Claims:1. A method for coloring a stainless steel, comprising the sequential steps of:
(a) imparting a mirror finish or a No. 8 surface finish to at least one surface of the stainless steel;
(b) cleaning the stainless steel with an organic solvent in order to remove all contamination from the at least one surface of the stainless steel, followed by air drying the stainless steel;
(c) coating a strippable paint on at least one surface of the stainless steel to form a strippable paint layer thereon;
(d) drying the strippable paint layer to attach the strippable paint layer to the at least one surface of the stainless steel;
(e) immersing the stainless steel in a coloring solution comprising a mixed aqueous solution of potassium dichromate and sulphuric acid for a period of time and at a temperature;
(f) washing the stainless steel with water, followed by air drying the stainless steel;
(g) hardening the stainless steel by immersing the stainless steel in a hardening solution comprising a mixed aqueous solution of phosphoric acid and chromic acid;
(h) washing the stainless steel with water, followed by air drying the stainless steel;
(i) peeling off the paint layer from the stainless steel; and
(j) washing the stainless steel with a buffer solution.

2. The method as claimed in claim 1, wherein the mirror finish or the No. 8 surface finish is imparted to the surface of the stainless steel by sisal buffing, color buffing or a combination thereof.

3. The method as claimed in claim 1, wherein the organic solvent comprises acetone.

4. The method as claimed in claim 1, wherein the strippable paint is adapted to provide wettability to the at least one surface of the stainless steel.
5. The method as claimed in claim 1, wherein the strippable paint is adapted to withstand a temperature of 80oC to 90oC.

6. The method as claimed in claim 1, wherein the strippable paint coating is performed in a single-step.

7. The method as claimed in claim 1, wherein the stainless steel is immersed in the coloring solution for a time period ranging from 15 to 17 minutes and at a temperature ranging from 85oC to 88oC.

8. The method as claimed in claim 1, wherein the hardening solution comprises phosphoric acid and chromic acid in a weight ratio of 1:1.

9. The method as claimed in claim 1, wherein the hardening the stainless steel comprises immersing an anode and cathode in the hardening solution and applying a voltage between the anode and the cathode to treat the stainless steel.

10. The method as claimed in claim 9, wherein a current of 3 to 5 amps is applied between the anode and the cathode.
, Description:FIELD OF THE INVENTION
[0001] The present disclosure pertains to technical field of stainless steel coloring techniques. In particular, the present disclosure pertains to a new, improved and highly reproducible method for producing colored stainless steel having improved color stability and brightness.

BACKGROUND OF THE INVENTION
[0002] The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] The techniques of coloring stainless steel have progressed rapidly, and in accordance with the increase in production of colored stainless steel, it is now used in various fields. In decorative applications such as mural, architecture, interior decoration, furniture, kitchen equipment and automobile manufacturing, there is an increasing demand for colored stainless steel. There exist many known methods for imparting a variety of color tones to stainless steel. The known methods typically involve two steps, "coloring" and "film hardening steps", which are separately carried out with individual solution composition, temperature, and treating condition.
[0004] British Patent Nos. 1,122,172 and 1,122,173 disclose coloring solutions that can be used for coloring stainless steel. When a stainless steel is immersed in such a coloring solution, colours of various shades may develop on surfaces of stainless steel depending upon the time of immersion. Japanese Patent Publication Nos. 52-32621, 52-25817, and 53-31817 disclose use of a mixed solution comprising sulfuric acid and chromic acid for coloring stainless steel.
[0005] Japanese Patent No. 52-25,817 discloses a method for controlling color shade over the surfaces of stainless steel. The principle underlying this method is such that the difference between the potential of steel, which is immersed in coloring solution and on which oxide films are being formed, and the potential of a reference electrode disposed in the coloring solution varies in accordance with the extent of the oxide films being formed on the steel, i.e. in accordance with the color shade, and therefore the color shade may be controlled by measuring and using the potential difference as a measure.
[0006] The above mentioned prior art methods, however, suffer from a number of disadvantages. For example, the prior art methods are capable only of producing a single color on treated steels, and they are difficult to control for reproducing exact and uniform color. The colored stainless steel products produced in accordance with the prior art methods lack color stability and exhibit low brightness. Further, the prior art coloring methods and materials are not conducive to a satisfactory control over operations for reproducing uniform and desired shade of color on stainless steel. Moreover, there has long been a desire among the manufacturers and users of stainless steel to provide different and attractively colored stainless steel for diverse applications.
[0007] Accordingly, it would be highly desirable and advantageous to provide a new and improved method for coloring stainless steel, by which stainless steel can be colored to a desired color tone with fair reproducibility. Another advantage would be to provide a highly reproducible method for producing colored stainless steel having improved color stability and brightness.
[0008] The present invention satisfies the existing needs, as well as others, and generally overcomes the deficiencies found in the prior art.
[0009] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[0010] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability.

