CA1280085C - Surface treated steel sheet for welded can material and method for its production - Google Patents
Surface treated steel sheet for welded can material and method for its productionInfo
- Publication number
- CA1280085C CA1280085C CA000502984A CA502984A CA1280085C CA 1280085 C CA1280085 C CA 1280085C CA 000502984 A CA000502984 A CA 000502984A CA 502984 A CA502984 A CA 502984A CA 1280085 C CA1280085 C CA 1280085C
- Authority
- CA
- Canada
- Prior art keywords
- steel sheet
- tin
- chromium
- plated
- amount
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 157
- 239000010959 steel Substances 0.000 title claims abstract description 157
- 238000000034 method Methods 0.000 title claims description 40
- 238000004519 manufacturing process Methods 0.000 title abstract description 14
- 239000000463 material Substances 0.000 title description 6
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 claims abstract description 133
- 239000011651 chromium Substances 0.000 claims abstract description 97
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims abstract description 92
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 92
- WGLPBDUCMAPZCE-UHFFFAOYSA-N Trioxochromium Chemical compound O=[Cr](=O)=O WGLPBDUCMAPZCE-UHFFFAOYSA-N 0.000 claims abstract description 49
- 229910000423 chromium oxide Inorganic materials 0.000 claims abstract description 49
- 230000015572 biosynthetic process Effects 0.000 claims abstract description 15
- 238000009713 electroplating Methods 0.000 claims abstract description 6
- KRVSOGSZCMJSLX-UHFFFAOYSA-L chromic acid Substances O[Cr](O)(=O)=O KRVSOGSZCMJSLX-UHFFFAOYSA-L 0.000 claims description 36
- AWJWCTOOIBYHON-UHFFFAOYSA-N furo[3,4-b]pyrazine-5,7-dione Chemical compound C1=CN=C2C(=O)OC(=O)C2=N1 AWJWCTOOIBYHON-UHFFFAOYSA-N 0.000 claims description 36
- 239000003792 electrolyte Substances 0.000 claims description 31
- 239000000654 additive Substances 0.000 claims description 23
- 230000008569 process Effects 0.000 claims description 17
- 238000007747 plating Methods 0.000 claims description 14
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 claims description 12
- 239000002253 acid Substances 0.000 claims description 12
- 229910052783 alkali metal Inorganic materials 0.000 claims description 10
- -1 thiosulfate ions Chemical class 0.000 claims description 7
- 150000001340 alkali metals Chemical class 0.000 claims description 6
- 150000003464 sulfur compounds Chemical class 0.000 claims description 6
- 230000000996 additive effect Effects 0.000 claims description 5
- QGZKDVFQNNGYKY-UHFFFAOYSA-O Ammonium Chemical compound [NH4+] QGZKDVFQNNGYKY-UHFFFAOYSA-O 0.000 claims description 4
- KRHYYFGTRYWZRS-UHFFFAOYSA-N Fluorane Chemical compound F KRHYYFGTRYWZRS-UHFFFAOYSA-N 0.000 claims description 4
- 150000002222 fluorine compounds Chemical class 0.000 claims description 4
- IUTCEZPPWBHGIX-UHFFFAOYSA-N tin(2+) Chemical compound [Sn+2] IUTCEZPPWBHGIX-UHFFFAOYSA-N 0.000 claims description 4
- KRHYYFGTRYWZRS-UHFFFAOYSA-M Fluoride anion Chemical compound [F-] KRHYYFGTRYWZRS-UHFFFAOYSA-M 0.000 claims description 3
- QAOWNCQODCNURD-UHFFFAOYSA-L Sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 claims description 3
- 150000001875 compounds Chemical class 0.000 claims description 3
- ZOMNIUBKTOKEHS-UHFFFAOYSA-L dimercury dichloride Chemical class Cl[Hg][Hg]Cl ZOMNIUBKTOKEHS-UHFFFAOYSA-L 0.000 claims description 3
- DDFHBQSCUXNBSA-UHFFFAOYSA-N 5-(5-carboxythiophen-2-yl)thiophene-2-carboxylic acid Chemical compound S1C(C(=O)O)=CC=C1C1=CC=C(C(O)=O)S1 DDFHBQSCUXNBSA-UHFFFAOYSA-N 0.000 claims description 2
- MIMUSZHMZBJBPO-UHFFFAOYSA-N 6-methoxy-8-nitroquinoline Chemical compound N1=CC=CC2=CC(OC)=CC([N+]([O-])=O)=C21 MIMUSZHMZBJBPO-UHFFFAOYSA-N 0.000 claims description 2
- PQUCIEFHOVEZAU-UHFFFAOYSA-N Diammonium sulfite Chemical compound [NH4+].[NH4+].[O-]S([O-])=O PQUCIEFHOVEZAU-UHFFFAOYSA-N 0.000 claims description 2
- 229910052936 alkali metal sulfate Inorganic materials 0.000 claims description 2
- BFNBIHQBYMNNAN-UHFFFAOYSA-N ammonium sulfate Chemical compound N.N.OS(O)(=O)=O BFNBIHQBYMNNAN-UHFFFAOYSA-N 0.000 claims description 2
- 229910052921 ammonium sulfate Inorganic materials 0.000 claims description 2
- 235000011130 ammonium sulphate Nutrition 0.000 claims description 2
- XYXNTHIYBIDHGM-UHFFFAOYSA-N ammonium thiosulfate Chemical compound [NH4+].[NH4+].[O-]S([O-])(=O)=S XYXNTHIYBIDHGM-UHFFFAOYSA-N 0.000 claims description 2
- GRWVQDDAKZFPFI-UHFFFAOYSA-H chromium(III) sulfate Chemical compound [Cr+3].[Cr+3].[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O GRWVQDDAKZFPFI-UHFFFAOYSA-H 0.000 claims description 2
- 229940044652 phenolsulfonate Drugs 0.000 claims description 2
- RCIVOBGSMSSVTR-UHFFFAOYSA-L stannous sulfate Chemical compound [SnH2+2].[O-]S([O-])(=O)=O RCIVOBGSMSSVTR-UHFFFAOYSA-L 0.000 claims description 2
- 229910000375 tin(II) sulfate Inorganic materials 0.000 claims description 2
- PXGOKWXKJXAPGV-UHFFFAOYSA-N Fluorine Chemical compound FF PXGOKWXKJXAPGV-UHFFFAOYSA-N 0.000 claims 2
- LSNNMFCWUKXFEE-UHFFFAOYSA-N Sulfurous acid Chemical compound OS(O)=O LSNNMFCWUKXFEE-UHFFFAOYSA-N 0.000 claims 2
- 229910052731 fluorine Inorganic materials 0.000 claims 2
- 239000011737 fluorine Substances 0.000 claims 2
- KJCVRFUGPWSIIH-UHFFFAOYSA-N 1-naphthol Chemical compound C1=CC=C2C(O)=CC=CC2=C1 KJCVRFUGPWSIIH-UHFFFAOYSA-N 0.000 claims 1
- IRPGOXJVTQTAAN-UHFFFAOYSA-N 2,2,3,3,3-pentafluoropropanal Chemical compound FC(F)(F)C(F)(F)C=O IRPGOXJVTQTAAN-UHFFFAOYSA-N 0.000 claims 1
- KLZUFWVZNOTSEM-UHFFFAOYSA-K Aluminum fluoride Inorganic materials F[Al](F)F KLZUFWVZNOTSEM-UHFFFAOYSA-K 0.000 claims 1
- 229910001512 metal fluoride Inorganic materials 0.000 claims 1
- 239000004922 lacquer Substances 0.000 abstract description 21
- 230000007797 corrosion Effects 0.000 abstract description 10
- 238000005260 corrosion Methods 0.000 abstract description 10
- 229940107218 chromium Drugs 0.000 description 72
- 235000012721 chromium Nutrition 0.000 description 72
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 18
- 230000000052 comparative effect Effects 0.000 description 14
- 238000000151 deposition Methods 0.000 description 14
- 230000008021 deposition Effects 0.000 description 14
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 14
- 229910001128 Sn alloy Inorganic materials 0.000 description 12
- 230000007423 decrease Effects 0.000 description 11
- 238000003466 welding Methods 0.000 description 11
- 238000010438 heat treatment Methods 0.000 description 10
- NNIPDXPTJYIMKW-UHFFFAOYSA-N iron tin Chemical compound [Fe].[Sn] NNIPDXPTJYIMKW-UHFFFAOYSA-N 0.000 description 10
- 230000008859 change Effects 0.000 description 9
- 238000005868 electrolysis reaction Methods 0.000 description 9
- 229910052759 nickel Inorganic materials 0.000 description 9
- 239000000243 solution Substances 0.000 description 9
- 239000005028 tinplate Substances 0.000 description 7
- 239000011248 coating agent Substances 0.000 description 6
- 238000000576 coating method Methods 0.000 description 6
- 239000005029 tin-free steel Substances 0.000 description 6
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 4
- 238000010306 acid treatment Methods 0.000 description 4
- 238000000137 annealing Methods 0.000 description 4
- 238000005238 degreasing Methods 0.000 description 4
