NZ555633A

NZ555633A - Clear-coated stainless steel sheet excelling in design and process for producing the material

Info

Publication number: NZ555633A
Application number: NZ555633A
Authority: NZ
Inventors: Masuhiro Fukaya; Naoto Ono; Haruki Ariyoshi
Original assignee: Nippon Steel & Sumikin Sst
Priority date: 2005-04-15
Filing date: 2006-04-14
Publication date: 2010-08-27
Also published as: AU2006237998A1; CN101098988A; TW200702453A; MY141238A; JP4606929B2; KR20070087630A; JP2006299299A; WO2006112376A1; TWI370847B; KR101018487B1; CN100585006C; TW201111523A; AU2006237998B2; TWI354029B

Abstract

A clear-coated stainless steel sheet excellent in whiteness which is a ferritic stainless steel sheet containing 0.2 to 0.8 mass percent of Si, 0.005 to 0.15 mass percent of Al and 0.1 to 0.7 mass percent of Nb, wherein the sheet has a surface oxide film where an atomic concentration ratio of Cr, Si, AI, and Fe ((Cr + Si +AI) / Fe) from its surface to a depth of 50 angstroms is 0.6 or more, a chemical conversion treatment agent of a coated film is composed of either one type or two types of an aminosilane type and/or epoxysilane type agent, a deposition amount of the chemical converting agent is between 2 and 20 mg/m2 (an amount of Si02 is measured by X-ray fluorescence), a clear coating material of the coated film is selected from a group consisting of polyester resin, acryclic resin, crylic urethane resin, and epoxy-modififed polyester resin, and a thickness of a clear coating material of the coated film is between 1 and 10 micrometers. Also disclosed is a method of manufacturing the above clear-coated stainless steel sheet.

Description

New Zealand Paient Spedficaiion for Paient Number 555633 555633 J DESCRIPTION CLEAR-COATED STAINLESS STEEL SHEET WITH EXCELLENT DESIGN PROPERTIES AND MANUFACTURING METHOD THEREOF TECHNICAL FIELD The present invention relates to a clear-coated stainless steel sheet with excellent design properties characteristic of metal materials, and which does not develop a yellow color at the time of coating, and is used for parts of home appliances, building materials, automobiles, and the like.

BACKGROUND ART Stainless steel has been used in a bare state in the fields of home appliances, kitchen utensils, automobiles, and building materials since it has excellent design properties and corrosion resistance. Recently, while making use of the excellent design properties of stainless steel such as metallic luster, application of clear-coated stainless steel sheets to which properties are added such as anti-staining properties and corrosion resistance has increased in the field of home appliances.

Because of their design properties, clear-coated stainless steel sheets dyed with pigments and the like and clear-coated stainless steel sheets to which pearlescent and metallic pigments are added are also being applied in addition to colorless clear-coated stainless steel sheets. However, the addition of these pigments reduces the transparency of a clear film and reduces the texture of the metallic luster characteristic of stainless steel.

On the other hand, with the colorless clear-coated stainless steel sheets, there is a problem in that they turn yellowish because of chromate treatment, which is a pretreatment, and design properties are impaired and the sheets may appear blackish depending on the viewing angle. To address these problems, Patent Documents 1 and 2 discuss the composition of a treatment solution and an amount of coating and Patent Document 3 discusses improvements in the outer appearance of ground stainless steel sheets and in anti-staining properties and seizure resistance of coated films due to phosphate and silica.

Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H05-106057 555633 2 HI 1-269660 Patent Document 3: Japanese Unexamined Patent Application, First Publication No. H08-281864 Patent Document 4: Japanese Unexamined Patent Application, First Publication No. S56-259 Patent Document 5: Japanese Unexamined Patent Application, First Publication No. S62-156253 Patent Document 6: Japanese Unexamined Patent Application, First Publication No. HI 1-254585 DISCLOSURE OF INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION Clear-coated stainless steel sheets excellent in whiteness and with high transparency cannot be achieved by improvements in the chromate treatment method alone as described above. The present invention addresses conditions for chemical conversion treatment, characteristics of material surface, and coating conditions, and to provide clear-coated stainless steel sheets excellent in whiteness, and a manufacturing method thereof.

MEANS FOR SOLVING THE PROBLEM As a result of studies by the present inventors it was revealed that yellowish coloring (yellowing) of colorless clear-coated stainless steel sheets is caused not only by chromate treatment but also by color development of temper color occurring on a surface of stainless steel sheets at the time of baking the clear coating film. It has also become clear that by changing the chemical conversion treatment from chromate treatment to non-chromate treatment and optimizing the thickness of the chemical conversion treatment film and further by using stainless steel sheets excellent in temper color resistance as stainless steels applied to clear-coated stainless steel sheets, it is possible to realize the prevention of yellowing at the time of clear coating.