OBJECTS OF THE INVENTION
[0011] It is an object of the present disclosure to provide a new and improved stainless steel coloring technique that overcomes one or more disadvantages associated with the previously known coloring techniques.
[0012] It is another object of the present disclosure to provide a new and improved method for coloring stainless steel, by which stainless steel can be colored to a desired color tone with fair reproducibility.
[0013] It is another object of the present disclosure to provide a highly reproducible method for producing colored stainless steel having improved color stability and brightness.
[0014] It is another object of the present disclosure to provide a method for coloring a stainless steel which can eliminate non-uniformity of color caused by poor surface conditions of a stainless steel to be colored.
[0015] It is another object of the present disclosure to provide a method which can produce different colours on stainless steel.
[0016] It is yet another object of the present disclosure to provide a simple method by which coloring of stainless steel can be precisely controlled with a satisfactory reproducibility.

SUMMARY
[0017] Aspects of the present disclosure relate to a method for coloring stainless steel, by which stainless steel can be colored to a desired color tone uniformly with fair reproducibility. The disclosed method can produce colored stainless steel having improved color stability and brightness.
[0018] In an aspect of the present disclosure, the method for coloring stainless steel can include the sequential steps of: (a) imparting a mirror finish or a No. 8 surface finish to at least one surface of the stainless steel; (b) cleaning the stainless steel with an organic solvent in order to remove all contamination from the at least one surface of the stainless steel, followed by air drying the stainless steel; (c) coating a strippable paint on at least one surface of the stainless steel to form a strippable paint layer thereon; (d) drying the strippable paint layer to attach the strippable paint layer to the at least one surface of the stainless steel; (e) immersing the stainless steel in a coloring solution comprising a mixed aqueous solution of potassium dichromate and sulphuric acid for a period of time and at a temperature; (f) washing the stainless steel with water, followed by air drying the stainless steel; (g) hardening the stainless steel by immersing the stainless steel in a hardening solution comprising a mixed aqueous solution of phosphoric acid and chromic acid; (h) washing the stainless steel with water, followed by air drying the stainless steel; (i) peeling off the paint layer from the stainless steel; and (j) washing the stainless steel with a buffer solution.
[0019] In an embodiment of the present disclosure, surface finishing of a stainless steel to be colored can be performed by imparting a mirror finish or a No. 8 surface finish to a surface of the stainless steel by sisal buffing, color buffing or a combination thereof.
[0020] In an embodiment, a surface finished stainless steel can be washed with an organic solvent in order to remove contaminations such as, grease stains, finger prints, oil marks, etc. from the polished surface of the stainless steel.
[0021] In an embodiment, a polished surface of the stainless steel can be coated with a strippable paint to form a strippable paint layer thereon. In certain preferred embodiments, the strippable paint can provide wettability to the polished surface of the stainless steel, and can withstand a temperature of 80oC to 90oC.
[0022] In a preferred embodiment, painted stainless steel can be immersed in the coloring solution for a time period ranging from 12 to 20 minutes, preferably 15 to 17 minutes, and at a temperature ranging from 70oC to 100oC, preferably 85oC to 88oC.
[0023] In a more preferred embodiment, the coloring solution can contain 350 gm of potassium dichromate and 265 cc of sulphuric acid in order to obtain a bright uniform color throughout the surface of the stainless steel.
[0024] In a more preferred embodiment, the hardening solution can include phosphoric acid and chromic acid in a weight ratio of 1:1, i.e. 50% by weight of phosphoric acid and 50% by weight of chromic acid.
[0025] According to embodiments of the present disclosure, hardening of a painted sheet can be performed by immersing an anode and cathode in the hardening solution and applying a voltage between the anode and the cathode to treat the stainless steel. A current of 3 to 5 amps can be applied between the anode and the cathode.
[0026] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.