- 238000005554 pickling Methods 0.000 description 4
- 230000010287 polarization Effects 0.000 description 4
- PUZPDOWCWNUUKD-UHFFFAOYSA-M sodium fluoride Chemical compound [F-].[Na+] PUZPDOWCWNUUKD-UHFFFAOYSA-M 0.000 description 4
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 3
- 239000003513 alkali Substances 0.000 description 3
- 230000004888 barrier function Effects 0.000 description 3
- 239000002585 base Substances 0.000 description 3
- 238000009792 diffusion process Methods 0.000 description 3
- 238000001035 drying Methods 0.000 description 3
- 238000004070 electrodeposition Methods 0.000 description 3
- 238000004826 seaming Methods 0.000 description 3
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 2
- 239000004677 Nylon Substances 0.000 description 2
- 239000000853 adhesive Substances 0.000 description 2
- 230000001070 adhesive effect Effects 0.000 description 2
- 239000000443 aerosol Substances 0.000 description 2
- 230000032683 aging Effects 0.000 description 2
- 239000010960 cold rolled steel Substances 0.000 description 2
- 229910052802 copper Inorganic materials 0.000 description 2
- 239000010949 copper Substances 0.000 description 2
- 229910052742 iron Inorganic materials 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 229920001778 nylon Polymers 0.000 description 2
- 230000001105 regulatory effect Effects 0.000 description 2
- 239000011775 sodium fluoride Substances 0.000 description 2
- 235000013024 sodium fluoride Nutrition 0.000 description 2
- 238000005476 soldering Methods 0.000 description 2
- 238000012360 testing method Methods 0.000 description 2
- WHOZNOZYMBRCBL-OUKQBFOZSA-N (2E)-2-Tetradecenal Chemical compound CCCCCCCCCCC\C=C\C=O WHOZNOZYMBRCBL-OUKQBFOZSA-N 0.000 description 1
- JHWIEAWILPSRMU-UHFFFAOYSA-N 2-methyl-3-pyrimidin-4-ylpropanoic acid Chemical compound OC(=O)C(C)CC1=CC=NC=N1 JHWIEAWILPSRMU-UHFFFAOYSA-N 0.000 description 1
- LSNNMFCWUKXFEE-UHFFFAOYSA-M Bisulfite Chemical compound OS([O-])=O LSNNMFCWUKXFEE-UHFFFAOYSA-M 0.000 description 1
- 229910001854 alkali hydroxide Inorganic materials 0.000 description 1
- 229910001515 alkali metal fluoride Inorganic materials 0.000 description 1
- 150000008044 alkali metal hydroxides Chemical class 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- KGBXLFKZBHKPEV-UHFFFAOYSA-N boric acid Chemical compound OB(O)O KGBXLFKZBHKPEV-UHFFFAOYSA-N 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000010924 continuous production Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000018109 developmental process Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000004090 dissolution Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000007654 immersion Methods 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 229910052738 indium Inorganic materials 0.000 description 1
- 230000006698 induction Effects 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- CLDVQCMGOSGNIW-UHFFFAOYSA-N nickel tin Chemical compound [Ni].[Sn] CLDVQCMGOSGNIW-UHFFFAOYSA-N 0.000 description 1
- 229940044654 phenolsulfonic acid Drugs 0.000 description 1
- 230000002265 prevention Effects 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 229910000679 solder Inorganic materials 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 238000009864 tensile test Methods 0.000 description 1
Abstract
ABSTRACT OF THE DISCLOSURE
A surface treated steel sheet having double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide are formed on a low tin plated steel sheet in which 30 to 80% of the surface of the steel sheet is covered with plated tin and the size of the exposed steel surface units, after tinplating, is 0.5 to 20 µm in diameter when expressing the irregularly exposed areas as circles, and a method for production of this surface treated steel sheet which comprises; (a) electro-plating with a small amount of tin under restricted condi-tions in order to obtain the low tin plated steel sheet described above, (b) after reflowing or without reflowing of the plated tin, formation of said double layer or formation of the metallic chromium layer followed by formation of hydrated chromium oxide under restricted conditions.
This surface treated steel sheet is suitable for producing a welded can body without removing the plated layer at high speed, since it is excellent in weldability, lacquer adhesion and corrosion resistance after lacquering.
A surface treated steel sheet having double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide are formed on a low tin plated steel sheet in which 30 to 80% of the surface of the steel sheet is covered with plated tin and the size of the exposed steel surface units, after tinplating, is 0.5 to 20 µm in diameter when expressing the irregularly exposed areas as circles, and a method for production of this surface treated steel sheet which comprises; (a) electro-plating with a small amount of tin under restricted condi-tions in order to obtain the low tin plated steel sheet described above, (b) after reflowing or without reflowing of the plated tin, formation of said double layer or formation of the metallic chromium layer followed by formation of hydrated chromium oxide under restricted conditions.
This surface treated steel sheet is suitable for producing a welded can body without removing the plated layer at high speed, since it is excellent in weldability, lacquer adhesion and corrosion resistance after lacquering.
Description
1~80085 SURFACE TREATED STEEL SHEET FOR WELDED CAN MATERIAL
AND METHOD FOR ITS PRODUCTION
FIELD GF THE INVENTION
The present invention relates to a surface treated steel sheet being excellent in weldability, lacquer adhesion and corrosion resistance after lacquering and a method for lts production. In detail, the present invention relates to a surface treated steel sheet having double layers consis-ting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide on a low tin plated steel sheet which is characterized by the state of plated tin, and a method for production of this surface treated steel sheet which is characterized by an electroplating with a small amount of tin on a steel sheet under restricted conditions and by the formation of a metallic chromium layer and hydrated chromium oxide layer on a low tin plated steel sheet under restricted conditions.
By using this surface treated steel sheet, a welded can body can be easily produced at high speed without removing the plated layer, in spite of the presence of a double layer consisting of metallic chromium and hydrated chromium oxide on a low tin plated steel sheet.
BACKGROUND AND OBJECTIVE
Generally, the seaming of a can body in a three piece can consisting of two can ends and a single can body is carried out soldering, adhesion with a nylon adhesive and electric welding.
. Recently, the electric welding has been widely used for the seaming of the tinplate can body in the field of food cans, aerosol cans and miscellaneous cans, instead of soldering with a solder of regulated lead content. In the seaming of the tinplate can body, it is desirable to de-crease the tin coating weight in tinplate, because tin used for the production of tinplate is very expensive. However, the weldability of tinplate gradually becomes poor with a decrease of the tin coating weight.
