AU2005201315B2

AU2005201315B2 - Clear-coated stainless steel sheet

Info

Publication number: AU2005201315B2
Application number: AU2005201315A
Authority: AU
Inventors: Haruki Ariyoshi; Masayoshi Furukawa; Osamu Oda; Naoto Ono
Original assignee: Nippon Steel Stainless Steel Corp
Current assignee: Nippon Steel Stainless Steel Corp
Priority date: 2004-03-31
Filing date: 2005-03-24
Publication date: 2006-11-02
Anticipated expiration: 2025-03-24
Also published as: JP2011136565A; AU2005201315A1; KR20060044811A; JP5289472B2; TW200538278A; CN100450770C; JP5542706B2; NZ539078A; TWI296566B; KR100642174B1; CN1676318A; JP2011136566A

Description

-la- Clear-Coated Stainless Steel Sheet BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to clear-coated stainless steel sheets for use in such applications as electrical household appliances, construction materials, and automotive components, which are treated by a non-chromate treatment and are excellent in press formability, weather resistance, rain streaking resistance, cleaner resistance, and chemical resistance.

Background Art It was formerly the practice, when press-forming sheet stock such as stainless steel sheet, to carry out forming after applying a lubricating oil to the sheet and to remove the lubricating oil with a solvent or an alkali degreaser following the forming operation.

However, the use of such lubricants degrades the pressing environment. In addition, because the fluorocarbons and organic solvents used as degreasers deplete the ozone layer, the use of such degreasers was restricted tighter and ultimately banned. As for the use of lubricating oils, due in part to problems associated with waste fluid treatment and the working environment, there has emerged a desire for clear-coated stainless steel sheets which can be press-formed without the application of oil thereto, eliminating the need for degreasing, thus take good care of the earth's environment, and yet have excellent lubricity. With regard to clear-coated stainless steel sheets that are used in such applications as electrical household appliances, owing to what is known as "sick house" syndrome and related problems, there exists a desire for clear-coated stainless steel sheets which are formaldehyde-free and chromate-free. Also, in connection with -2construction materials, there exists a desire for clear-coated stainless steel sheets which are excellent in weather resistance, rain streaking resistance, and resistance to acid rain.

Clear-coated stainless steel sheets having excellent press formability in current use include those which are obtained, as described in Japanese Patent Application, First Publication No. 2001-149860, by carrying out chromate treatment involving an tr application of a chromate solution, and thermally curing an epoxy-modified polyester r, resin, ether- and ester-based urethane resin having a bisphenol skeleton, an ester skeleton, or carboxyl groups, or acrylic resin with a curing agent such as melamine resin to disperse a polyolefin wax within a resulting coating. Because such clear-coated stainless steel sheets of excellent press formability contain a bisphenol-type skeleton, they have a poor weather resistance, limiting their applications. Moreover, in indoor applications, these clear-coated stainless steel sheets of prior-art cause problems such as sick house syndrome because the melamine resin serving as the curing agent contains formaldehyde, which formaldehyde is released in the curing process and is thus present within the coating, and because hexavalent chromium, albeit in a very small amount, is mixed in the chromate treatment.

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

n -2a- SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an environmentally friendly clear-coated stainless steel sheet which is excellent in coat clarity, weather n resistant, rain streaking resistance, scratch resistance, cleaner resistance, and chemical resistance, and a manufacturing method therefore.

O

As a result of investigations on the properties of thermosetting resins, waxes, chemical conversion agents, and coating conditions, we have arrived at an environmentally friendly clear-coated stainless steel sheet and a manufacturing methoc thereof.

Accordingly, the present invention provides a clear-coated stainless steel sheet including a stainless steel sheet, a bottom layer formed on the steel sheet, and a top layer formed on the bottom layer, wherein the bottom layer is made of a chemical conversion coating including an aminosilane and/or an epoxysilane, the chemical conversion coating has a coating weight of 2 to 50 mg/m 2 the top layer is made of a clear coating including a thermosetting resin which includes an acrylic resin having cross-linkable functional groups as a major constituent and a blocked isocyanate compound for cross-linking and curing the acrylic resin, the acrylic resin has a glass transition point of 30 to 90 0 C and a number-average molecular weight of 3,000 to 50,000, and the clear coating has a thickness of 1 to 10 rtm.