It is known that the temper color of stainless steel sheets can be suppressed by forming an oxide film containing SK>2 and AI2O3 on the sheet surface by subjecting stainless steel sheets with an increased Si or A1 content to bright annealing. In other words, an oxide film with concentrated Si and A1 is readily formed accompanied by an increase in Si or A1 content and temper color resistance improves. As such stainless steel, high Si steel (Patent Documents 4 and 5) and high A1 steel (Patent Document 5) 555633 3 and the like are known. Moreover, Patent Document 6 describes a stainless sheet known as a heat resistant transparent coating stainless steel sheet where the content of A1 or Si on a surface of ferritic stainless steel sheet is controlled between 4.5 and 6 atomic %.

As a result of extensive study on means for solving the aforementioned problems, the present inventors established clear-coated stainless steel sheets using ferritic stainless steel sheets excellent in whiteness and a manufacturing method thereof by examining conditions for chemical conversion treatment, characteristics of the material surface, and coating conditions.

The essence of the present invention is as follows. (1) A clear-coated stainless steel sheet excellent in whiteness which is a ferritic'stainless steel sheet containing 0.2 to 0.8 mass% of Si, wherein the sheet has a surface oxide film where an atomic concentration ratio of Cr, Si, Al, and Fe, that is, (Cr + Si + Al) / Fe, from its surface to a depth of 50 A is 0.6 or more, a chemical conversion treating agent of a coating film is formed of either one type or two types of an aminosilane type and/or epoxysilane type agent, a deposition amount of the chemical conversion treating agent is between 2 and 20 mg/m2 (an amount of SiC>2 is measured by X-ray fluorescence), and a thickness of a clear coating material of the coating film is between 1 and 10 \mx. (2) The clear-coated stainless steel sheet excellent in whiteness according to (1) wherein the stainless steel sheet further contains 0.005 to 0.15 mass% of Al. (3) The clear-coated stainless steel sheet excellent in whiteness according to (1) wherein the stainless steel sheet further contains 0,1 to 0.7 mass% of Nb. (4) A clear-coated stainless steel sheet excellent in whiteness which is a ferritic stainless steel sheet containing 0.2 to 0.8 mass% of Si, 0.005 to 0.15 mass% of Al and 0.1 to 0.7 mass% of Nb, wherein the sheet has a surface oxide film where an atomic concentration ratio of Cr, Si, Al, and Fe, that is, (Cr + Si + Al) / Fe, from its surface to a depth of 50 A is 0.6 or more, a chemical conversion treating agent of a coating film is formed of either one type or two types of an aminosilane type and/or epoxysilane type agent, a deposition amount of the chemical conversion treating agent is between 2 and 20 mg/m2 (an amount of SiC>2 is measured by X-ray fluorescence), and a thickness of a clear coating material of the coating film is between 1 and 10 jam. (5) A manufacturing method of the clear-coated stainless steel sheet excellent in whiteness according to (1) wherein annealing is carried out where a final bright annealing temperature T1 (°C) and an annealing atmosphere dew point T2 (°C) satisfy conditions described by formulae 1 and 2. 750 < T1 < 5 x T2 + 1200 (formula 1) -70 <T2 <-30 (formula 2) (6) A method for manufacturing a material for a clear-coated stainless steel sheet excellent in whiteness which is a ferritic stainless steel sheet containing 0.2 to 0.8 mass% of Si and 0.005 to 0.15 mass% of Al, wherein the sheet has a surface oxide film where an atomic concentration ratio of Cr, Si, Al, and Fe ((Cr + Si + Al) / Fe) from its surface to a depth of 50 A is 0.6 or more, a chemical conversion treatment agent of a coated film is composed of either one type or two types of an aminosilane type and/or epoxysilane type agent, a deposition amount of the chemical converting agent is between 2 and 20 mg/m2 (an amount of SiC>2 is measured by X-ray fluorescence), and a thickness of a clear coating material of the coated film is between 1 and 10 |jm, and wherein annealing is carried out under a condition where a final bright annealing temperature T1 (°C) and an annealing atmosphere dew point T2 (°C) satisfy formulae 1 and 2. 750 < T1 < 5 x T2 + 1200 (formula 1) -70 < T2 < -30 (formula 2) (7) A method for manufacturing a material for a clear-coated stainless steel sheet excellent in whiteness which is a ferritic stainless steel sheet containing 0.2 to 0.8 mass% of Si, 0.005 to 0.15 mass% of Al and 0.1 to 0.7 mass% of Nb, wherein the sheet has a surface oxide film where an atomic concentration ratio of Cr, Si, Al, and Fe ((Cr + r Si + Al) / Fe) from its surface to a depth of 50 A is 0.6 or more, a chemical conversion treatment agent of a coated film is composed of either one type or two types of an aminosilane type and/or epoxysilane type agent, a deposition amount of the chemical converting agent is between 2 and 20 mg/m2 (an amount of Si02 is measured by X-ray fluorescence), and a thickness of a clear coating material of the coated film is between 1 and 10 (am, and wherein annealing is carried out under a condition where a final bright annealing temperature T1 (°C) and an annealing atmosphere dew point T2 (°C) satisfy formulae 1 and 2. 750 < T1 < 5 x T2 + 1200 (formula 1) -70<T2 <-30 (formula 2) (8) A method for manufacturing a material for a clear-coated stainless steel sheet excellent in whiteness which is a ferritic stainless steel sheet containing 0.2 to 0.8 mass% of Si, wherein the sheet has a surface oxide film where an atomic concentration ratio of Cr, Si, Al, and Fe ((Cr + Si + Al) / Fe) from its surface to a depth of 50 A is 0.6 or more, a chemical conversion treatment agent of a coated film is composed of either one type or two types of an aminosilane type and/or epoxysilane type agent, a deposition amount of the chemical converting agent is between 2 and 20 mg/m2 (an 555633 amount of Si02 is measured by X-ray fluorescence), and a thickness of a clear coating material of the coated film is between 1 and 10 jim, wherein annealing is carried out under a condition where a final bright annealing temperature T1 (°C) and an annealing atmosphere dew point T2 (°C) satisfy formulae 1 and 2, and wherein the ferritic stainless steel sheet, which is cold rolled, is subjected to final bright annealing after surface polishing. 750 < T1 < 5 x T2 + 1200 (formula 1) -70 < T2 < -30 (formula 2) (9) The method for manufacturing the material for the clear-coated stainless steel sheet excellent in whiteness according to (6) wherein the ferritic stainless steel sheet, which is cold rolled, is subjected to final bright annealing after surface polishing. (10) The method for manufacturing the material for the clear-coated stainless steel sheet excellent in whiteness according to (7) wherein the ferritic stainless steel sheet, which is cold rolled, is subjected to final bright annealing after surface polishing.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a relationship between a deposition amount of the chemical conversion treating agent and a Ab* value indicating a degree of discoloration of a clear-coated stainless steel sheet.