From the background described above, the development of 1~8008~;
a welded can material, which is cheaper than conventional electrotinplate, is easily welded without removing the plated layer at high speed and is excellent in lacquer adhesion and corrosion resistance after lac~uering, has been required in the field of food cans.
Within the last few years, various surface treated steel sheets have been proposed as welded can materials having the characteristics described above. For instance, low tin plated steel sheet (LTS) with below about 1000 mg/m2 of tin which is reflowed or unreflowed after tinplating has ~een proposed. However, this LTS has a narrower current range for s~und welding than that for tinplate. The reason is considered to be that the amount of ~ree metallic tin in this LTS is smaller than that in tinplate and also further decreases because of the change of plated free metallic tin to iron-tin alloy by heating for lacquer curing or reflowing after tinplating. For the improvement in the weldability of this LTS, the following three methods have been proposed.
The first method is one in which a steel sheet is plated with a small amount of nickel before tinplating. In this method, a decrease in the amount of plated free metallic tin, that is, the change of plated metallic tin to iron-tin alloy by heating for lacquer curing, is suppressed because a dense nickel-tin alloy layer formed during aging at room temperature or a dense iron-tin alloy containing nickel formed by reflowing after tinplating, acts as a barrier for the diffusion of iron to plated tin. The second method is one in which nickel is plated on a steel sheet before annealing and then all or a part of the plated nickel is diffused on the surface of the steel sheet by heating for the annealing of the steel sheet, after which a small amount of tin is plated on the steel sheet covered with a nickel diffusion layer. The third method is one in which tin is plated on a steel sheet before annealing, instead of nickel in the second method.
In the second and third methods, a nickel diffusion layer or an iron-tin alloy layer formed on the steel sheet by heating for the annealing of the steel sheet acts as a barrier for the change of the plated metallic tin to iron-1~ 80085 tin alloy by heating for the lacquer curing or reflowingafter tinplating.
Although the weldability and the corrosion resistance after lacquering of the LTS by these methods described above are improved, the excellent lacquer adhesion required for a can material is not obtained. The reason is considered to be that the surface of the LTS is oxidized during aging in an ordinary atmosphere because the surface of the LTS is not sufficiently covered with the film formed by an electric chromic acid treatment. If the surface of the LTS is suffi-ciently covered with this film, the weldability becomes poor, although the lacquer adhesion of the LTS may be improved.
Accordingly, it is the first obiective of the present invention to provide a surface treated steel sheet having excellent weldability, excellent lacquer adhesion and excellent corrosion resistance after lacquering for a welded can material.
It is the second objective of the present invention to provide a method for the continuous production of a surface treated steel sheet having ~xcellent characteristics as described above.
BRIEF DESCRIPTION OF THE INVENTION
The first objective of the present invention can be accomplished by providing a surface treated steel sheet having double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide on a low tin plated steel sheet in which 30 to 80% of the surface of the steel sheet is covered with plated tin and the size of the exposed steel surface units after tinplating is 0.5 to 20 um in diameter, when expressing the irregularly ex-posed areas as circles.
The second objective of the present invention can be accomplished by an electroplating with a small amount of tin `~
on a steel sheet under special electroplating conditions which is characterized by a lower current density and lower amount of additives in the tinplating electrolyte compared with these in conventional electrotinplating and by the deposition of metallic chromium on plated tin and the exposed area of steel sheet which is not plated with tin lX800l35 under special conditions which is characterized by a cath-rlic electrolysis under higher current density regulated bycathodic potential for the electrodeposition of metallic chromium on said tin plated steel sheet.
It is a very important point and an inventive feature in the present invention that the exposed steel surface lies scattered after tinplating and metallic chromium is posit-ively deposited on the surfaces of p:Lated tin and the ex-posed steel which is not plated with tin, and furthermore that the surface of metallic chromiwn is uniformly covered with hydrated chromium oxide layer. That is to say, it is considered that the surface treated steel sheet according to the present invention is a hybrid of a tin free steel (TFS) wherein a steel sheet is covered with double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide and a tin plated steel sheet, in which demerits are overcome and merits are retained in both surface treated steel sheets.
The surface treated steel sheet according to the pre-sent invention can be used in applications wherein excellent weldability, i.e. easily being welded without the removal of the plated layer at high speed, is required, such as food can bodies and aerosol can bodies which are lacquered, except for the welded part before welding. Furthermore, the surface treated steel sheet of the present invention can also be used in applications wherein excellent lacquer adhesion and excellent corrosion resistance after lacquering are required such as can ends, drawn cans and drawn and redrawn cans (DR cans), besides can bodies.
DETAILED DESCRIPTION OF THE INVENTION
The steel sheet used for the production of the surface treated steel sheet according to the present invention can be any cold rolled steel sheet customarily used in manu-facturing electrotinplate and TFS. Preferably, the thick-ness of the steel sheet is from 0.1 to about 0.35 mm.
The surface treated steel sheet according to the present invention is produced by the foilowing processes:
(1) degreasing with an alkali and pickling with an acid ~ water rinsing ~ tinplating under special conditions -~ water rinsing , chromium plating under 1'~8008S
special conditions 3 water rinsing -~ formation of hydrated chromium oxide > water rinsing -> drying or (2) degreasing with an alkali and pickling with an acid - ~ water rinsing - > tinplating under special conditions > water rinsing - -~ simultaneous ~ormation of metallic chromium and hydrated chromium oxide under special conditions > water rinsing ~ drying. In both methods reflowing after tinplating may be carried out.
Furthermore, water rinsing after chromium plating may be omitted in method (1).
In the surface treated steel sheet according to the present invention, the state of tin plated on a steel sheet is very important.
30 to 80% of the surface of the steel sheet should be covered with plated tin and the size of the exposed steel surface which lies scattered after tinplating should be 0.5 to 20 l~m, more preferably 1 to 10 ~m in diameter when expressing the irregularly exposed areas as circles. In the case where above 80~ of the surface of the steel sheet is covered with plated tin or the size of the exposed steel surface is below 0.5 ~m in diameter, weldability and lacquer adhesion are not improved because the greater part of the surface of the steel sheet is covered with plated tin and the greater part of the plated tin changes to iron-tin alloy by heating for lacquer curing or reflowing after tinplating.
If the size of the exposed steel surface after tinplating is above 20 ~m in diameter, the exposed steel surface units become continuous and a greater part of plated tin becomes a granular deposit of 0.1 to 1 ~m in diameter. As a result, plated tin is easily peeled off from the surface of the steel sheet. If the surface of the steel sheet covered with plated tin is below 30%, excellent weldability is not obtained, particularly in the case of a small amount of plated tin.
Tin coating weight is also one of the important factors in the surface treated steel sheet according to the present invention. The optimum range of the tin coating weight is from 50 to 900 mg/m , more preferably 100 to 600 mg~m . At below 50 mg/m2 of tin coating weight, excellent weldability 1~80085 is not obtained because the amount of metallic tin decreases remarkably by the change to iron-tin alloy by heating for lacquer curing or reflowing after tinplating.
At above 900 mg/m2 of plated tin, lacquer adhesion becomes poor as with electrotinplate because the greater part of the steel surface is uniformly covered with plated tin, although excellent weldability is obtained.