According to the present invention, there can be obtained a environmentally friendly clear-coated stainless steel sheet which is formaldehyde-free and chromate-free, and yet is excellent in coat clarity, weather resistance, rain streaking resistance, scratch resistance, cleaner resistance, and chemical resistance. Therefore, clear-coated stainless steel sheets which lend themselves well to use in electrical household appliances and as construction materials, both as interior finishing materials and also as exterior finishing materials, can be provided.

In the clear-coated stainless steel sheets of the invention, the clear coating may include 0.25 to 5 parts by weight of solid contents of polyolefin wax per 100 parts by weight of solid contents of the thermosetting resin.

A particle diameter of the polyolefin wax may be in a range of 0.1 to 7 [tm.

PREFERRED EMBODIMENTS In the practice of the invention, the chemical conversion coating includes an aminosilane and/or an epoxysilane, and is formed by subjecting a treatment in which a coating weight is controlled to be 2 to 50 mg/m 2 (weight of SiO 2 as measured by fluorescent x-ray analysis), then baking and drying at a stainless steel sheet material surface temperature (MT) of 60 to 140 0 C. In the case in which the coating weight is more than 50 mg/m 2 amount of water-soluble components in the chemical conversion coating increases, leading to blister formation on the surface of the coating. The coating weight is preferably 2 to 10 mg/m 2 Chemical conversion treatment is carried out using a chemical conversion solution containing an aminosilane coupling agent and/or an epoxysilane coupling agent.

Examples of the aminosilane coupling agent include N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, and 3-aminopropyltrimethoxysilane.

Examples of the epoxysilane coupling agent include 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, and 3-glycidoxypropylmethyldiethoxysilane. The chemical conversion solution may also contain a polyethylene wax and a silica aggregate.

The chemical conversion solution can be coated by a suitable technique such as spraying, roll coating, curtain flow coating, or electrostatic coating.

Drying is at least for vaporizing moisture, and is typically carried out at a temperature of 60 to 140 0 C. If necessary, a known pretreatment such as alkali degreasing or an acid or alkali etching may be carried out at the time of this chemical conversion treatment.

Chromate conversion coatings contain hexavalent chromium, albeit in a small amount. Not only is this harmful, when a chromate conversion coating is formed, surface of the stainless steel takes on a yellow tinge. In contrast, the chemical conversion coating in the present invention is clear and colorless. Moreover, when forming a clear coating having a thickness of 1 to 10 pm on this chemical conversion coating and baking at a stainless steel material surface temperature (MT) of 190 to 240 0 C, there can be obtained a clear-coated steel sheet which has a high degree of whiteness and also has excellent weather resistance and excellent rain streaking resistance required of construction material products. This rain streaking resistance, while not fully understood, is thought to arise as follows. Trace amounts of water-soluble components in the chemical conversion coating is oriented in the coating surface. At the same time, the coating surface has a high hardness. As a result, even when contaminants such as waste ash adhere on the surface, substantially no penetration into the coating interior occurs. Instead, the contaminants are washed away together with the water-soluble components oriented in the coating surface, resulting in an excellent resistance to rain streaking.

The clear coating in the present invention includes a thermosetting resin which includes an acrylic resin having one or more cross-linkable functional group selected from among hydroxyl group, carboxyl group, and alkoxysilane groups, as a major constituent. The acrylic resin can be obtained by a known method of preparing coating resins.

Examples of the acrylic resin that may be used include aliphatic or cyclic acrylates such as methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, n-hexyl methacrylate, cyclohexyl acrylate, cyclohexyl methacrylate, and lauryl methacrylate. These resins can be obtained by reacting one or more non-functional monomer selected from among vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, and n-butyl vinyl ether; styrene monomers such as styrene and ca-methylstyrene; and acrylamide monomers such as acrylamide, N-methylolacrylamide, and diacetone acrylamide with one or more polymerizable monomer having a cross-linkable functional group such as a hydroxyl group, carboxyl group, or alkoxysilane group.