Fig. 2 is a diagram showing a relationship between an atomic concentration ratio of Cr, Si, Al, and Fe, that is (Cr + Si + Al) / Fe, in an oxide film from its surface to a depth of 50 A and the discoloration Ab* value of a clear-coated stainless steel sheet due to baking at 224°C.

Fig. 3 is a diagram showing a relationship between baking temperatures of a polished stainless steel sheet and a stainless steel sheet subjected to bright annealing within a scope of the present invention after being polished and the discoloration Ab* value of a clear-coated stainless steel sheet.

Fig. 4 is a diagram showing a relationship among annealing temperature T1 (°C) and dew point T2 (°C) of bright annealing, and the atomic concentration ratio of Cr, Si, Al, and Fe, that is (Cr + Si + Al) / Fe, in the oxide film from its surface to the depth of 50 A.

BEST MODE FOR CARRYING OUT THE INVENTION As a result of studying pretreatment and characteristics of a material surface in design properties of a clear-coated stainless steel sheet, the present inventors discovered two important points described below in order to achieve a clear-coated stainless steel sheet excellent in whiteness. Firstly, an aminosilane type and/or epoxysilane type chemical conversion treating agent should be used instead of a chromate treatment agent as a chemical conversion treating agent for the pretreatment and the deposition amount of the chemical conversion treating agent should also be within an optimized range. Secondly, in a material for a stainless steel sheet, Cr, Si, and/or Al components are enriched on a surface thereof to prevent discoloration of the material due to temper coloring at the time of baking coating. Note that any of polyester resins, acrylic resins, acrylic urethane resins, epoxy-modified polyester resins, and the like may be adopted as a colorless clear coating material.

A clear chemical conversion treatment of the present invention will be described.

The clear chemical conversion treating agent used in the present invention is composed of either one type or two types of an aminosilane type and/or epoxysilane type agent. The reason for selecting an aminosilane type and/or epoxysilane type is that when they are used, adhesive property is satisfactory and the development of a yellow color can be prevented compared to a case where a chromate treatment agent is used. A stainless steel sheet is treated so that a deposition amount of the chemical conversion treating agent will be 2 to 20 mg/m2 (the amount of SiC>2 is measured by X-ray fluorescence) and is baked and dried when a surface temperature (MT) of the stainless steel sheet material is between 60 and 140°C.

Fig. 1 shows the results of an investigation of changes in color tones of clear-coated stainless steel sheet where the deposition amount of E-206 (aminosilane type) manufactured by Nihon Parkerizing Co., Ltd. is changed from 2 mg/m2 to 50 mg/m2 at 100°C, roll-coated with an acrylic urethane resin clear coating material so that the resulting coating film thickness will be 2 jj.m, and coated at a baking temperature of 200°C. By using a SUS430/bright annealing member where atomic concentration ratio (Cr + Si + Al) / Fe from the surface thereof to a depth of 50 A (described later) is 0.6 or more as a material for the stainless steel sheet used for clear coating, yellowing caused by temper coloring is reduced as much as possible. The vertical axis (Ab*) in Fig. 1 indicates values calculated by subtracting b* values of a steel sheet material (2) from b* values when the deposition amount of the chemical conversion treating agent is changed as shown by the horizontal axis and the clear coating material is coated by baking (1). In other words, Ab* = b*(l) - b*(2).