In order to obtain the tin plated steel sheet having features described above, the steel sheet is plated with tin under the following conditions after degreasing with an alkali and pickling with an acid:
Tinplating electrolyte Stannous phenolsulfonate bath or stannous sulfate bath Concentration of stannous ion 30 to 80 g/l Concentration of acid as sulfuric acid 15 to 60 g/l Concentration of additives 0.2 to 2 g/l Temperature of the electrolyte 40 to 60C
Cathodic current density 2 to 10 A/dm2 Generally, in the case that tinplating electrolyte having higher concentrations of stannous ion, acid and additives is used, a higher current density and higher temp-erature of the electrolyte should be selected. On the contrary, in lower concentrations of stannous ion, acid and additives, lower current density and lower temperature should be selected in order to insure that the exposed steel surface of the tin plated steel sheet lies scattered after tinplating. However, even if tinplating is carried out under the conditions limited in the present invention, an increase in the amount of plated tin leads to a decrease in the exposed steel surface after tinplating. Therefore, it is indispensable in the present invention that the amount of plated tin is maintained below 900 mg/m . In the tinplating conditions described above, the range of current density and the range in the concentration of additives in the tinplating electrolyte are particularly important factors for the production of the surface treated steel sheet according to the present invention.
~80085 The concentration of additives below 0.2 g/1 is not suitable in the present invention because the adhesion of plated tin to the steel sheet becomes poor and plated tin is easily peeled off from the surface of the steel sheet. At above 2 g/1 in the concentration of additives, excellent weldability and excellent lacquer adhesion are not obtained because the greater part of the steel surface is uniformly covered with plated tin. Above 10 A/dm of current density is not preferable in the present invention for the formation of a uniform plated tin layer on the steel sheet. A current density below 0.2 A/dm2 is not suitable for high speed production of the surface treated steel sheet according to the present invention.
In the present invention, an a-naphthol additive such as ethoxylated a-naphthol and ethoxylated a-naphthol sulfonic acid, which are used as additives in tinplating electrolyte for the production of conventional electrotin-plate, are suitable.
The tin plated steel sheet produced under the condi-tions described above is covered with metallic chromium layer and hydrated chromium oxide layer. The amount of metallic chromium and hydrated chromium oxide formed on the tin plated steel sheet are also important factors in the present invention. The amount of metallic chromium should be controlled in the range of 7 to 100 mg/m2, more pre-ferably 20 to 70 mgtm2. If the amount of metallic chromium is below 7 mg/m2, the surface treated steel sheet being excellent in weldabilityr lacquer adhesion and corrosion resistance after lac~uering is not obtained because the surfaces of plated tin and the exposed steel which is not plated with tin are not sufficiently covered with deposited metallic chromium. At above 100 mg/m of metallic chromium, weldability becomes poor, although the corrosion resistance after lacquering is improved, if the surface of metallic chromium is uniformly covered with hydrated chromium oxide.
The optimum range of hydrated chromium oxide formed on the metallic chromium layer is 5 to 50 mg/m2 as chromium, more preferably 7 to 30 mg/m2 as chromium.
If the amount of hydrated chromium oxide formed on the , . - 7 -~80085 metallic chromium layer is below 5 mg/m2, the corrosion resistance after lacquering and the lacquer adhesion become poor, although the weldability is excellent, because the surface of the metallic chromium layer is not sufficiently covered with -the formed hydrated chromium oxide. It is not also preferable that the amount of hydrated chromium oxide is above 50 mg/m2 as chromium because the weldability becomes remarkably poor by an increase of hydrated chromium oxide which has high electric resistance.
For the formation of double layer consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide or metallic chromium layer followed by the formation of hydrated chromium oxide layer on the tin plated steel sheet obtained under the conditions described above, the following two methods, which are used for the production of TFS, are utilized. One is a tWO step process in which metallic chromium is plated by a cathodic electrolysis in a known chromium plating electrolyte such as a Sargent bath or a highly concentrated chromic acid electrolyte containing additives such as fluorine compounds and sulfur compounds and then hydrated chromium oxide is formed on the metallic chromium layer by a cathodic electrolysis in a dilute con-centrated chromic acid electrolyte containing additives described above. The other is a one step process in which said double layer is simultaneously formed on the tin plated steel sheet by a cathodic electrolysis in a dilute concent-rated chromic acid electrolyte containing additives des-cribed above.
However, the conditions for the electrodeposition of metallic chromium in the one step process or two step process are very important in the present invention.
Namely, it is indispensable in the present invention that the electrode potential of the tin plated steel sheet in the chromic acid electrolyte used for the electrodeposition of metallic chromium is kept to less noble potential than that for the deposition of metallic chromium from chromic acid.
Therefore, it is preferable that the tin plated steel sheet is potentiostatically electrolyzed at a less noble potential than that for the deposition of metallic chromium ~R~08~
from chromic acid. However, conven-tional electrotinplate and TFS are industrially produced by a galvanostatic electrolysis. If -the tin p]ated steel sheet is gal-vanostatically electrolyzed under a ca-thodic current density in which the electrode potential of the tin plated steel sheet is kep-t noble with respec-t -to tha-t for the deposition of metallic chrolnium, the surface treated steel shee-t having excellent weldability, excellent lacquer adhesion and excellent corrosion resistance after lacquering is not obtained because a large amount of hydrated chromium oxide containing a little amount of metallic chromium is formed on the tin plated steel sheet.
In the case of simultaneous formation of metallic chromium and hydrated chromium oxyde on the tin plated steel sheet by using a one step process used for the production of TFS, the conditions for the deposition of metallic chromium should be preferentially decided from the electrode potential of the tin plated steel sheet in the chromic acid electrolyte; after that, the amount of hydrated chromium oxide should be controlled.
In the present invention, it is preferable to employ the following electrolytic chromium plating conditions for the formation of a metallic chromium layer on a tin plated steel sheet by using a one step method or a two step method:
Concentration of chromic acid: 30 to 300 g/l, more preferably 30 to 100 g/1 in the one step method and 100 to 300 g/1 in the two step method.
Concentration of S042 and F additives:
1.0 to 5.0 weight~, more preferably 1.0 to 3.0 weight%
of the concentration of chromic acid.
1~8008~;
Additives: at least one compound selected from the group consisting of fluorine compounds, such as hydrofluoric acid, fluoboric acid, fluosilicic acid, ammonium bifluoride, an alkali metal bifluoride, ammonium fluoride, an alkali metal fluoride, ammonium fluoborate, an alkali metal fluoborate, ammonium fluosili-cate, an alkali metal fluosilicate, aluminum fluorideand sulfur compounds such as sulfuric acid, ammonium sulfate, an alkali metal sulfate, chromium sulfate, ammonium sulfite, an alkali metal sulfite, ammonium thiosulfate, an alkali metal thiosulfate.
Temperature of the electrolyte: 30 to 60C.
Cathodic current density: higher than that shown when the elec,rode potential of th~ .in plated steel sheet is kep. -~o -0.8 to -1.0 V vs SCE in the chromic acid electrolyte described above.
Generally, the amount of hydrated chromium oxide.formed during chromium plating decreases with an increase in the concentration of chromic acid in a suitable weight ratio of additives to chromic acid.
It is not preferable to use an electrolyte having below 30 g/l of chromic acid for the chromium plating, because the current efficiency for the deposition of metallic chromium decreases remarkably. The concentration of chromic acid above 300 g/l is also not suitable from an economical point of view. The presence of additives such as fluorine compounds and sulfur compounds in the chromium plating electrolyte is indispensable for a uniform chromium deposition. If the weight % of fluor-ide ion or sulfate ion in the additives to chromic acid is below 1.0 or above 5.0, the current efficiency for the deposition of metallic chromium remarkably decreases, in addition to a decrease in the uniformity of the de-posited metallic chromium and hydrated chromium oxide.
Particularly, at below a 1.0 value for the weight % of additives to chromic acid, the formed insoluble hydrated ,., -- 10 --1~80085 chromium oxide to chromic acid, an insoluble hydrated chromium oxide is formed on the metallic chromium layer and the weldability becomes remarkably poor. The amount of hydrated chromium oxide formed on the metallic chromium layer decreases with an increase in the temper-ature of the electrolyte. The temperature of the elec-trolyte above 60C is not suitable from an industrial point of view, because the current efficiency for the deposition of metallic chromium decreases remarkably.
