US3616299A

US3616299A - Process for producing steel products having a hydrated chromium oxide film excellent in the surface appearance

Info

Publication number: US3616299A
Application number: US809891A
Authority: US
Inventors: Akira Hata; Tamotu Hinami
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1968-03-28
Filing date: 1969-03-24
Publication date: 1971-10-26
Anticipated expiration: 1988-10-26
Also published as: BE730531A; GB1253422A; DE1916049B2; NL162974B; NL162974C; FR2004968A1; DE1916049A1; NL6908669A

Abstract

A process for obtaining a hydrated chromium oxide treated steel article having a beautiful surface appearance without any roll marks by pressing a steel product subjected to the chromic acid cathode electrolytic treatment, while a chromic acid film thus electrodeposited on the surface of the steel product is still wet, immediately after the electrolytic treatment by the help of a leveling roll as an auxiliary holddown roll having a surface hardness of 40* to 90* (A scale) without need of frequently exchanging a holddown roll installed for preventing the formation of arc spots.

Description

United States Patent Inventors Akira Hata;

Tamotu Hinami, both of Kitakyushu, Japan Appl. No. 809,891 Filed Mar. 24, 1969 Patented Oct. 26, 1971 Assignee Nippon Steel Corporation Tokyo, Japan Priority Mar. 28, 1968 Japan 43/ 197 17 PROCESS FOR PRODUCING STEEL PRODUCTS HAVING A HYDRATED CHROMIUM OXIDE FILM EXCELLENT IN THE SURFACE APPEARANCE 7 Claims, 1 Drawing Fig.

US. Cl 204/35 N Int. Cl C23b 11/00 Field of Search 204/35, 36,

[56] References Cited UNITED STATES PATENTS 488,499 12/1892 Hollingshead 204/36 1,488,553 4/1924 Peacock 204/35 Primary Examinerl-loward S. Williams Assistant Examiner-W, 1. Solomon Attorney-Wenderoth, Lind & Ponack ABSTRACT: A process for obtaining a hydrated chromium oxide treated steel article having a beautiful surface appearance without any roll marks by pressing a steel product subjected to the chromic acid cathode electrolytic treatment, while a chromic acid film thus electrodeposited on the surface of the steel product is still wet, immediately after the electrolytic treatment by the help of a leveling roll as an auxiliary holddown roll having a surface hardness of 40 to 90 (A scale) without need of frequently exchanging a holddown roll installed for preventing the formation of arc spots.

PATENTEDUET 2B |97l 3,616.2 9 9 INVENTOR Akiro Hafo Tomofu Hinami 1 A BY ZUQMJJLM ATTORNEY PROCESS FOR PRODUCING STEEL PRODUCTS HAVING A HYDRATED CHROMIUM OXIDE FILM EXCELLENT IN THE SURFACE APPEARANCE This invention relates to a process for producing steel products having a film of hydrated chromium oxide excellent in the surface appearance by a chromic acid cathodic electrolysis treatment.

In order to electrolyze hydrated chromium oxide onto the surface of an iron or steel product, there is usually practiced a method in which such material to be treated as, for example, a properly pretreated steel plate is electrolytically treated in a treating bath mainly composed of such hexavalent chromium ions as chromic acid ions or dichromic acid ions, and additionally sulfuric acid or another inorganic acid to be added, if required.

Due to excellent properties of films formed by the chromic acid series treatment as above-mentioned, particularly in the anticorrosion and paint adhesion, an increasing importance has been attached to the said treatment as an antirusting treatment of iron and steel products. However, chromic acid series films, especially the hydrated chromium oxide film obtained by the above-mentioned treatment, have a disadvantage that they are so soft as to be likely to be flawed particularly by rolls when they are still wet immediately after electrodeposition.

In addition thereto, since an anticorrosion film obtained by a cathodic treatment with an electrolyte containing mostly chromic acid ions has an electric insulating property, there occur often are spots particularly on conductor rolls.

The above-mentioned problems of the chromic acid series coated surface formed by a cathodic treatment shall be explained with reference to a drawing.

The drawing is a schematic view of an improved electrolysis treatment section for working the present invention.

This electrolysis treatment section is connected with degreasing, water-rinsing and pickling steps as the preliminary stage thereof and is followed by a heating and drying step as the latter stage thereof.