As a result, as shown in Fig. 1, when the deposition amount of the chemical conversion treating agent exceeds 20 mg/m2, gloss of the clear-coated stainless steel sheet is reduced and the sheet turns yellowish to some extent. When the deposition 555633 7 amount is 20 mg/m2 or less, the Ab* value will be 0.3 or less and changes in color tone will be small. On the other hand, when the deposition amount of the chemical conversion treating agent is less than 2 mg/m2, the film will be nonuniform and adhesion between the clear coating film and stainless steel is reduced. For this reason, the deposition amount of the chemical conversion treating agent in the present invention is set between 2 mg/m2 and 20 mg/m2. Note that by measuring the amount of Si02 by X-ray fluorescence on the surface of stainless steel sheet, the deposition amount of the chemical conversion treating agent can be determined.

A thickness of the clear coating film is in a range between 1 |im and 10 jxm. This is because when the thickness is 1 jam or less, it is difficult to achieve a stable color tone and when the thickness is 10 j.im or more, it is difficult to manufacture the film by one coating process and the cost of the coating film will be higher. Favorably, the thickness is in a range between 2 jam and 5 |am.

Materials for clear-coated stainless steel sheet of the present invention will be described next.

The present invention is characterized by using a steel sheet having a surface oxide film where the atomic concentration ratio of Cr, Si, Al, and Fe, that is, (Cr + Si + Al) / Fe, from its surface to a depth of 50 A is 0.6 or more to prevent temper coloring at the time of baking coating and to prevent yellowing. By using a ferritic stainless steel sheet containing 0.2 mass% or more of Si and by subjecting this steel sheet to bright annealing under a predetermined temperature and atmosphere, it is possible to achieve an atomic concentration ratio of the steel sheet surface within the favorable range described above.

The stainless steel for a clear-coated stainless steel sheet is a ferritic stainless steel sheet containing Si of 0.2 to 0.8 mass%. Since the diffusion rate of elements in the ferritic stainless steel is faster compared to that in an austenitic stainless steel, it is easy to concentrate Cr and Si in the film. Accordingly, the present invention is limited to a ferritic stainless steel sheet. The concentration is relatively easy to achieve especially in special kinds of steel (Cr-Al steel and Cr-Si steel) and is disclosed in Patent Documents 4 and 5. However, in the ferritic stainless steel of the present invention, in other words, the steel containing 0.2 to 0.8 mass% of Si, a technique described next in order to concentrate Cr, Si, and Al in the film will be essential.

In the present invention, since a surface oxide film is formed where Si is concentrated on the surface of the stainless steel sheet, it is essential that 0.2 mass% or more of Si is contained and in this case, the amount of Si includes that used as a deoxidizing element at the time of melting and manufacturing steel. An upper limit of 0.8% Si content particularly indicates that the present invention is a general purpose ferritic stainless steel. When more Si is contained than this, the component range of general purpose ferritic stainless steel is exceeded, for example, SUS430 or SUS 430 J1L where Nb is added. In other words, the addition of Si exceeding 0.8 mass% deteriorates workability due to an effect of solid solution hardening and is not suited for general purpose.

In the present invention, it is desirable to contain small quantities of Al between 0.005 mass% and 0.15 mass%. This is because, similar to Si, Al has an effect of suppressing an increase in the b* value at the time of baking a clear coating film by being concentrated in a film at the time of bright annealing as described below. Al equal to or more than 0.005 mass% is added as a deoxidizing element. The addition of Al in a large amount deteriorates workability. Accordingly, the desirable amount of Al to be added is between 0.005 and 0.15 mass%.

In the present invention, it is possible to set the content of Cr, which is a basic component of a ferritic stainless steel sheet, within a range of 16.0 to 20.0 mass%. Moreover, to make a clear-coated stainless steel sheet, the content of Cr should be as high as possible. This is because there is an effect to accelerate the concentration of Cr in the film at the time of bright annealing. Furthermore, in the present invention, it is desirable to contain 0.1 to 0.7 mass% of Nb. This is because oxides of Nb are considered to be more stable thermodynamically than those of Cr and have similar effects to those of Cr.

The technique in order to concentrate Cr, Si, and Al on the surface oxide film in the ferritic stainless steel containing 0.2 to 0.8 mass% of Si will be described next.