The temperature of the electrolyte below 30C is also not suitable because a large amount of hydrated chromium oxide is formed.
In some cases of the production of the sur-face treated steel sheet according to the present in-vention, the tin plated steel sheet is reflowed before the deposition of metallic chromium or electrolytic chromic acid treatment. Reflowing the tin plated steel sheet gives good effects for the adhesion of the plated tin to the steel sheet and the prevention to the incre-ment of ircn t'n alloy during heating for lacquer curing because iron-tin alloy layer formed between the plated tin and the steel sheet acts as a barrier for the change of the plated tin to the iron-tin alloy.
The known reflowing method in which a temper-ature above the melting point of tin is maintained for /' a short time by resistance heating and/or induction heating can be used for reflowing of the tin plated steel sheet in the present invention.
It is suitable in the present invention thatthe tin plated steel sheet is heated to the melting point of tin to 350C for 0.5 to 3 seconds and then immediately quenched into water.
Reflowing at higher temperature for longer time is not desirable because of the poor weldability caused by the change of a large part of plated tin to iron-tin alloy, particularly in the case of lower amount of plated tin.
1~800~S
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows cathodic polarization curves of a steel sheet and a tin sheet by a potentiostatic electrolysis wherein the steel sheet and the tin sheet are polarized to less potential from the rest potential of each sheet at 50 mV/min of polarization speed;
Figures 2 and 3 each show a magnified sche-matic diagram in cross section of a surface treated steel sheet according to the present invention;
Figures 4 and 5 each show a magnified photo-graph by a scanning electronmicroscope and the inten-sities of Sn.K~ and Cr.K~ of a surface treated steel sheet according to the present invention.
Figure 1 shows cathodic polarization curves of steel sheet and tin sheet by a potentiostatic elec-trolysis in which steel sheet and tin sheet are polar-ized to less noble potential from the rest potential of each sheet at 50 mV/min of polarization speed in an aqueous solution consisting of 50 g/l of chromic acid, 0.5 g/1 of sulfuric acid and 5 g/l of sodium fluoride under 120 m/min of flow speed of the solution at a 50C
solution temperature.
It is found from Figure 1 that metallic chromium is deposited at less noble potential than -1.0 V versus Saturated Calomel Electrode (vs SCE) and the formation and the dissolution of hydrated chromium oxide is repeated in -0.8 to -1.0 V vs SCE on the steel sheet and the tin sheet, and furthermore the current density on the tin sheet shown in -0.8 to -1.0 V vs SCE
of the potential range is remarkably largerthan that on the steel sheet. In the case of potentiostatic electrolysis, it is possible to deposit metallic chro-mium on the steel sheet and the tin sheet, if the electrode potential of the steel sheet and the tin sheet is kept to less noble potential than -1.0 V vs SCE in the chromic acid solution described above.
However, it is found from Figure 1 that metallic chro-mium is deposited on the steel sheet, but hydrated 3008~i chromium oxide is formed on the tin sheet without the deposition of metallic chromium, if the steel sheet and the tin sheet are galvanostatically electrolyzed under the same cathodic current density of 30 A/dm2, because the electrode potential of the steel sheet is kept to about -1.5 V vs SCE but that of the tin sheet moves to -0.8 to -1.0 vs SCE. The behavior of the tin plated steel sheet is the same as the tin sheet. There-fore, the tin plated steel sheet should be electrolyzed under a cathodic current density above 30 A/dm2 for the deposition of metallic chromium on the tin plated steel sheet.
Generally, current density for the reduction of chromic acid at -0.8 to -1.0 V vs SCE increases with increases in the concentration of chromic acid, temper-ature of chromic acid solution and the flow speed of the solution. For instance, the tin plated steel sheet should be electrolyzed under a current density above 50 A/dm2 for the deposition of metallic chromium on the tin plated steel sheet, if the concentration of chromic acid in the electrolyte increases to 250 g/l at the same temper-ature and the same flow speed of the electrolyte as shown in Figure 1.
Therefore, it is indispensable in the present invention that the tin plated steel sheet in which the exposed steel s~rface lies scattered after tinplating be galvanostatically electrolyzed under a higher current density than that encountered when the electrode potential of the tin plated steel sheet is kept to -0.8 to -1.0 V vs SCE in the chromic acid electrolyte for the deposition of metallic chromium.
Figure 2 shows the state wherein the surfaces of plated tin 3 and the expose~ steel base 5 are covered wlth metallic chromium 2 and hydrated chromium oxide 1. Figure 3 shows the state wherein iron-tin alloy 4 is formed in the intersurface between plated tin 3 and steel base 5 by reflowing after tinplating and the surfaces of plated tin and the exposea steel lZ80085 base are covered with metallic chromium 2 and hydrated chromium oxide 1 the same as shown in Figure 2.
Figure 4 shows a magnified photograph by a scanning electronmicorscope and the intensities of Sn.K~ and Cr.K~ of a surface treated steel sheet according to the present invention which is produced by the following conditions: A steel sheet was plated with 470 mg/m2 of tin under the conditions of the present invention and then it was covered with metallic chromium of 45 mg/m2 and hydrated chromium oxide of 13 mg/m2 as chromium by a potentiostatic electrolysis under the conditions wherein the electrode potential of the tin plated steel sheet was kept to -1.5 V vs SCE in the chromic acid electrolyte containing 50 g/1 of chromic acid, 0.5 g/l of sulfuric acid and 5 g/l of sodium fluoride at 50C and 120 m/min of flow speed of the electrolyte. In this case, the measured cathodic current density was 30 to 32 A/dm2.
Figure 5 also shows a magnified photograph by a scanning electronmicroscope and the intensity of Sn.K~ and Cr.K~ of a surface treated steel sheet which is produced by the following conditions: A steel sheet was plated with 470 mg/m2 of tin under the conditions of the present invention and then it was treated galvanostatically in the chromic acid electrolyte used for the sample of Figure 4 under 28 A/dm2 of cathodic current density. In this case, the measured electrode potential of the tin plated steel sheet was -0.9 to -1.0 V vs SCE and the tin plated steel sheet was covered with the film having 60 mg/m2 of total chromium.
In Figure 4 and Figure 5, a white straight line shows the scanned position for the measurement of the intensities of Sn.K~ and Cr.K~.
- 13a -The changes in the intensities of Sn Ka and Cr Ka were shown in the upper part and lower part of the scanning line, respectively.
In Figure 4 and Figure 5, it was found from the change of the intensity of Sn Ka that the white part was the tin plated part and the black part was the exposed steel surface after tinplating.
Furthermore, it was found from the change of the intensity of Cr~Ka that a film containing chromium was uniformly formed on the plated tin and the exposed steel surface in Figure 4 and a thicker film containing chromium was formed only on the plated tin in Figure 5.
By using a known chemical method in which hydrated chromium oxide was dissolved in an alkali hydroxide solution, it was found the film containing chromium consists of metallic chromium and hydrated chromium oxide in Figure 4 and only hydrated chromium oxide in Figure 5.
EXAMPLES OF THE PRESENT INVENTION
The present invention is illustrated by the following Examples.
In Example 1 to Example 7 and Comparative example 1 to Comparative example 7, a cold rolled steel sheet having a thickness of 0.21 mm was basically treated by the following process after electrolytically degreasing in a solution of 70 g/1 of sodium hydroxide under 10 A/dm2 of cathodic current density for 2 seconds at 70 C, water rinsing, pickling by an immersion into 70 g/l of sulfuric acid for 3 seconds at 25 C and then water rinsing.
In Example 1 to Example 5 and Comparative example 1 to Comparative example 5, a steel sheet pretreated under the conditions described above was treated by the following process.
Tinplating ~ water rinsing , formation of metallic chromium and hydrated chromium oxide ~ water rinsing ~ drying.
