions, Fe, which is a main component of stainless steel, is not oxidized very much, but easily oxidizable elements such as Cr, Si, Al, etc. are selectively oxidized, so that these elements are present in the surface oxide film. Concentrate. In particular, in the case of stainless steel melted on an industrial scale, the surface concentration of Si is remarkable. In the case of pickling finishes, these surface enrichments have been removed.

It has been found that the surface concentration of Cr and Si makes it difficult to obtain good coating film adhesion in the bright annealed material. In other words, the chemically inert Cr and Si oxides are concentrated on the surface of the stainless steel plate, so the reactivity with the chromate treatment liquid and non-Cr treatment liquid, which are pre-coating treatment liquids, is low The adhesion at the film interface tends to be insufficient. Further, Si oxide is water-repellent, and when the Si oxide is segregated locally, the treatment liquid is repelled, so that the treatment tends to be uneven.

The present invention has been devised to solve such a problem, and without damaging the design of the bright annealed stainless steel sheet, surface concentration of Cr, Si, etc., which are factors that inhibit coating film adhesion. It aims at providing the method of reducing and improving coating-film adhesiveness.

The pre-coating method for the bright annealed stainless steel sheet of the present invention is characterized in that anodic electrolytic treatment is performed in an alkaline aqueous solution having a pH of 12 or more in order to achieve the desired improvement in coating film adhesion.
Further, by performing the anodic electrolysis treatment of the present invention, a bright annealed stainless steel plate for coating having a surface oxide film having a composition of Fe ratio 0.4 or more is obtained.

Prior to the conventional coating pretreatment, by performing the electrolytic treatment of the present invention, it is possible to obtain a coated stainless steel sheet having good coating adhesion even if any of the non-reactive non-Cr treatment and the chromate treatment is used. I can do it.

The present invention is applied to a bright annealed stainless steel plate, and the stainless steel plate used as a coating original plate may be any of ferrite, austenite, martensite, and the like. In order to ensure designability after painting, the surface glossiness (based on JISZ8741) 60 degree reflection of the bright annealed material is preferably 4000% or more, more preferably 5000% or more for the BA finish. No. In the case of 4MAT finishing material, the gloss is low due to the polishing marks, but the glossiness and polishing streaks have a unique balance and have a unique appearance. The gloss is generally 3000 to 4000% when light is incident in the direction of the polishing streak, and 1500 to 2500% when light is incident in the direction direct to the polishing streak. Changing is undesirable for clear paint.

The usual coating process for stainless steel sheets is as follows. Receiving original plate → Alkaline immersion degreasing → Surface adjustment (may not be) → Chromate treatment or non-Cr treatment → Painting and baking. In the case of the present invention, anodic electrolytic treatment is carried out in an alkaline aqueous solution, instead of the usual degreasing degreasing. Alkaline immersion degreasing has the effect of removing dirt such as oil adhering to the surface of the stainless steel plate, but has no effect of modifying the oxide film enriched with Cr, Si, etc. on the surface of the stainless steel plate. When a stainless steel plate is immersed in an acidic aqueous solution, Fe-based oxides present on the surface of the steel plate can be removed, but oxides such as Cr and Si that are poor in reactivity tend to remain. If all of these oxides are removed, it becomes easy to perk-wash and the surface becomes rough, so that the design properties of the bright annealed material are lost. Electrolytic pickling is also unsuitable for pre-coating of the BA material for the same reason regardless of whether it is anodized or cathodic. Patent Document 2 cited above discloses a technique for selectively removing Fe-based oxides by electrolytic pickling and actively leaving Si-based oxides.

Patent Document 4 discloses a technique for cathodic electrolysis of a stainless steel plate in an alkaline aqueous solution as a pretreatment for coating. In this case, the effect of cathodic electrolysis is removal of dirt adhering to the surface due to generation of hydrogen bubbles and mechanical destruction of the surface oxide film, and there is no action of dissolving the oxide film. In the case of stainless steel, in particular, the bright annealed material, the surface oxide film is in very close contact with the steel substrate, and the effect of removing the surface oxide film by the hydrogen foaming action is small.

In contrast, when a stainless steel plate is anodically electrolyzed in an alkaline aqueous solution, most of the reaction that occurs is oxygen generation, but some of the Cr oxide in the oxide film is eluted. This is presumably because Cr is changed to a soluble hexavalent compound by the oxidation reaction at the anode. The elution rate varies depending on the electric potential, but becomes remarkable when the pH is 12 or more. Therefore, in the present invention, the bright annealing treatment material is subjected to an anodic electrolysis treatment in an alkaline aqueous solution having a pH of 12 or more as a pretreatment for coating. Further, in this pH range, Fe oxide is also dissolved, though very little. Although Si oxide is considered to be basically insoluble, it is considered that it physically drops off as Cr oxide and Fe oxide are dissolved. Depending on the steel type, Al or the like may be concentrated in the surface oxide, but these inactive oxides are considered to fall off during the anodic electrolysis. In addition, the value of pH used in this specification is a value measured at room temperature.