After cold-rolling annealing of SUS430, which is a ferritic stainless steel, a material subjected to a No.4 polish finish is bright-annealed and by changing the temperature and atmosphere in the bright annealing, the ratio of Cr, Si, Al, and Fe in the oxide film was changed. An aminosilane type chemical conversion treating agent is coated as a chemical conversion treatment onto the material whose atomic concentration ratio (Cr + Si + Al) / Fe is adjusted within a range of 0.15 to 3.5 so that the deposition amount of the agent will be 10 mg/m2. After baking and drying the resultant sheet at 100°C, an acrylic urethane resin clear coating material was roll-coated thereon so that the resulting coating film thickness will be 2 pm. Fig. 2 shows a relationship between atomic concentration ratio (Cr + Si + Al) / Fe and the Ab* value when baking temperature after coating with the clear coating material was 224°C. The degree of discoloration was evaluated by the Ab* value, which is a difference between b* values of the material after baking and those of a reference material (clear-coated stainless steel 555633 # sheet material). As is apparent from Fig. 2, in a material whose atomic concentration ratio (Cr + Si + Al) / Fe is adjusted to 0.6 or more, changes in color tone due to coating/baking are small and the degree of discoloration is low as indicated by the Ab* value, which is less than 1.

Subsequently, a material whose atomic concentration ratio (Cr + Si + Al) / Fe is adjusted to 0.6 or more and a material whose atomic concentration ratio (Cr + Si + Al) / Fe is adjusted to less than 0.6 were prepared and changes in color tones thereof were observed by changing the baking temperature from 150°C to 250°C. Apart from the differences in baking temperature and material type, other conditions are similar to those in the case of Fig. 2 described above. In Fig. 3, the atomic concentration ratio (Cr + Si + Al) / Fe of materials indicated by open circles (o) and open triangles (A) is set to approximately 3. For materials indicated by filled circles (•) and filled triangles (A ), the atomic concentration ratio (Cr + Si + Al) / Fe thereof is set to approximately 0.4 and 0.2, respectively. As a result, as shown in Fig. 3, in the material whose atomic concentration ratio (Cr + Si + Al) / Fe is adjusted to 0.6 or more by bright annealing, changes in color tone due to coating/baking are small. On the other hand, a material not subjected to bright annealing after being polish finished or a material treated under bright annealing conditions outside the scope of the present invention and having its atomic concentration ratio (Cr + Si + Al) / Fe adjusted to less than 0.6 exhibited more yellowish discoloration at the time of coating/baking and marked discoloration was observed especially when the baking temperature was 200°C or higher.

It should be noted that components in the surface oxide film are analyzed by Auger Electron Spectroscopy (AES) and the value of atomic concentration ratio (Cr + Si + Al) / Fe was defined as that at the point where oxygen concentration was the highest. In the present invention, it is extremely important to control the atomic concentration ratio (Cr + Si + Al) / Fe to 0.6 or more and to achieve a surface oxide film concentrated with Cr and/or Si. In this case, Si is concentrated in an outer layer of the surface oxide film and Al is concentrated in an inner layer, which is in a vicinity of bulk. This is because the bright annealing condition is a reducing condition for Cr whereas it is an oxidizing condition for Si and Al and dissociation pressure of AI2O3 is lower than that of Si02. Moreover, although an appropriate range of atomic concentration ratio (Cr + Si + Al) / Fe may be different depending on the measuring method of the surface oxide film, the required structure of the surface oxide film is the same as that described above.

The atomic concentration ratio (Cr + Si + Al) / Fe in the surface oxide films of a material subjected to cold-rolling and annealing and a polish finished material is 555633 approximately 0.2. Thus, the present inventors conducted bright annealing on a stainless steel sheet and studied the bright annealing condition to form an oxide film where the atomic concentration ratio of Cr, Si, Al, and Fe (Cr + Si + Al) / Fe from its surface to the depth of 50 A is 0.6 or more.

Although bright annealing is generally carried out in a reducing atmosphere where hydrogen and nitrogen are mixed in a proportionate range, which is H2:N2 = (1 to 9):1, the components in the oxide film change depending on the combinations of annealing temperature T1 (°C) and atmospheric dew point T2 (°C) at the time.

Fig. 4 shows the results where cold-rolling annealing or mechanical polishing corresponding to aNo.4 finish (US G 0203 No.5349) is carried out on cold-rolled steel sheets of SUS430J1L and SUS430 followed by bright annealing at various annealing temperatures T1 (°C) and atmospheric dew points T2 (°C). The horizontal axis indicates the annealing temperature T1 whereas the vertical axis indicates the atmospheric dew point T2 and ranges of T1 and T2 where the respective atomic concentration ratio (Cr + Si + Al) / Fe of the steel sheets from the surface to the depth of t 50 A is stable at 0.6 or more are described by the following formulae. 750 < T1 < 5 x T2 + 1200 (formula 1) -70 < T2 < -30 (formula 2) Within ranges described by the formulae 1 and 2, since it is a condition to reduce Cr on the one hand while it is a condition to oxidize Si and Al on the other, the concentration of active elements (Si, Al) in the film due to selective oxidation is carried out effectively. When the temperature T1 (°C) of bright annealing is lower than 750°C, diffusion of active elements (Si, Al) relative to Cr will be delayed and an oxide film with a sufficient atomic concentration ratio will not be achieved. Moreover, when the temperature T1 (°C) exceeds a temperature calculated by (5 * T2 + 1200)°C, selective oxidation of Si and Al will be suppressed since it will be a more reducing condition than the range described by the formula 1 and an oxide film with a sufficient atomic concentration ratio will not be achieved. Therefore, it is better that the range of the bright annealing temperature T1 (°C) be that described by the formula 1 determined from the relationship with the dew point. A favorable range of the temperature T1 is between 800°C and 1000°C and even more favorable between 800°C and 900°C.