As compared with a known electrolysis treatment section the improvement of the present invention is characterized only by providing a leveling roll 7 as an auxiliary holddown roll in such manner as shown in the drawing, but is quite the same as the former in other respects.

in the known electrolysis treatment a steel plate is led with conductor rolls 4 and 4' which are provided with

holddown rolls

3 and 3 respectively, and with sink rolls 5 which are installed as dipped in an electrolysis bathtub 2, in which there is held a treating solution containing water-soluble hexavalent chromium ions, and meanwhile a required film is formed on the surface of the steel plate. 6 is an anode provided as dipped in the electrolyte and opposed to the surface of the passing plate.

Heretofore, when carrying out an electrodepositing treatment with known equipments a nonuniform contact was unavoidably formed between the steel plate and the conductor rolls 4 and 4', whereby an electric discharge occurred between them, resulting in the production of perforations" called arc spots or discharge traces on the surface of the steel plate. The

holddown rolls

3 and 3 attached to the conductor rolls 4 and 4' respectively are originally provided for the purpose of attaining sufficient contact of the steel plate 1 with the conductor rolls 4 and 4'. These

holddown rolls

3 and 3 are coated with material which has a high hardness above 90in the Shore Durometer hardness (A scale) and is difficult to elastically deform as, for example, a hard vinyl chloride, or may be coated with an elastic material ofa comparatively low hardness as of 40to 90in the Shore Durometer hardness (A scale) so that the pressing force may be elevated and the contact between the steel plate and conductor rolls may be improved to prevent the production of arc spots.

However, the arrangement ofthe holddown rolls 3 and 3' as above-mentioned is attended by the following difficulties:

As the surface layer of the anticorrosion film electrodeposited on the steel plate, which is made a cathode, is made of hydrated chromium oxide in the form ofa sol, it is so soft that, if the surface of the contacting roll is defective, there is produced an unevenness in coloring, or else the electrodeposited film is easily scraped off and roll marks are left on the film surface while the steel plate is advancing.

In the case of using the

holddown rolls

3 and 3 made of a material having a high hardness the steel plate must suffer from a deterioration of the surface flatness as a result of the fluctuation of the tension of the steel plate during its passage at a high speed as, for example, about 200 to 290 m./min. That is, there appears a phenomenon of the steel plate being drawn by deformation, whereby the surface of the holddown rolls are flawed, again causing the soft electrodeposited film to scrape off by irregular flaws thus formed on the roll surface.

Heretofore, when the holddown roll was thus flawed, the flawed roll was immediately replaced by a new one. This replacement was, however, attended with various disadvantages, for instance, the line had to be stopped and the frequent replacement resulted not only in complicating the operation, but also in remarkably reducing the operation efficiency.

Further, a special consideration was required for the sake of the lining materials of the rolls, because the treating solution was mainly composed of chromic acid series chemical having a strong oxidizing property. Consequently, a higher cost had to be paid for the lining materials.

in view of these facts as above-mentioned the present inventors have made various investigations to eliminate such disadvantages and have confirmed as the result of the investigations that it is almost impossible to obtain the treated steel plate having a uniform film on its surface when selecting the elastic material for lining the surface of the holddown rollfrom those comparatively low in hardness to elevate the pressing force only, because in this case the holddown roll is largely elastically deformed and the contact area between the steel plate and the holddown roll surface expanded, whereby the formed corrosion-resisting film is often easily scraped off, but that the corrosion-resisting film on the side in contact with the conductor roll is substantially not flawed, irrespective of the hardness of the surface lining material of the holddown roll, because the conductor roll is plated on the surface with hard chromium.

As regards tin-free steels practiced at present as steel plates for making cans those having chromium series coatings occupy almost the monopolistic position, and it is thought that, when the coating is ofa two-layer structure made ofa metallic chromium inner layer high in the anticorrosion and a nonmetallic chromium outer layer, said coating would be superior to a coating made of a single layer of metallic chromium in the paint adhesiveness, workability, productivity, electrodepositing efficiency at the time of electrodepositing the coating and other respects. However, it is to be mentioned as a disadvantage of the coating of a two-layer structure that on the nonmetallic chromium outer layer, for example, a hydrated chromium oxide layer of a plating amount of 0.l to 2.5 mg./dm. or preferably 0.2 to 0.6 mg./dm. the color irregularity due to the nonuniformity of the plating amount or socalled roll mark is conspicuous on account of the presence of the inner layer of metallic chromium of a plating amount of 0.5 to 2 mg./dm."' having a white metallic lustrous surface, which is not desirable from the viewpoint of the commodity appearance.