The alkaline aqueous solution used as the electrolytic solution in the present invention is prepared by dissolving one or more of alkali metal hydroxides, carbonates, phosphates and silicates. Here, Na, K or Li is preferable as the alkali metal from the viewpoint of availability. Moreover, the concentration of these alkali metal salts is preferably 0.2 to 2 mol / L (hereinafter referred to as M) in terms of alkali metal. If it is less than 0.2M, the concentration change due to taking out of the liquid is large during continuous processing, management is complicated, and processing efficiency may be reduced. If it exceeds 2M, the effect of the treatment is saturated. A more preferable alkali metal salt concentration range is 0.5 to 1.5M. In the case of the electrolytic solution of the present invention, alkaline earth salts, surfactants, corrosion inhibitors and the like may be added within a range that does not cause a decrease in pH.

The electrolytic treatment conditions are: liquid temperature: 40 to 70 ° C. (more preferably 50 to 60 ° C.), current density: 2 to 25 A / dm 2 (more preferably 5 to 20 A / dm 2), treatment time: 3 to 60 seconds (more Preferably, it is performed for 5 to 20 seconds. If the liquid temperature is less than 40 ° C., the effect of improving the adhesion is small, and if it exceeds 70 ° C., the evaporation of water is intense and the concentration control of the electrolyte is difficult. Further, when the current density is less than 2 A / dm 2 and the processing time is less than 3 seconds, the effect of improving the adhesion is small, and even when the current density exceeds 25 A / dm 2 and the processing time exceeds 60 seconds, the effect is saturated.

The present invention is an electrolytic treatment in an alkaline aqueous solution, and unlike the case of an acidic solution, there is little dissolution of the steel base, so that the glossiness due to the treatment hardly occurs. In addition, since oxides such as Cr and Si are preferentially removed by electrolytic treatment, the surface oxide film has a Fe-rich composition, and the reactivity with the pretreatment liquid for coating is improved. When the electrolytic treatment of the present invention is applied to a bright annealed stainless steel plate, a stainless steel plate having a surface oxide film with an Fe ratio of 0.4 or more is obtained with almost no decrease in surface gloss, and this is coated according to a conventional method. When the coating is performed, a coated stainless steel sheet having good coating film adhesion and excellent design can be obtained. Here, the Fe ratio is the total of all metal elements (Fe, Cr, Ni, Si, Al, Mn, etc.) among the detected elements obtained by analyzing the surface layer by X-ray photoelectron spectroscopy (XPS). It refers to the ratio of Fe (mass%) to (mass%).

Using a SUS430BA finish with a plate thickness of 0.4 mm (Fe ratio of steel plate surface before electrolysis: 0.11, surface gloss of 5500) in an aqueous solution of sodium hydroxide 0.5M + sodium carbonate 0.2M (pH 13.2), temperature 60 Anodization was carried out at 60 ° C. and a current density of 5 A / dm 2 for 60 seconds, followed by washing with water and drying, followed by a coating type chromate treatment. The deposited amount in terms of Cr was 20 mg / m2. After drying, a polyester resin-based clear paint was applied to a dry film thickness of 10 μm, and baked at an ultimate plate temperature of 230 ° C. for 45 seconds.

The same steel plate as used in Example 1 was used, and anodic electrolysis was performed in an aqueous solution of sodium hydroxide 0.5M + sodium orthosilicate 0.1M (pH 13.4) at a temperature of 60 ° C. and a current density of 10 A / dm 2 for 15 seconds. . Thereafter, washing with water and drying were carried out in the same manner as in Example 1, followed by coating-type chromate treatment. After drying, a polyester resin-based clear paint was applied in a dry film thickness of 10 μm and baked.

Using the same steel plate as used in Example 1, anodic electrolysis in the same electrolytic solution as in Example 1 at a current density of 5 A / dm 2 for 60 seconds, followed by washing with water and drying, Zr coating type non-Cr treatment was applied to the film An amount of 60 mg / m @ 2 (Zr equivalent adhesion amount 24 mg / m @ 2) was applied, and after drying, a polyester resin-based clear paint was applied in a dry film thickness of 10 .mu.m and baked.

SUS430 No. with a plate thickness of 0.4 mm. 4MAT finishing material (Fe ratio of steel plate surface before electrolysis: 0.11, surface gloss: polishing direction 3000, polishing direction 1650) was used in the same electrolytic solution as in Example 1 at a temperature of 60 ° C. and a current density of 5 A / dm 2. A 60-second anodic electrolysis treatment was performed. Thereafter, a coating-type chromate treatment was applied in an amount of 15 mg / m @ 2 as a Cr conversion adhesion amount, and after drying, a polyester resin-based clear paint was applied at a dry film thickness of 10 [mu] m and baked.