Polymerizable monomers having hydroxyl group are monomers having one or more hydroxyl group and one or more polymerizable unsaturated double bond per one molecule, and examples of which include hydroxyalkyl esters such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl acrylate, and hydroxypropyl methacrylate, and polymerizable vinyl monomers having lactone-modified hydroxyl group such as Praccel FM1 to FM5 and Praccel FA-1 to FA-5 (available from Daicel Chemical Industries, Ltd.).

Polymerizable monomers having carboxyl group are compounds having one or more carboxyl group and one or more polymerizable unsaturated double bond per one molecule, and examples of which include acrylic acid, methacrylic acid, itaconic acid, maleic acid and fumaric acid.

Polymerizable monomers having alkoxysilane group are compounds having one or more alkoxysilane group and one or more polymerizable unsaturated double bond per one molecule, and examples of which include vinyltrimethoxysilane, vinyltriethoxysilane, and methacryloxypropyltrimethoxysilane.

As noted above, the acrylic resin has a cross-linkable functional group such as hydroxyl group, carboxyl group, or alkoxysilane group. The acrylic resin may have two or more cross-linkable functional groups per one molecule. Moreover, a glass transition point of this resin is preferably in a range of 30 to 90 0 C, and more preferably 50 to 90 0

C.

In the case in which the glass transition point of the resin is less than 30 0 C, when a temperature at a surface of a steel sheet having a coating of the resin rises to 80 to 100°C on account of heat generated by friction and plastic working of the steel sheet during continuous pressing, the coating will soften and the resin from the coating will stick to the die. On the other hand, in the case in which the resin glass transition point is more than 90 0 C, the resin has a poor workability during coating, resulting in problems such as pinhole formation and poor leveling. A number-average molecular weight of the acrylic resin is preferably in a range of 3,000 to 50,000, and more preferably 4,000 to 10,000.

Since the number-average molecular weight of the acrylic resin is coupled with the glass transition point of the resin, the glass transition point can fall within the preferred range mentioned above by setting the molecular weight in the foregoing range.

The cross-linking agent serving as another constituent of the acrylic resin-based thermosetting resin component is a blocked isocyanate compound.

The blocked isocyanate compound is a compound having two or more isocyanate groups per one molecule, and examples of which include any of the following polyisocyanates which has been blocked with a blocking agent such as a phenol compounds, oxime compounds, activated methylene compounds, s-caprolactam compounds, triazole compounds, or pyrazole compounds: aromatic diisocyanates such as tolylene diisocyanate, diphenylmethane diisocyanate, xylylene diisocyanate, and naphthalene diisocyanate; aliphatic diisocyanates such as hexamethylene diisocyanate and dimer acid diisocyanate; alicyclic diisocyanates such as isophorone diisocyanate and cyclohexane diisocyanate; and the biuret-type addition products or isocyanurate ring-type addition products of the foregoing isocyanates. Organotin catalysts such as dibutyltin dilaurate may be used as a dissociation accelerator for the blocking agent. Examples of commercially available products include products of Sumika Bayer Urethane Co., Ltd., such as Desmodur BL1100, BLl265MPA/X, VPLS2253, BL3475BS/SN, BL3272MPA, BL3370MPA, BL4265SN, Desmosam 2170 and Sumidur 3175; products of Asahi Chemical Industry Co., Ltd., such as Duranate 17B-60PX, TPA-B80X, MF-B60X and products of Dainippon Ink and Chemicals, Inc., such as Burnock DB-980K, D-550, B3-867, and B7-887-60; and products of Nippon Polyurethane Industry Co., Ltd., such as Coronate 2515, 2507, and 2513, and one or a mixture of two or more selected from foregoing products can be used.

The relative proportions of the resin and the cross-linking agent in the acrylic resin-based thermosetting resin composition can be varied within a broad range according to the intended purpose. Specifically, it is desirable for the amount of isocyanate groups to be set in a range of 0.3 to 2.0 molecules per one molecule of total of OH group and COOH group in the acrylic resin. End groups and side chains in one molecule of a base resin can be selected from hydroxyl groups, carboxyl groups, and alkoxysilane groups.

The clear coating may also include a curing catalyst to promote curing.