The object of the present invention is to provide a method of continuously electrolytically treating steel plates to fonn thereon a surface layer of a hydrated chromium oxide film having a beautiful and uniform surface by erasing flaws or holddown roll marks unavoidably produced on the electrodeposited film surface owing to such flawed holddown rolls as is mentioned above.

That is, according to the present invention flaws on the film surface may be erased by pressing the film surface on the side of the holddown roll 3' using a leveling roll (in the drawing this roll is indicated by a number 7), said leveling roll being lined with an elastic material having a surface hardness of 40to in the Shore Durometer (A scale) and being provided in the rear of the holddown roll 3 a hardness of more than 90or generally lOin the Shore Durometer (A scale), said holddown roll 3' being provided as attached to the final conductor roll 4' on the peripheral surface thereof, the steel plate 1 passing between said holddown roll 3' and the final conductor roll 4' after having finally left the electrolytic bath 2, or being provided as a roll separate from the above-mentioned conductor roll between said conductor roll 4 and a film heating and drying section (not illustrated).

In other words, in the present invention the leveling roll 7 is provided on the final conductor roll 4 as is shown in the drawing or is provided in the part within drag-out tanks No. l, 2 ..arranged in connection with the latter part of the electrolytic treating section but so adjacent to the electrolytic treating section as possible, or a deflector roll of the said dragout tanks is utilized as a leveling roll, though not illustrated. in the above-mentioned case the hardness of the elastic member applied to line the surface of the leveling roll is 40to 90in the Shore Durometer (A scale), that is, lower than that of the above-described holddown roll. Needless to say, the position of fitting the leveling roll is not limited to only those mentioned above, but any effective position may be found within the range covering from the final conductor roll to the heating and drying section. Further, it is to be noted, that at least either one of the leveling rolls, when they are arranged in a pair, must have a surface of an elastic member.

in the present invention, the steel plate means a cold-rolled steel sheet, chromium-plated steel plate, zinc-plated steel plate, tin-plated steel plate or any other iron or steel product, a continuous steel strip of a thickness of about'O. l 2 to 1.2 mm. being preferred.

it is most favorable not only from the viewpoint of the equipment but also from that of the operation to provide the leveling roll 7 in the rear of the holddown roll 3' so as to be opposed to the surface of the conductor roll 4' where the film on the steel plate 1 is still wet just after it is electrodeposited until it is heated and dried. However, so long as the film on the steel plate having passed the holddown roll 3' is wet, the leveling roll need not always be opposed to the conductor roll 4' and can be provided independently in the rear of the conductor roll 4' together with an opposed roll (not illustrated).

Further, it is necessary to adapt the pressing force of the leveling roll 7 to the thickness of the film. According to the finding by the present inventors, a favorable result can be obtained with a pressing force of less than 30 kgjcrn? or preferably 4 to 6 kg./cm.

In the present invention, the leveling roll 7 is made to act on the electrodeposited film while it is still wet. Once the film dries, it becomes very difficult to erase the flaws made on the film surface.

The effect of leveling the film surface with the leveling roll 7 can also be adjusted by regulating the wet state of the film surface by applying such means as spraying the electrolyte onto the film surface between the holddown roll 3' and leveling roll 7.

The leveling roll 7 is desirably to be of a hardness of 40 to 90 or preferably 60 to 75 in the Shore Durometer hardness on the A scale.

As the treating bath relates to such Cr -containing aqueous solutions having strong oxidizing property as, for example, an aqueous solution containing dichromic acid ions or chromic acid ions, the elastic material is selected from a group consisting of a chromic acidproof rubber, chloroprene rubber, polypropylene, vinylidene chloride, chlorinated polyethylenevinyl chloride and chlorosulfonated polyethylene resin which will not deteriorate in contact with said solution. However, when taking the rollability, heatproofness and treating bath contaminating property into consideration, a roll lined with a rubber material made mainly of a chlorinated polyethylene resin is most preferable.

As described above, the surface hardness of the leveling roll is limited to within the range of 40 to 90 in the Shore Durometer hardness on the A scale. This is because, when the hardness is more than the roll surface will be too hard and will be likely to be flawed and when the hardness is less than 40 it will be difficult to erase flaws on the film surface.

If this leveling roll 7 is arranged in the rear of the

holddown roll

3 or 3 so that it may work together with said'holddown

roll

3 or 3 lined for example, with a hard vinyl chloride resin of a high hardness of more than 90 in the Shore Durometer hardness on the A scale, it is possible to obtain a steel plate having a uniform smooth anticorrosion film free from are spots and roll marks or scratches.