In Example 6 and Example 7, chromium plating was carried out after tinplating and then hydrated chromium oxide was formed.
In Comparative example 6 which shows an example of electrotinplate, tin plated steel sheet was treated by using sodium dichromate solution. In Comparative example 7 which , - 14 -1~800~35 shows an example of tin free steel, a steel sheet was directly treated by chromic acid solution containing additives.
In Example 2, Example 4, Example 7, Comparative example 2, Comparative example 4 and Comparative example 6, the tin plated steel sheet was immediately quenched in water after raising the temperature of the tin plated steel sheet to 280 ~C during 1.6 seconds before chromium plating or electro-lytic chromic acid treatment. In Comparative example 5, nickel is plated on a steel sheet by using a Watt's bath containing 250 g/l of NiSO4-6H20, 30 g/l of NiC12-6H2O and 40 g/l of H3BO3 under 5 A/dm2 of cathodic current density at Furthermore, phenolsulfonic acid as the acid and ethoxylated a-naphthol as the additive in the tinplating electrolyte are respectively used in Example 1 to Example 7 and Comparative example 1 to Comparative example 6.
In each Example and Comparative example, the conditions for tinplating, chromium plating and electrolytic chromic acid treatment are shown in detail in the Table.
The weldability and lacquer adhesion of the thus treated steel sheet in the above described Examples and Comparative examples were evaluated by the following testing methods after the measurement of the amounts of plated nickel, plated tin, metallic chromium and chromium in hydrated chromium oxide by the fluorescent X-ray method, the results of which are shown in the attached Table.
(1) Weldability The weldability is evaluated by an available range of secondary current in welding as shown in the report by N.T.
Williams (Metal Construction, April 1977, pages 157-160), that is to say, the wider the secondary current range in welding, the better the weldability. The upper limit in the available secondary current range corresponds to the welding conditions in which some defect such as splashing is found and the lower limit corresponds to the welding conditions in which the breakage occurs in the welded part tearing tests.
In order to obtain data wherein the available range of secondary current in welding is decided in each sample, large amounts of samples are necessary.
1~80S)85 Therefore, the weldability was evalua-ted by an electric contact resistance according to the following method, because an electric contact resistance has an apparent correlation with an available range of secondary current in welding as shown in the report by T. Fujimura (Journal of the Iron and Steel Institute of Japan, Vol. 69, No. 13, Sept. 1983, page 181), that is, the lower the electric contact resistance, the wider the secondary current range in welding. Accordingly, if the electric contact resistance is lower, the weldability is better.
At first, the sample was treated on both sides cut to a size of 20 mm x 100 mm after baking at 210 C for 20 minutes. The electric contact resistance of the sample was calculated from the change of voltage in a pair of copper disk electrodes (diameter: 65 mm, thickness: 2 mm) to which 5 amperes of direct current were supplied and 50 kg of load was added, when two sample pieces were inserted between a pair of the copper disk electrodes rotating at Sm/min.
(2) Lacguer adhesion The sample was baked at 210 C for 12 minutes after coating with 50 mg/dm2 of an epoxy-phenolic type of lacquer.
The two coated sample pieces, which were each cut to a size of 5 mm x 150 mm, were bonded together using a nylon adhesive having a thickness of 100 ~m at 200 C for 30 seconds under 3 kg/cm2 of pressure by a Hot Press.
The bonding strength of the assembly, which is shown as kg/5mm, was measured by a conventional tensile testing machine.
~8008S
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~.
AND METHOD FOR ITS PRODUCTION
FIELD GF THE INVENTION
The present invention relates to a surface treated steel sheet being excellent in weldability, lacquer adhesion and corrosion resistance after lacquering and a method for lts production. In detail, the present invention relates to a surface treated steel sheet having double layers consis-ting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide on a low tin plated steel sheet which is characterized by the state of plated tin, and a method for production of this surface treated steel sheet which is characterized by an electroplating with a small amount of tin on a steel sheet under restricted conditions and by the formation of a metallic chromium layer and hydrated chromium oxide layer on a low tin plated steel sheet under restricted conditions.
By using this surface treated steel sheet, a welded can body can be easily produced at high speed without removing the plated layer, in spite of the presence of a double layer consisting of metallic chromium and hydrated chromium oxide on a low tin plated steel sheet.
BACKGROUND AND OBJECTIVE
Generally, the seaming of a can body in a three piece can consisting of two can ends and a single can body is carried out soldering, adhesion with a nylon adhesive and electric welding.
. Recently, the electric welding has been widely used for the seaming of the tinplate can body in the field of food cans, aerosol cans and miscellaneous cans, instead of soldering with a solder of regulated lead content. In the seaming of the tinplate can body, it is desirable to de-crease the tin coating weight in tinplate, because tin used for the production of tinplate is very expensive. However, the weldability of tinplate gradually becomes poor with a decrease of the tin coating weight.
From the background described above, the development of 1~8008~;
a welded can material, which is cheaper than conventional electrotinplate, is easily welded without removing the plated layer at high speed and is excellent in lacquer adhesion and corrosion resistance after lac~uering, has been required in the field of food cans.
Within the last few years, various surface treated steel sheets have been proposed as welded can materials having the characteristics described above. For instance, low tin plated steel sheet (LTS) with below about 1000 mg/m2 of tin which is reflowed or unreflowed after tinplating has ~een proposed. However, this LTS has a narrower current range for s~und welding than that for tinplate. The reason is considered to be that the amount of ~ree metallic tin in this LTS is smaller than that in tinplate and also further decreases because of the change of plated free metallic tin to iron-tin alloy by heating for lacquer curing or reflowing after tinplating. For the improvement in the weldability of this LTS, the following three methods have been proposed.
The first method is one in which a steel sheet is plated with a small amount of nickel before tinplating. In this method, a decrease in the amount of plated free metallic tin, that is, the change of plated metallic tin to iron-tin alloy by heating for lacquer curing, is suppressed because a dense nickel-tin alloy layer formed during aging at room temperature or a dense iron-tin alloy containing nickel formed by reflowing after tinplating, acts as a barrier for the diffusion of iron to plated tin. The second method is one in which nickel is plated on a steel sheet before annealing and then all or a part of the plated nickel is diffused on the surface of the steel sheet by heating for the annealing of the steel sheet, after which a small amount of tin is plated on the steel sheet covered with a nickel diffusion layer. The third method is one in which tin is plated on a steel sheet before annealing, instead of nickel in the second method.
In the second and third methods, a nickel diffusion layer or an iron-tin alloy layer formed on the steel sheet by heating for the annealing of the steel sheet acts as a barrier for the change of the plated metallic tin to iron-1~ 80085 tin alloy by heating for the lacquer curing or reflowingafter tinplating.
Although the weldability and the corrosion resistance after lacquering of the LTS by these methods described above are improved, the excellent lacquer adhesion required for a can material is not obtained. The reason is considered to be that the surface of the LTS is oxidized during aging in an ordinary atmosphere because the surface of the LTS is not sufficiently covered with the film formed by an electric chromic acid treatment. If the surface of the LTS is suffi-ciently covered with this film, the weldability becomes poor, although the lacquer adhesion of the LTS may be improved.
Accordingly, it is the first obiective of the present invention to provide a surface treated steel sheet having excellent weldability, excellent lacquer adhesion and excellent corrosion resistance after lacquering for a welded can material.
It is the second objective of the present invention to provide a method for the continuous production of a surface treated steel sheet having ~xcellent characteristics as described above.
BRIEF DESCRIPTION OF THE INVENTION
The first objective of the present invention can be accomplished by providing a surface treated steel sheet having double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide on a low tin plated steel sheet in which 30 to 80% of the surface of the steel sheet is covered with plated tin and the size of the exposed steel surface units after tinplating is 0.5 to 20 um in diameter, when expressing the irregularly ex-posed areas as circles.