Examples of the curing catalysts include dibutyltin dilaurate and tetra-n-butyl-1,3-diacetoxy distannoxane. If necessary, such catalysts may be used in admixture. A coating preparation used to form the clear coating may also include additives such as leveling agents, foam inhibitors, antioxidants, ultraviolet absorbers, delusterants, and silane coupling agents. Moreover, a pigment or dye may additionally be dispersed in the coating preparation to give a colored clear coating. If necessary, other resin such as epoxy resin, silicone resin, fluororesin, or polyester resin may also be included.

In the present invention, it is preferable for the above thermosetting resin composition to include 0.25 to 5.0 parts by weight of solid contents of polyolefin wax per 100 parts by weight of solid contents of the above thermosetting resin composition.

Examples of the polyolefin waxes include hydrocarbon waxes such as paraffin, 9 microcrystalline waxes, polyethylene waxes, and fluorine-containing polyethylene waxes. Since when working the clear-coated stainless steel sheet of the present invention, a temperature of the clear coating rises on account of heat generated by the working operation and friction, a melting point of the wax is preferably in a range of to 160 0 C. In the case in which the melting point is less than 70 0 C, the wax softens and melts when the clear-coated stainless steel sheet is worked, making it impossible for the wax to exhibit good properties as a solid lubricating additive. On the other hand, in the case in which the melting point is higher than 160 0 C, hard particles are present in the surface, lowering the friction characteristics, and thus making it impossible for the clear-coated stainless steel sheet to achieve a high workability.

An acid value of the polyolefin wax is preferably in a range of 0 to 30. In the case in which the value is more than 30, the wax has a good compatibility with the resin and does not rise uniformly to the surface of the coating, thus making it impossible for the clear-coated stainless steel sheet to achieve a sufficient workability.

A particle diameter of these wax particles is preferably in a range of 0.1 to gm. Wax particles of which the diameter is more than 7.0 pm should be avoided because distribution of the solidified wax does not become uniform. On the other hand, in the case in which the particle diameter of less than 0.1 pm, the clear-coated stainless steel sheet does not achieve a sufficient workability. The amount of solid contents of polyolefin wax is preferably in a range of 0.25 to 5.0 parts by weight per 100 parts by weight of resin solids of the thermosetting resin composition. In the case in which the amount of solid contents of polyolefin wax is less than 0.25 part by weight, the clear-coated stainless steel sheet does not achieve a sufficient workability. In the case in which the amount of solid contents of polyolefin wax is more than 5.0 parts by weight, the surface of the coating is uneven, compromising the appearance of the clear-coated stainless steel sheet.

Examples Examples of the present invention and comparative examples are explained below. In the examples, "parts" and "percent" are by weight.

A four-neck flask provided with a thermometer, a reflux condenser, a stirrer, a dropping funnel, and a nitrogen inlet was charged with specific amounts of toluene and butyl acetate as shown in Table 1. The temperature was then raised to 110°C and, while blowing in nitrogen and stirring the flask contents, a mixture of methyl methacrylate, n-butyl methacrylate, 2-hydroxyethyl methacrylate, methyl acrylate and azobisisobutyronitrile (AIBN) was added in drops into the flask over a period of three hours. After addition was completed, azobisisobutyronitrile was further added and the reaction was continued at the same temperature for three hours, ultimately yielding an acrylic copolymer having a non-volatiles content of A coating material for a top layer was prepared by formulating a clear coating material as shown in Table 1.

Table 1 Thermosetting Resin A-1 A-2 A-3 A-4 R-1 Ai A-2 A-3 A-4 R-1 Toluene 25.0 25.0 25.0 25 0 25 0 250 Acrylic resin Butyl acetate 24.0 24.0 24.3 24.3 23.0 24.5 Methyl methacrylate 16.0 16.0 29.0 29.0 5.0 40.5 Styrene 5.0 5.0 5.0 5.0 n-butyl methacrylate 19.5 19.5 6.5 6.5 35.5 methacrylate 2-hydroxyethyl methacrylate 9.0 9.0 9.0 9.0 2-hydroxyethyl acrylate Methyl acrylate 0.5 0.5 0.5 0.5 0.5 Azobisisobutyro- 1.0 1.0 0.7 0.7 2.0 0.5 nitrile (AIBN) 1.0 1.0 07 0.7 20 Average molecular weight 5,000 5,000 20,000 20,000 2,500 60,000 Glass transition Glass transition 60 60 80 80 20 100 point 0

C

Blocked Blocked Desmodur componad VPLS2253 (NCO 20 40 20 40 20 compound content, 10.5%) (crosslinker) content A polyester/melamine resin coating material (Kawakami Toryo KK) was used as the clear coating material in some of the comparative examples.