When the dew point T2 (°C) of the bright annealing exceeds -30°C, oxidation of Fe will be severe and since the concentration of Fe in the surface oxide film cannot be avoided and discoloration occurs due to temper color, it is not suited as the material for a clear-coated stainless steel sheet. Moreover, a gas dew point lower than -70°C is technically extremely difficult to achieve. Therefore, the dew point T2 (°C) of the 555633 11 bright annealing is favorably within the range described by the formula 2 and is favorably -60°C or higher and lower than -35°C.

Although the material described in document 5 is described to have less temper coloring under Si concentration of 0.3 %, examples in the same document under a Si concentration of 0.3 % are graded 3 (yellow) when temper color was evaluated by 5 scales and cannot be used for a clear-coated stainless steel sheet excellent in whiteness. In the present invention, by clearly specifying the atomic concentration ratio (Cr + Si + Al)/Fe in the oxide film as 0.6 or more and by making the manufacturing conditions capable of achieving this atomic concentration ratio clear, it became possible to achieve a clear-coated stainless steel sheet excellent in whiteness even at a low Si concentration of 0.8 % or lower.

A polish-finished material on which a surface pattern unique to stainless steel is applied by polishing marks is useful as a member in areas where design properties are important. However, since the extent of concentration of Cr, Si, and Al in the surface film of the stainless steel where surface polishing is applied is considerably small, yellowing occurs at the time of applying the clear coating film and it is impossible to achieve a clear-coated stainless steel sheet excellent in whiteness and with high design properties. Accordingly, there is a need for carrying out bright annealing in order to concentrate Cr, Si, and Al in the surface film. However, when performing bright annealing after surface polishing of a ferritic stainless steel sheet, which is subjected to normal softening annealing, the manufacturing process will be complex causing an increase in cost. Accordingly, in the present invention, it is desirable to conduct a final bright annealing after surface polishing of the cold-rolled ferritic stainless steel sheet. By the manufacturing process of cold rolling and surface polishing followed by bright annealing, softening annealing and concentration of Cr, Si, and Al in the surface film (high (Cr + Si + Al)/Fe value) for achieving a desired material quality can be carried out simultaneously by one bright annealing process. Especially in the ferritic stainless steel sheet of the present invention, it is also possible to control and adjust the recrystallization temperature depending on the content of Nb so that the softening temperature and temperature for concentrating Cr, Si, and Al in the surface film coincide. Moreover, in the case of austenitic stainless steel, since solid solution annealing is normally conducted at 1000°C or higher, this does not coincide with the temperature at which Cr, Si, and Al concentrate in the surface film. Therefore, it is necessary to perform the annealing process twice after cold-rolling and polishing and the annealing processes to be carried out are solid solution annealing and bright annealing for the formation of an oxide film. 555633 12 EXAMPLES Table 1 shows examples of the present invention and comparative examples. Three kinds of surface finish, which are No. 4 (fine polish finish), HL (hair line polish finish), and No. 2B (surface lightly rolled after pickling), are applied to ferritic stainless steel sheets. These ferritic stainless steel sheets are subjected to bright annealing under the conditions described in Table 1. Note that while a No. 2B polish finished ferritic stainless steel sheet goes through a manufacturing process where annealing is performed after cold rolling, since those whose surfaces are finished by other polishing are not subjected to annealing after cold-rolling, softening will also be carried out simultaneously by bright annealing. 13 Table 1 No, STEEL TYPE COMPONENT (mass%) SURFACE FINISH BRIGHT ANNEALING CONDITION ATOMIC CONCENTRATION RATIO FROM SURFACE TO 50A DEPTH (Cr+Si+Al)/Fe CHEMICAL CONVERSION TREATMENT COLOR TONE b* WHITENESS REMARKS Cr Si Al TEMPERATURE TlpC) DEW POINT T2fC) CONDITION COMPATIBILITY TYPE DEPOSITION AMOUNT (mg/m2) CLEAR COATING MATERIAL THICKNESS (fun) EXAMPLE OF PRESENT INVENTION 1 SUS430 16.24 0.59 0.09 No,4 850 -50 O 1.8 AMINOSILANE 2 2.5 1,1 O 2 SUS430 16.24 0.59 0.09 No.4 850 -50 o 1.8 EPOXYSILANE 2 2.5 0.9 O 3 SUS430 16.24 0.59 0.09 No.4 850 -50 o 1,8 AMINOSILANE 2.5 1.3 O 4 SUS430 16.24 0.59 0.09 No.4 850 -50 o 1.8 AMINOSILANE 19 2,5 1,7 O SUS430 16.24 0.59 0,09 No.4 850 -50 o 18 EPOXYSILANE 19 2.5 !.8 Q 6 SUS430 16.24 0.59 0.09 No,4 850 -50 o 1.8 AMINOSILANE 1,1 1.2 o 7 SUS430 16.24 0.59 0.09 No.4 850 -50 o 1.8 AMINOSILANE 1-1 o S SUS430 1624 0.59 0.09 No.4 850 -50 o 1.8 AMINOSILANE 1.5 9 SUS430 16.24 0.59 0.09 No,4 750 -70 o 08 AMINOSILANE 2,5 1.4 o SUS430 16.24 0.59 0t09 No,4 750 -50 o 1 AMINOSILANE "2.5 1.8 o 11 : SUS430 16.24 0,59 0.09 No,4 750 -30 0 1.4 AMINOSILANE 2.5 1.5 ; 12 SUS430 16,24 059 0.09 No.4 850 -70 o 0,9 AMINOSILANE 2.5 0.8 13 SUS430 16.24 0.59 0.09 No.4 850 •30 r\ 1.6 AMINOSILANE 2,5 1.2 O 14 SUS430 16.24 0.S9 0.09 No.4 ' 950 -50 o 1,8' AMINOSILANE 2,5 0.9 0 14 SUS430 16.24 0.59 0.09 No.4 1050 -30 O 1.2 AMINOSILANE 2.5 j 1.7 o 16 SUS430 16.24 0.59 0.09 No.4 850 -50 O 1.8 AMINOSILANE 8 1 1 ■ 1? SUS430JIL 19.22 0.44 0.02 No.4 900 -47 O 3 AMINOSILANE S 2.5 0.9 "w" 18 SUS430J1L 19.22 0.44 0.02 2B 900 -55 O 2 AMINOSILANE 4.5 1.2 \.y 19 SUS430J1L 19.22 0.44 0.02 HL 900 -50 O Z5 EPOXYSILANE 2.5 1.1 ss COMPARATIVE EXAMPLE SUS430J1L 19.40 0.07 0.02 No.4 850 -50 O 0,41 AMINOSILANE t.l 3.9 X 21 SUS430 16.24 0.59 0.09 No.4 WITHOUT ANNEALING (AS POLISHED) X 0.15 AMINOSILANE 2.5 4.5 & 22 SUS430 16.24 0.59 0.09 No.4 700 -60 X 0.3 AMINOSILANE 2.5 3.2 X 23 SUS43G 16.24 0.59 0.09 No,4 1100 -30 X 0.21 EPOXYSILANE 2.5 .1 X 24 SUS430 16.24 0.59 0.09 No.4 900 -70 X 0.4 EPOXYSILANE 2.5 4.1 * SUS430 16.24 0.59 0.09 No.4 800 - - - - - MATERIAL TEMPER COLOR 26 SUS430 16.24 0.59 0.09 No.4 850 -50 o 1.8 AMINOSILANE 1 - - - POOR ADHESION 27 SUS430 1624 0 59 0.09 No.4 850 -50 o i.8 AMINOSILANE 2 5 2,4 28 SUS430 16,24 0.59 0.09 No.4 850 -50 o 1.8 CHROMATE 2.5 2.9 y 29 SUS430 16.24 0,59 0,09 No.4 •850 _<;n Q 1.8 AMINOSILANE 0,5 - .