The second objective of the present invention can be accomplished by an electroplating with a small amount of tin `~
on a steel sheet under special electroplating conditions which is characterized by a lower current density and lower amount of additives in the tinplating electrolyte compared with these in conventional electrotinplating and by the deposition of metallic chromium on plated tin and the exposed area of steel sheet which is not plated with tin lX800l35 under special conditions which is characterized by a cath-rlic electrolysis under higher current density regulated bycathodic potential for the electrodeposition of metallic chromium on said tin plated steel sheet.
It is a very important point and an inventive feature in the present invention that the exposed steel surface lies scattered after tinplating and metallic chromium is posit-ively deposited on the surfaces of p:Lated tin and the ex-posed steel which is not plated with tin, and furthermore that the surface of metallic chromiwn is uniformly covered with hydrated chromium oxide layer. That is to say, it is considered that the surface treated steel sheet according to the present invention is a hybrid of a tin free steel (TFS) wherein a steel sheet is covered with double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide and a tin plated steel sheet, in which demerits are overcome and merits are retained in both surface treated steel sheets.
The surface treated steel sheet according to the pre-sent invention can be used in applications wherein excellent weldability, i.e. easily being welded without the removal of the plated layer at high speed, is required, such as food can bodies and aerosol can bodies which are lacquered, except for the welded part before welding. Furthermore, the surface treated steel sheet of the present invention can also be used in applications wherein excellent lacquer adhesion and excellent corrosion resistance after lacquering are required such as can ends, drawn cans and drawn and redrawn cans (DR cans), besides can bodies.
DETAILED DESCRIPTION OF THE INVENTION
The steel sheet used for the production of the surface treated steel sheet according to the present invention can be any cold rolled steel sheet customarily used in manu-facturing electrotinplate and TFS. Preferably, the thick-ness of the steel sheet is from 0.1 to about 0.35 mm.
The surface treated steel sheet according to the present invention is produced by the foilowing processes:
(1) degreasing with an alkali and pickling with an acid ~ water rinsing ~ tinplating under special conditions -~ water rinsing , chromium plating under 1'~8008S
special conditions 3 water rinsing -~ formation of hydrated chromium oxide > water rinsing -> drying or (2) degreasing with an alkali and pickling with an acid - ~ water rinsing - > tinplating under special conditions > water rinsing - -~ simultaneous ~ormation of metallic chromium and hydrated chromium oxide under special conditions > water rinsing ~ drying. In both methods reflowing after tinplating may be carried out.
Furthermore, water rinsing after chromium plating may be omitted in method (1).
In the surface treated steel sheet according to the present invention, the state of tin plated on a steel sheet is very important.
30 to 80% of the surface of the steel sheet should be covered with plated tin and the size of the exposed steel surface which lies scattered after tinplating should be 0.5 to 20 l~m, more preferably 1 to 10 ~m in diameter when expressing the irregularly exposed areas as circles. In the case where above 80~ of the surface of the steel sheet is covered with plated tin or the size of the exposed steel surface is below 0.5 ~m in diameter, weldability and lacquer adhesion are not improved because the greater part of the surface of the steel sheet is covered with plated tin and the greater part of the plated tin changes to iron-tin alloy by heating for lacquer curing or reflowing after tinplating.
If the size of the exposed steel surface after tinplating is above 20 ~m in diameter, the exposed steel surface units become continuous and a greater part of plated tin becomes a granular deposit of 0.1 to 1 ~m in diameter. As a result, plated tin is easily peeled off from the surface of the steel sheet. If the surface of the steel sheet covered with plated tin is below 30%, excellent weldability is not obtained, particularly in the case of a small amount of plated tin.
Tin coating weight is also one of the important factors in the surface treated steel sheet according to the present invention. The optimum range of the tin coating weight is from 50 to 900 mg/m , more preferably 100 to 600 mg~m . At below 50 mg/m2 of tin coating weight, excellent weldability 1~80085 is not obtained because the amount of metallic tin decreases remarkably by the change to iron-tin alloy by heating for lacquer curing or reflowing after tinplating.
At above 900 mg/m2 of plated tin, lacquer adhesion becomes poor as with electrotinplate because the greater part of the steel surface is uniformly covered with plated tin, although excellent weldability is obtained.
In order to obtain the tin plated steel sheet having features described above, the steel sheet is plated with tin under the following conditions after degreasing with an alkali and pickling with an acid:
Tinplating electrolyte Stannous phenolsulfonate bath or stannous sulfate bath Concentration of stannous ion 30 to 80 g/l Concentration of acid as sulfuric acid 15 to 60 g/l Concentration of additives 0.2 to 2 g/l Temperature of the electrolyte 40 to 60C
Cathodic current density 2 to 10 A/dm2 Generally, in the case that tinplating electrolyte having higher concentrations of stannous ion, acid and additives is used, a higher current density and higher temp-erature of the electrolyte should be selected. On the contrary, in lower concentrations of stannous ion, acid and additives, lower current density and lower temperature should be selected in order to insure that the exposed steel surface of the tin plated steel sheet lies scattered after tinplating. However, even if tinplating is carried out under the conditions limited in the present invention, an increase in the amount of plated tin leads to a decrease in the exposed steel surface after tinplating. Therefore, it is indispensable in the present invention that the amount of plated tin is maintained below 900 mg/m . In the tinplating conditions described above, the range of current density and the range in the concentration of additives in the tinplating electrolyte are particularly important factors for the production of the surface treated steel sheet according to the present invention.
~80085 The concentration of additives below 0.2 g/1 is not suitable in the present invention because the adhesion of plated tin to the steel sheet becomes poor and plated tin is easily peeled off from the surface of the steel sheet. At above 2 g/1 in the concentration of additives, excellent weldability and excellent lacquer adhesion are not obtained because the greater part of the steel surface is uniformly covered with plated tin. Above 10 A/dm of current density is not preferable in the present invention for the formation of a uniform plated tin layer on the steel sheet. A current density below 0.2 A/dm2 is not suitable for high speed production of the surface treated steel sheet according to the present invention.
In the present invention, an a-naphthol additive such as ethoxylated a-naphthol and ethoxylated a-naphthol sulfonic acid, which are used as additives in tinplating electrolyte for the production of conventional electrotin-plate, are suitable.
The tin plated steel sheet produced under the condi-tions described above is covered with metallic chromium layer and hydrated chromium oxide layer. The amount of metallic chromium and hydrated chromium oxide formed on the tin plated steel sheet are also important factors in the present invention. The amount of metallic chromium should be controlled in the range of 7 to 100 mg/m2, more pre-ferably 20 to 70 mgtm2. If the amount of metallic chromium is below 7 mg/m2, the surface treated steel sheet being excellent in weldabilityr lacquer adhesion and corrosion resistance after lac~uering is not obtained because the surfaces of plated tin and the exposed steel which is not plated with tin are not sufficiently covered with deposited metallic chromium. At above 100 mg/m of metallic chromium, weldability becomes poor, although the corrosion resistance after lacquering is improved, if the surface of metallic chromium is uniformly covered with hydrated chromium oxide.
The optimum range of hydrated chromium oxide formed on the metallic chromium layer is 5 to 50 mg/m2 as chromium, more preferably 7 to 30 mg/m2 as chromium.
If the amount of hydrated chromium oxide formed on the , . - 7 -~80085 metallic chromium layer is below 5 mg/m2, the corrosion resistance after lacquering and the lacquer adhesion become poor, although the weldability is excellent, because the surface of the metallic chromium layer is not sufficiently covered with -the formed hydrated chromium oxide. It is not also preferable that the amount of hydrated chromium oxide is above 50 mg/m2 as chromium because the weldability becomes remarkably poor by an increase of hydrated chromium oxide which has high electric resistance.