DETAILED DESCRIPTION OF THE INVENTION
[0027] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0028] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the “invention” may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the “invention” will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0029] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[0030] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[0031] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
[0032] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.
[0033] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein.
[0034] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0035] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
[0036] Various terms are used herein. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0037] Reference will now be made in detail to the exemplary embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
[0038] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.
[0039] The present disclosure provides a method for coloring stainless steel, by which stainless steel can be colored to a desired color tone uniformly with fair reproducibility. The disclosed method can produce colored stainless steel having improved color stability and brightness.
[0040] In an aspect of the present disclosure, the method for coloring stainless steel can include the sequential steps of: (a) imparting a mirror finish or a No. 8 surface finish to at least one surface of the stainless steel; (b) cleaning the stainless steel with an organic solvent in order to remove all contamination from the at least one surface of the stainless steel, followed by air drying the stainless steel; (c) coating a strippable paint on at least one surface of the stainless steel to form a strippable paint layer thereon; (d) drying the strippable paint layer to attach the strippable paint layer to the at least one surface of the stainless steel; (e) immersing the stainless steel in a coloring solution comprising a mixed aqueous solution of potassium dichromate and sulphuric acid for a period of time and at a temperature; (f) washing the stainless steel with water, followed by air drying the stainless steel; (g) hardening the stainless steel by immersing the stainless steel in a hardening solution comprising a mixed aqueous solution of phosphoric acid and chromic acid; (h) washing the stainless steel with water, followed by air drying the stainless steel; (i) peeling off the paint layer from the stainless steel; and (j) washing the stainless steel with a buffer solution.
[0041] In an embodiment of the present disclosure, surface finishing of a stainless steel to be colored can be performed by imparting a mirror finish or a No. 8 surface finish to a surface of the stainless steel by sisal buffing, color buffing or a combination thereof. The quality of surface finish may be dependent on the quality of stainless steel being polished. In certain embodiments, color-motion can be done by moving a work piece in the direction the wheel is turning. By using light-to-medium pressure, an ultra-bright and shiny finish can be achieved.
[0042] In an embodiment, quality of the surface finish, after color motion buffing, can be determined by image distortion method. According to this embodiment, an object can be placed before a finished surface, and distortions in reflected image can be observed to determine the quality of the surface finish. In one preferred embodiment, the stainless steel to be colored can be austenitic with ferrite (2 to 1). To obtain best possible results, stainless steels having minimum nonmetallic inclusions can be chosen. For the purpose of the present invention, stainless steel sheet having a chemical composition in weight percent, Sulfur 0.008 max and Aluminium 0.015 max, can be chosen so as to obtain a highly advantageous and desired end result.
[0043] In an embodiment, a surface finished stainless steel can be washed with an organic solvent in order to remove contaminations such as, grease stains, finger prints, oil marks, etc. from the polished surface of the stainless steel. After washing, the stainless steel can be air dried. In a preferred embodiment, surface finished stainless steel can be washed with acetone.
[0044] In an embodiment, the polished surface of the stainless steel can be coated with a strippable paint to form a paint layer thereon. Preferably, the paint coating can be performed in a single step. A paint layer formed by the strippable paint is easily and completely strippable from the surface of the steel at any desired time. In certain preferred embodiments, the strippable paint can provide wettability to the polished surface of the stainless steel, and can withstand a temperature of 80oC to 90oC. Further, the paint can have good flow so that it can be easily spread across the surface of the stainless steel. After painting, the stainless steel can be subjected to air drying to cure and attach the strippable paint layer to the surface of the stainless steel. The dried paint layer can be further heat treated for 5-10 minutes, if desired.
[0045] After painting, the stainless steel can be immersed in an acid resistant and high temperature resistant vat containing a coloring solution comprising a mixed aqueous solution of potassium dichromate and sulphuric acid. In a more preferred embodiment, the painted stainless steel can be immersed in the coloring solution for a time period ranging from 12 to 20 minutes, preferably 15 to 17 minutes, and at a temperature ranging from 70oC to 100oC, preferably 85oC to 88oC. The acid solution in vat can be heated slowly by electrical heating arrangement. After attaining a desired temperature, the painted stainless sheet can be immersed vertically in the coloring solution. A support arrangement can be used for hanging the painted sheet vertically in the acid solution. After being treated with coloring solution, the stainless steel can be rinsed with water so as to remove any of the acid solution which remains on the surface, and then be air dried.
[0046] In an embodiment, the coloring solution can contain 300-350grams of potassium dichromate and 250-265cc of sulphuric acid. In a more preferred embodiment, the coloring solution can contain 350 gm of potassium dichromate and 265 cc of sulphuric acid in order to obtain a bright uniform color throughout a surface of stainless steel.
[0047] After being treated with coloring solution, the painted stainless steel can be subjected to hardening by immersing the painted stainless steel in a hardening solution comprising a mixed aqueous solution of phosphoric acid and chromic acid. In a preferred embodiment, the hardening solution can include 35% to 40% chromic acid and 65% to 65% phosphoric acid. In a more preferred embodiment, the hardening solution can include phosphoric acid and chromic acid in a weight ratio of 1:1, i.e. 50% by weight phosphoric acid and 50% by weight chromic acid.
[0048] In one embodiment, hardening of a painted steel sheet can be performed by immersing an anode and cathode in the hardening solution and applying a voltage between the anode and the cathode to treat the steel sheet. A current of 3 to 5 amps can be applied between the anode and the cathode. After being treated with hardening solution, the stainless steel can be washed with water and air dried so as to remove water stains or dirt, if any, on the surface of the stainless steel.
[0049] After hardening, the paint layer can be peeled off from the surface of the stainless steel by hand, and then cleaned with a smooth buffer to remove dust or paint residues, if any, on the surface to obtain a ready-to-use colored stainless steel.
[0050] While the foregoing description discloses various embodiments of the disclosure, other and further embodiments of the invention may be devised without departing from the basic scope of the disclosure. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE INVENTION
[0051] The present disclosure provides a method for coloring stainless steel, by which stainless steel can be colored to a desired color tone with fair reproducibility.
[0052] The present disclosure provides a highly reproducible method for producing colored stainless steel having improved color stability and brightness.
[0053] The present disclosure provides a method for coloring a stainless steel which can eliminate non-uniformity of color caused by poor surface conditions of a stainless steel to be colored.
[0054] The present disclosure provides a method which can produce different colours on stainless steel.
[0055] The present disclosure provides a simple method by which coloring of stainless steel can be precisely controlled with a satisfactory reproducibility.
[0056] The present disclosure provides a method for coloring stainless steel which is environment friendly and cost effective.
[0057] The present disclosure provides a method for coloring stainless steel which is suitable for small scale application.