Using the same steel plate as in Example 4, anodic electrolysis was performed in the same electrolytic solution as in Example 2 at a temperature of 60 ° C. and a current density of 10 A / dm 2 for 15 seconds. Thereafter, it was washed with water and dried, and a coating-type chromate treatment was applied in an amount of 20 mg / m @ 2 as a Cr conversion deposit. After drying, a polyester resin-based clear paint was applied in a dry film thickness of 10 [mu] m and baked.

Using the same steel plate as in Example 4, anodic electrolysis was performed in the same electrolytic solution as in Example 1 at a temperature of 60 ° C. and a current density of 15 A / dm 2 for 10 seconds. Then, after washing with water and drying, the same Zr-based non-Cr treatment as in Example 3 was applied as a film adhesion amount of 60 mg / m 2 (Zr conversion adhesion amount 24 mg / m 2). Was applied and baked.

Comparative Example 1

The steel plate used in Example 1 was immersed in the same alkaline aqueous solution as in Example 1 at 60 ° C. for 30 seconds, and then the same coating-type chromate treatment and coating as in Example 1 were performed.

Comparative Example 2

The steel plate used in Example 1 was subjected to cathodic electrolysis in the same alkaline aqueous solution as in Example 2 for 60 seconds at a current density of 5 A / dm2. Thereafter, the same coating-type chromate treatment and coating as in Example 1 were performed.

Comparative Example 3

The steel plate used in Example 1 was immersed in the same alkaline aqueous solution as in Example 1 at 60 ° C. for 30 seconds, then washed with water and dried, and then subjected to the same non-Cr coating type treatment and coating as in Example 3.

Comparative Example 4

The steel plate used in Example 4 was immersed in the same alkaline aqueous solution as in Example 1 at 60 ° C. for 30 seconds, then washed with water and dried, and then subjected to the same non-Cr coating type treatment and coating as in Example 3.

[Measurement of Fe ratio]
The measurement of the Fe ratio was determined from the composition of the oxide film on the surface of the stainless steel plate after electrolytic treatment obtained by an XPS apparatus (ESCA5500MC manufactured by ULVAC-PHI Co., Ltd.). Measurement was performed at 15 kV-30 mA using MgKα rays as the X-ray source.
[Measurement of surface gloss]
For the measurement of the surface glossiness, a gloss meter (VG2000 manufactured by Nippon Denshoku Industries Co., Ltd.) was used, and the 60-degree specular gloss of the clear coated steel sheet was measured according to the specular gloss measurement method defined in JISZ8741.
[Evaluation of coating film adhesion]
For coating film adhesion, the sample piece after coating is bent 180 degrees with a stainless steel plate with the same thickness (0.4 mm) as the sample (1 t), and then an adhesive tape is applied to the coating film outside the bent part. Evaluation was made based on the state of adhesion of the coating film when peeled off instantaneously. Evaluation was made on a 6-point scale, with 5 indicating no peeling of the coating film and 0 indicating peeling of the entire surface.

As shown in the measurement results in Table 1, when alkaline anodic electrolysis is performed under the conditions of the present invention, the Fe ratio, which was initially 0.11, increases to 0.4 or more, and the coating adhesion is 4 to 4 However, the surface gloss is hardly lowered. This is because chemically inactive oxides such as Cr and Si concentrated on the surface of the stainless steel plate were selectively removed, and the ratio of chemically active Fe oxide was relatively increased. On the other hand, in the case of the comparative example, there is almost no decrease in the surface glossiness, but there is no increase in the Fe ratio, and no improvement in coating film adhesion is observed. In the case of alkaline cathode electrolysis (Comparative Example 2), a slight increase in the Fe ratio is observed, but the effect of improving coating film adhesion is insufficient.

As described above, when the stainless steel plate subjected to the bright annealing treatment as the final step is subjected to an anodic electrolysis treatment in an alkaline aqueous solution having a pH of 12 or higher, the reactivity with the pretreatment liquid is good, and as a result, the coating film adhesion is good. A bright annealed stainless steel sheet is obtained. When this stainless steel plate is subjected to clear coating, a coated stainless steel plate excellent in design properties suitable for fields such as exterior materials, interior materials, and home appliances can be obtained.

Claims (2)

A pre-painting method for a bright annealed stainless steel sheet, characterized by performing an anodic electrolytic treatment in an alkaline aqueous solution having a pH of 12 or more. A bright annealed stainless steel sheet for coating, comprising a surface oxide film having a composition with an Fe ratio of 0.4 or more, obtained by the method of claim 1.