SUS 430 stainless steel sheets with a No. 4 polished finish were used as metal sheets.

The chemical conversion coating solution was applied onto the stainless steel sheet using a roll coater under the condition that a SiO 2 weight on the stainless steel sheet be 2 to 50 mg/m 2 as determined by fluorescent x-ray analysis, then dried at a steel sheet temperature (MT) of 100 0 C. Next, the clear coating material was applied thereon using a bar coater and baked at a sheet material temperature (MT) of 193 0 C, giving the clear-coated stainless steel sheets in Examples No. 1 to 25 of the present invention shown in Tables 2-1 and 2-2.

For the sake of comparison, clear-coated stainless steel sheets with clear coatings that lack the characteristics of the present invention, clear-coated stainless steel sheets having chromate conversion coatings as chemical conversion coatings, and clear-coated stainless steel sheets lacking chemical conversion coatings were treated in the same way as in the above-described examples of the present invention, thereby giving the stainless steel sheets in Comparative Examples No. 26 to 37 shown in Tables 3-1 and 3-2.

The resulting specimens were all subjected to tests of the coat clarity, scratch resistance, rain streaking resistance, cleaner resistance, chemical resistance, and weather resistance, and to an Erichsen test. The results are shown below in Tables 2-1, 2-2, 3-1, and 3-2.

2005201315 24 Mar 2005 Table 2-1 Examples of the Invention S Chemical conversion coating Clear coating Polyolefin wax No. YsNCotnmaeil Coating weight T Thickness Amount of wax Particle YsNCotnmaeil(mg/inyp 2 (pbw) size 1 yes aminosilane 2 A-i 2 2 yes aminosilane 10 A-i 2 3 yes aminosilane 30 A-i 2 4 yes aminosilane 50 A-i 2- yes epoxysilane 2 A-i 2 6 yes epoxysilane 10 A-i 2 7 yes* epoxysilane 30 A-1 8 yes epoxysilane 50 A-i 2 9 yes aminosilane +epoxysilane 2 A-i 2- yes amninosilane +epoxysilane 10 A-i 2 11 yes aminosilane +epoxysilane 30 A-i 2 12 yes aminosilane +epoxysilane 50 A-i 13 yes aminosilane 10 A-2 2- 1.4 yes aminosilane 10 A-3 2 yes amninosilane 10 A-4 2 16 yes aminosilane 10 A-i I 17 yes aminosilane 10 A-2 I 18 yes aminosilane 10 A-3 10- 19 yes aminosilane 10 A-4 10 yes aminosilane 10 A-1 2 0.3 21 yes epoxysilane 10 A-i 2 2 22 yes aminosilane +epoxysilane 10 A-i 2 5 23 yes aminosilane 10 A-2 2 0.3 2.S 24 yes epoxysilane 10 A-3 2 2 yes aninosilane +epoxysilane 10 A-4 2 5 2005201315 24 Mar 2005 Table 2-2 Examples of the Invention Scratc C peties Coat clarity 0.6 0.5 resistance 5H Rain streaking resistance 5 41 Cleanser resistance Chemical resistance ~1* Weather resistance Formability Salt water spray Overall rating Release of formaldehyde, hexavalent Remarks 2 3 41 resistance i 1 A1 F I 2, ood (Joodnone