The surfaces of the rolls located in the plate passing section from the final conductor roll to the heating and drying step should have a hardness which is the same as or lower than the hardness of the above-described leveling roll. But their pressing force can be higher than that of said leveling roll in some cases.

An example of the present invention shall be described in the following. However, the object of the present invention is not limited to the hydrated chromium oxide coated steel plate as shown in the following example, but also includes chromic acid coated materials made with any other bath composition, and the means of erasing roll marks in the chromium oxide coating, and in addition thereto the position of setting the leveling roll, the material for lining the leveling roll, the pressing force and others may be altered freely within the scope of the claims.

EXAMPLE In an electrolytic treating apparatus, in which holddown rolls, conductor rolls, dipping rolls and a leveling roll are arranged as is shown in the drawing, the hardness of the surface of the holddown roll was made l00 in the Shore Durometer hardness (A scale), the hardness of the surface of the leveling roll was made 65 also in the Shore Durometer hardness (A scale) and a steel strip on the final conductor roll was pressed with a pressing force of IOkg/cm. by the leveling roll.

The electrolytic treating solution was an aqueous solution of 45g./l. of chromic anhydride and 0.4g./l. of sulfuric acid at a bath temperature of 55C. The above-mentioned steel strip was electrolytically treated as a cathode at a current density of 30 amp./dm. for 3seconds and was advanced at a high speed through the above-mentioned final conductor roll, drag-out tanks and wringer rolls to the heating and drying step. The steel plate was water-washed and was then dried to obtain a product having a uniform electrodeposited film rich in luster and having no color irregularity, roll mark or coating defect.

What is claimed is:

l. A process for producing steel products having a hydrated chromium oxide film excellent in the surface appearance by electrodepositing an anticorrosion film by electrolytically treating an iron or steel product as a cathode in an electrolyte containing hexavalent chromium ions, comprising a step of pressing the electrodeposited film surface with a leveling roll of a surface hardness of 40 to 90 in the Shore Durometer hardness (A Scale) while said electrodeposited film is still wet after the steel product coated with the electrodeposited film has passed a holddown roll provided on a final conductor roll.

2. A process according to claim 1 wherein said iron or steel product is a continuous cold-rolled steel sheet, steel plate, chromium-plated steel plate, zinc-plated steel plate or tinplated steel plate.

3. A process according to claim 1 wherein said leveling roll is lined with a chromic acid-proof elastic rubber or resin.

4. A process according to claim 3, wherein said iron or steel product is a continuous cold rolled steel sheet, steel plate. chromium-plated steel plate, zinc-plated steel plate or tinplated steel plate.

5. A process for producing steel products having a hydrated chromium oxide film excellent in the surface appearance by electrodepositing an anticorrosion film by electrolytically treating an iron or steel product as a cathode in an electrolyte containing hexavalent chromium ions, comprising a step of pressing the electrodeposited film surface with a leveling roll of a surface hardness of 40 to 90 in the Shore Durometer hardness (A scale) under a maximum pressure of 30kg./cm." while said electrodeposited film is still wet after the steel product coated with the electrodeposited film has passed a 5 holddown roll provided on a final conductor roll.

6. A process according to claim 5, wherein said leveling roll I e ll

Claims

2. A process according to claim 1, wherein said iron or steel product is a continuous cold-rolled steel sheet, steel plate, chromium-plated steel plate, zinc-plated steel plate or tin-plated steel plate.
3. A process according to claim 1, wherein said leveling roll is lined with a chromic acid-proof elastic rubber or resin.
4. A process according to claim 3, wherein said iron or steel product is a continuous cold-rolled steel sheet, steel plate, chromium-plated steel plate, zinc-plated steel plate or tin-plated steel plate.
5. A process for producing steel products having a hydrated chromium oxide film excellent in the surface appearance by electrodepositing an anticorrosion film by electrolytically treating an iron or steel product as a cathode in an electrolyte containing hexavalent chromium ions, comprising a step of pressing the electrodeposited film surface with a leveling roll of a surface hardness of 40* to 90* in the Shore Durometer hardness (A scale) under a maximum pressure of 30kg./cm.2 while said electrodeposited film is still wet after the steel product coated with the electrodeposited film has passed a holddown roll provided on a final conductor roll.
6. A process according to claim 5, wherein said leveling roll is lined with a chromic acidproof elastic rubber or resin.
7. A process according to claim 5, wherein said iron or steel product is a continuous cold-rolled steel sheet, steel plate, chromium-plated steel plate, zinc-plated steel plate or tin-plated steel plate.