UNSTABLE COLOR TONE SUS430 16.24 0,59 0.09 No.4 850 -50 o 1.8 AMINOSILANE - - DIFFICULT TO MANUFACTURE 31 SUS430 16.24 0.59 0.09 No.4 850 -50 o 1.8 EPOXYSILANE 0.5 - - UNSTABLE COLOR TONE 32 SUS43Q DIFFICULT TO 1124 0,59 0.09 Na,4 m -SO © W EPOXYSILANE : MANUFACTURE ITALIC LETTER INDICATES DEVIATION FROM SCOPE OF PRESENT INVENTION, 555633 16 The atomic concentration of Cr, Si, Al, and Fe in the oxide film from the surface to the depth of 50 A of each ferritic stainless steel sheet after being subjected to bright annealing was measured by Auger Electron Spectroscopy (AES) and the atomic concentration ratio (Cr + Si + Al)/Fe was calculated.

As a chemical conversion treatment, a chemical conversion treating agent (E-206) of an aminosilane type and epoxysilane type manufactured by Nihon Parkerizing Co., Ltd. were used for each ferritic stainless steel sheet after being subjected to bright annealing and their deposition amount was changed between a range of 2 mg/m2 and 50 mg/m2. Chromate treatment was applied to the ferritic stainless steel sheet as the chemical conversion treatment for comparison. Subsequently, 1 to 10 (am of a clear film of acrylic urethane resins was treated at a baking temperature of 220°C.

Color tone after clear coating was measured by b* values (JIS Z8729) and the degree of discoloration was evaluated.