A

0 4 5 n^A ~none 0.3 5H 5 5 5 5 8.6 4 Good none 6 0.5 6H 5 5 5 5 8.5 4 Good none_____ 7 0.4 5H1 5 5 5 5 8.4 5 Good none_____ 8 0.3 5H1 5 4 5 5 8.5 5 Good none 9 0.5 6H 5 5 5 5 8.4 5 Good none 0.3 511 5 5 5 5 8.6 5 Good none_____ 11 0.6 5H 5 4 5 5 8.5 5 Good none 12 0.8 5H1 5 5 5 -5 8.4 5 Good none 13 0.6 5H 5 4 5 5 8.5 5 Good none 14 0.5 6H 5 5 5 5 8.3 5 Good none 0.8 5H4 5 5 4 8.2 4 Good none 16 0.5 5H 5 5 5 5 8.5 5 Good none_____ 17 0.6 511 5 5 5 5 8.3 5 God none_____ 18 0.4 611 5 5 5 5 8.4 5 Good none_____ 19 0.2 611 4 4 5 5 10 5 Very good none 0.5 5H 5 5 5 5 9.7 5 Very good none 21 .0.4 5H 5 5 5 5 9.8 4 Very good -none 22 0.6 511 4 5 5 5 10 4 Very good none 23 0.5 5H 5 5 5 5 9.9 5 Very good none 24 10.4 5H1 5 5 5 5 9.5 4 Very good none 125 10.6 1 S4 5. 9.5 4 Very good none 2005201315 24 Mar 2005 Table 3-1 Comparative Examples Chemical conversion coating No. CoatingCotn Yes/No Cating weight material (mg/rn 2 26 no 27 no 28 yes chromate 10 29 yeEhEmte 3 chromate 130 31 yes chromate 10 32 yes chromate 20 33 yes chromate 30 34 yes chmsiae 10 34 yes aminosilane 10 36 yes aminosilane 10 37 yes aminosilane 10 Clear.

Type A-1 A-2 A-1 A-i polyester/ melamine polyester/ -melamine polyester/ melamine polyester/ melamine polyester/ melamine polyester/ melamine B-i coating Thickness (gm) 2 2 2 2 Polyolefin wax Amount of wax Particle size 2 2 2 2 5 2 B-2 2 2005201315 24 Mar 2005 Table 3-2 Comparative Examples Coat properties___ No. Coat -waeRar No. Coaty resistance streaking Cleanser Chemical Weather Form- alt wat Overall clarity resistance resistance resistance resistance resistance ability spray rating 26--resistance Poor 26 -Poor Release of formaldehyde, hexavalent chromium I I Remarks poor adhesion poor adhesion none I I t -f Poor none 28 1.5 5H 5 5 5 5_ r9u ye -oor yes 29 2.8 5H 5 5 5 5 9.8 5 Poor yes 2 3H 3 3 1 2 9.3 3 Poor yes 31 1.8 3H 2 3 1 2 9.5 3 Poor yes 32 2.4 3H 3 3 1 2 9.7 3 Poor yes 33 3.1 4H 2 3 1 1 9.5 4 Poor yes 34 0.5 3H 2 3 1 2 9.5 3 Poor yes 0.6 3H 2 3 1 1 9.4 4 Poor yes 36 0.4 3H 5 5 5 5 9.8 5 Poor none pinholes, 37 0.3 6H 5 5 5 5 9.4 5 Poor none poor leveling The following methods were used to evaluate the coatings.

Coat Clarity A colored state of the coating was measured by a CR-300 color-difference meter (Minolta Co., Ltd.) as a color difference Ab value between a steel sheet stock baked at a sheet temperature of 193 0 C and the clear-coated stainless steel sheet. Here, a larger Ab indicates greater yellowness.

Scratch Resistance The test specimens were rated by the pencil scratch test method described in JIS K5400. Here, a rating of 5H or higher by the peel method was regarded as a conforming article.

Rain Streaking Resistance The test specimens were subjected to a atmospheric exposure test for three months, and a degree to which rain streaks formed on the coating was rated.

No marks whatsoever (conforming) 4: Faint marks are visible (conforming) 3: Marks are rather conspicuous (defective) 2: Heavy marks remain (defective) 1: Peeling occurred (defective) Cleanser Resistance The test specimens were immersed in Magiclean (a cleaner made by Kao Corporation) at 40 0 C for 72 hours, after which a condition of the coating was examined and rated.

No marks whatsoever (conforming) 4: Faint marks are visible (conforming) 3: Marks are rather conspicuous (defective) 2: Heavy marks remain (defective) 1: Peeling occurred (defective) Chemical Resistance After dripping 2 mL of 5% sulfuric acid and 5% NaOH onto the test specimen, a cover was placed over the specimen, and the specimen were left for 16 hours. Then a condition of the coating was observed and rated.

No marks whatsoever (conforming) 4: Faint marks are visible (conforming) 3: Marks are rather conspicuous (defective) 2: Heavy marks remain (defective) 1: Peeling occurred (defective) Weather Resistance The gloss retention after 1,000 hours was rated by a sunshine weatherometer (SWOM) test.

5: Gloss retention of 80 to 100% (conforming) 4: Gloss retention of 60 to 80% (conforming) 3: Gloss retention of 40 to 60% (defective) 2: Gloss retention of 20 to 40% (defective) 1: Gloss retention of 0 to 20% (defective) Erichsen Value Erichsen Value was rated in accordance with JIS Z2247. Since an Erichsen value of 9.3 mmn was obtained when graphite grease was used for SUS430 stainless sheet, values higher than this were regarded as acceptable.

Salt Water Spray Resistance Test Crosscuts were made in the coating, and a salt water spray test was carried out for 500 hours according to the method described in JIS Z2371, followed by visually examining and rating a degree of peeling of the coating.

5: Degree of peeling of less than 5% (conforming) 4: Degree of peeling of 5%/o or more and less than 10% (conforming) 3: Degree of peeling of 10% or more and less than 15% (defective) 2: Degree of peeling of 15% or more and less than 20% (defective) 1: Degree of peeling of 20% or more (defective) As is apparent from the above description and examples, the present invention provides a clear-coated stainless steel sheets which releases no formaldehyde or hexavalent chromium and takes good care of the earth's environment, and yet is excellent in coat clarity, scratch resistance, rain streaking resistance, cleaner resistance, chemical resistance, and weather resistance. Moreover, by including 0.25 to 5 parts by weight of solid contents of polyolefin wax per 100 parts by weight of solid contents of thermosetting resin, there can be obtained a clear-coated stainless steel sheet of excellent lubricity which have a better formability than a stainless steel sheet coated with an oil-based lubricant.

In contrast, in the comparative examples, adhesion of the clear coating is poor in the absence of chemical conversion treatment. Chromate conversion-coated steel sheets take on a yellow tinge and contain hexavalent chromium, making them environmentally undesirable. Furthermore, a steel sheet of prior-art coated with a polyester/melamine resin coating is inferior in scratch resistance, rain streaking resistance, cleaner resistance, chemical resistance, and weather resistance, and may also release formaldehyde. In the case in which the acrylic resin in the thermosetting resin has an average molecular weight outside a range of 3,000 to 50,000, or a glass transition point outside a range of 30 to 900C, the clear-coated steel sheet has a number of drawbacks, including a poor scratch resistance and a poor manufacturability.

The present invention provides a environmentally friendly clear-coated stainless steel sheet which is formaldehyde-free and chromate-free, and yet is excellent in coat clarity, weather resistance, rain streaking resistance, scratch resistance, cleaner resistance, and chemical resistance. The clear-coated stainless steel sheet lends themselves well to use in electrical household appliances and as construction materials, both as interior finishing materials and also as exterior finishing materials.

Claims

1. A clear-coated stainless steel sheet comprising: a stainless steel sheet; a bottom layer formed on the steel sheet; and a top layer formed on the bottom layer, wherein the bottom layer is made of a chemical conversion coating comprising an aminosilane and/or an epoxysilane, the chemical conversion coating has a coating weight of 2 to 50 mg/m 2 the top layer is made of a clear coating comprising a thermosetting resin including an acrylic resin having cross-linkable functional groups as a major constituent and a blocked isocyanate compound for cross-linking and curing the acrylic resin, the acrylic resin has a glass transition point of 30 to 90 0 C and a number-average molecular weight of 3,000 to 50,000, and the clear coating has a thickness of 1 to 10 rim.

2. The clear-coated stainless steel sheet according to claim 1, wherein the clear coating comprises 0.25 to 5 parts by weight of solid contents ofpolyolefin wax per 100 parts by weight of solid contents of the thermosetting resin.

3. The clear-coated stainless steel sheet according to claim 2, wherein a particle diameter of the polyolefin wax is in a range of 0.1 to 7 pm. 22

4. A clear-coated stainless steel sheet substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples. DATED this 24 h Day of March 2005 Shelston IP Attorneys for: Nippon Steel Sumikin Stainless Steel Corporation