In the examples of the present invention, a clear-coated stainless steel sheet with the b* value of 2 or smaller and excellent in whiteness can be obtained by setting the atomic concentration ratio in the oxide film on ferritic stainless steel sheet surface to 0.6 or more and by setting the deposition amount of the chemical conversion treating agent composed of either one type or two types of an aminosilane type and/or epoxysilane type agent between 2 and 20 mg/m2. On the other hand, in comparative examples, a yellowish clear-coated stainless steel sheet with the b* value larger than 2 is obtained and it appears to have a blackish color tone depending on the viewing angle.

In comparative example 20, since the content of the Si component of the stainless steel sheet is deviated from the lower limit, (Cr + Si + Al)/Fe in the oxide film is deviated from the lower limit and the b* value will be 2 or larger and the sheet will be inferior in whiteness. In comparative examples 21 to 25, since one of temperature T1 and dew point T2 of the bright annealing conditions is deviated from the upper or lower limit, (Cr + Si + Al)/Fe in the oxide film is deviated from the lower limit and the b* value will be 2 or larger and the sheet will be inferior in whiteness. Especially in comparative example 25, since temper color of stainless steel sheet material was clearly observed, (Cr + Si + Al)/Fe was not measured. In comparative examples 26 to 32, although (Cr + Si + Al)/Fe of the material is 0.6 or more, the condition for the chemical conversion treating agent is deviated from the scope of the present invention. In comparative example 26, the deposition amount of the chemical conversion treating agent is deviated from the lower limit and poor in adhesion. In comparative example 27, the deposition amount of the chemical conversion treating agent is deviated from the

Claims

555633 17 upper limit. In comparative example 28, since the chemical conversion treating agent is a chromate agent and is not a non-chromate type agent of the present invention, the b* value will be 2 or larger and the sheet will be inferior in whiteness. In comparative examples 29 and 31, the thickness of the clear coating material is deviated from the lower limit and color tone is unstable. In comparative examples 30 and 32, the thickness of the clear coating material is deviated from the upper limit and the sheet will be difficult to manufacture. While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims. INDUSTRIAL APPLICABILITY According to the present invention, which investigates the relationship between design properties and color tone of a clear-coated stainless steel sheet, it is possible to manufacture a clear-coated stainless steel sheet excellent in whiteness whose b* value (chromaticity, degree of yellow/blue according to JIS Z 8729) is in a range between 0 and 2. In a general clear-coated stainless steel sheet, the b* value is approximately from 4 to 6 and the sheet is yellowish and appears to have a dark tone depending on the viewing angle. On the other hand, a clear-coated stainless steel sheet of the present invention is whitish and does not appear to have a dark tone regardless of the viewing angle. 555633 18 CLAIMS

1. A clear-coated stainless steel sheet excellent in whiteness which is a ferritic stainless steel sheet containing 0.2 to 0.8 mass% of Si, 0.005 to 0.15 mass% of Al and 0.1 to 0.7 mass% ofNb, wherein the sheet has a surface oxide film where an atomic concentration ratio of Cr, Si, Al, and Fe ((Cr + Si + Al) / Fe) from its surface to a depth of 50 A is 0.6 or more, a chemical conversion treatment agent of a coated film is composed of either one type or two types of an aminosilane type and/or epoxysilane type agent, a deposition amount of the chemical converting agent is between 2 and 20 mg/m2 (an amount of Si02 is measured by X-ray fluorescence), a clear coating material of the coated film is selected from a group consisting of polyester resin, acryclic resin, crylic urethane resin, and epoxy-modififed polyester resin, and a thickness of a clear coating material of the coated film is between 1 and 10 (am.

2. A method for manufacturing a clear-coated stainless steel sheet, comprising: performing annealing of a ferritic stainless steel sheet containing 0.2 to 0.8 mass% of Si, 0,005 to 0.15 mass% of Al and 0.1 to 0.7 mass% ofNb such that the sheet has a surface oxide film where an atomic concentration ratio of Cr, Si, Al, and Fe ((Cr + Si + Al) / Fe) from its surface to a depth of 50 A is 0.6 or more; performing chemical conversion treatment of the stainless sheet using a chemical conversion treatment agent composed of either one type or two types of an aminosilane type and/or epoxysilane type agent such that a deposition amount of the chemical converting agent is between 2 and 20 mg/m2 (an amount of SiC>2 is measured by X-ray fluorescence); and performing coating a clear coating material selected from a group consisting of polyester resin, acrylic resin, acrylic urethane resin, and epoxy-modified polyester resin on the stainless sheet such that a thickness of a clear coating material of the coated film is between 1 and 10 Jim, mid wherein annealing is carried out under a condition where a final bright annealing temperature T1 (°C) and an annealing atmosphere dew point T2 (°C) satisfy formulae 1 and 2 750 < T1 < 5 x T2 + 1200 (formula 1) -70 <T2 < -30 (formula 2)

3. The method for manufacturing the clear-coated stainless steel sheet according to claim 2, wherein the ferritic stainless steel sheet is subjected to cold roiling, surface- 555633 19 polishing, and the final bright annealing before the chemical conversion treatment. Nippon Steel & Sumiki Stainless Steel Corporation By their Attorneys James & Wells Intellectual Property