For the formation of double layer consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide or metallic chromium layer followed by the formation of hydrated chromium oxide layer on the tin plated steel sheet obtained under the conditions described above, the following two methods, which are used for the production of TFS, are utilized. One is a tWO step process in which metallic chromium is plated by a cathodic electrolysis in a known chromium plating electrolyte such as a Sargent bath or a highly concentrated chromic acid electrolyte containing additives such as fluorine compounds and sulfur compounds and then hydrated chromium oxide is formed on the metallic chromium layer by a cathodic electrolysis in a dilute con-centrated chromic acid electrolyte containing additives described above. The other is a one step process in which said double layer is simultaneously formed on the tin plated steel sheet by a cathodic electrolysis in a dilute concent-rated chromic acid electrolyte containing additives des-cribed above.
However, the conditions for the electrodeposition of metallic chromium in the one step process or two step process are very important in the present invention.
Namely, it is indispensable in the present invention that the electrode potential of the tin plated steel sheet in the chromic acid electrolyte used for the electrodeposition of metallic chromium is kept to less noble potential than that for the deposition of metallic chromium from chromic acid.
Therefore, it is preferable that the tin plated steel sheet is potentiostatically electrolyzed at a less noble potential than that for the deposition of metallic chromium ~R~08~
from chromic acid. However, conven-tional electrotinplate and TFS are industrially produced by a galvanostatic electrolysis. If -the tin p]ated steel sheet is gal-vanostatically electrolyzed under a ca-thodic current density in which the electrode potential of the tin plated steel sheet is kep-t noble with respec-t -to tha-t for the deposition of metallic chrolnium, the surface treated steel shee-t having excellent weldability, excellent lacquer adhesion and excellent corrosion resistance after lacquering is not obtained because a large amount of hydrated chromium oxide containing a little amount of metallic chromium is formed on the tin plated steel sheet.
In the case of simultaneous formation of metallic chromium and hydrated chromium oxyde on the tin plated steel sheet by using a one step process used for the production of TFS, the conditions for the deposition of metallic chromium should be preferentially decided from the electrode potential of the tin plated steel sheet in the chromic acid electrolyte; after that, the amount of hydrated chromium oxide should be controlled.
In the present invention, it is preferable to employ the following electrolytic chromium plating conditions for the formation of a metallic chromium layer on a tin plated steel sheet by using a one step method or a two step method:
Concentration of chromic acid: 30 to 300 g/l, more preferably 30 to 100 g/1 in the one step method and 100 to 300 g/1 in the two step method.
Concentration of S042 and F additives:
1.0 to 5.0 weight~, more preferably 1.0 to 3.0 weight%
of the concentration of chromic acid.
1~8008~;
Additives: at least one compound selected from the group consisting of fluorine compounds, such as hydrofluoric acid, fluoboric acid, fluosilicic acid, ammonium bifluoride, an alkali metal bifluoride, ammonium fluoride, an alkali metal fluoride, ammonium fluoborate, an alkali metal fluoborate, ammonium fluosili-cate, an alkali metal fluosilicate, aluminum fluorideand sulfur compounds such as sulfuric acid, ammonium sulfate, an alkali metal sulfate, chromium sulfate, ammonium sulfite, an alkali metal sulfite, ammonium thiosulfate, an alkali metal thiosulfate.
Temperature of the electrolyte: 30 to 60C.
Cathodic current density: higher than that shown when the elec,rode potential of th~ .in plated steel sheet is kep. -~o -0.8 to -1.0 V vs SCE in the chromic acid electrolyte described above.
Generally, the amount of hydrated chromium oxide.formed during chromium plating decreases with an increase in the concentration of chromic acid in a suitable weight ratio of additives to chromic acid.
It is not preferable to use an electrolyte having below 30 g/l of chromic acid for the chromium plating, because the current efficiency for the deposition of metallic chromium decreases remarkably. The concentration of chromic acid above 300 g/l is also not suitable from an economical point of view. The presence of additives such as fluorine compounds and sulfur compounds in the chromium plating electrolyte is indispensable for a uniform chromium deposition. If the weight % of fluor-ide ion or sulfate ion in the additives to chromic acid is below 1.0 or above 5.0, the current efficiency for the deposition of metallic chromium remarkably decreases, in addition to a decrease in the uniformity of the de-posited metallic chromium and hydrated chromium oxide.
Particularly, at below a 1.0 value for the weight % of additives to chromic acid, the formed insoluble hydrated ,., -- 10 --1~80085 chromium oxide to chromic acid, an insoluble hydrated chromium oxide is formed on the metallic chromium layer and the weldability becomes remarkably poor. The amount of hydrated chromium oxide formed on the metallic chromium layer decreases with an increase in the temper-ature of the electrolyte. The temperature of the elec-trolyte above 60C is not suitable from an industrial point of view, because the current efficiency for the deposition of metallic chromium decreases remarkably.
The temperature of the electrolyte below 30C is also not suitable because a large amount of hydrated chromium oxide is formed.
In some cases of the production of the sur-face treated steel sheet according to the present in-vention, the tin plated steel sheet is reflowed before the deposition of metallic chromium or electrolytic chromic acid treatment. Reflowing the tin plated steel sheet gives good effects for the adhesion of the plated tin to the steel sheet and the prevention to the incre-ment of ircn t'n alloy during heating for lacquer curing because iron-tin alloy layer formed between the plated tin and the steel sheet acts as a barrier for the change of the plated tin to the iron-tin alloy.
The known reflowing method in which a temper-ature above the melting point of tin is maintained for /' a short time by resistance heating and/or induction heating can be used for reflowing of the tin plated steel sheet in the present invention.
It is suitable in the present invention thatthe tin plated steel sheet is heated to the melting point of tin to 350C for 0.5 to 3 seconds and then immediately quenched into water.
Reflowing at higher temperature for longer time is not desirable because of the poor weldability caused by the change of a large part of plated tin to iron-tin alloy, particularly in the case of lower amount of plated tin.
1~800~S
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows cathodic polarization curves of a steel sheet and a tin sheet by a potentiostatic electrolysis wherein the steel sheet and the tin sheet are polarized to less potential from the rest potential of each sheet at 50 mV/min of polarization speed;
Figures 2 and 3 each show a magnified sche-matic diagram in cross section of a surface treated steel sheet according to the present invention;
Figures 4 and 5 each show a magnified photo-graph by a scanning electronmicroscope and the inten-sities of Sn.K~ and Cr.K~ of a surface treated steel sheet according to the present invention.
Figure 1 shows cathodic polarization curves of steel sheet and tin sheet by a potentiostatic elec-trolysis in which steel sheet and tin sheet are polar-ized to less noble potential from the rest potential of each sheet at 50 mV/min of polarization speed in an aqueous solution consisting of 50 g/l of chromic acid, 0.5 g/1 of sulfuric acid and 5 g/l of sodium fluoride under 120 m/min of flow speed of the solution at a 50C
solution temperature.
It is found from Figure 1 that metallic chromium is deposited at less noble potential than -1.0 V versus Saturated Calomel Electrode (vs SCE) and the formation and the dissolution of hydrated chromium oxide is repeated in -0.8 to -1.0 V vs SCE on the steel sheet and the tin sheet, and furthermore the current density on the tin sheet shown in -0.8 to -1.0 V vs SCE
of the potential range is remarkably largerthan that on the steel sheet. In the case of potentiostatic electrolysis, it is possible to deposit metallic chro-mium on the steel sheet and the tin sheet, if the electrode potential of the steel sheet and the tin sheet is kept to less noble potential than -1.0 V vs SCE in the chromic acid solution described above.
However, it is found from Figure 1 that metallic chro-mium is deposited on the steel sheet, but hydrated 3008~i chromium oxide is formed on the tin sheet without the deposition of metallic chromium, if the steel sheet and the tin sheet are galvanostatically electrolyzed under the same cathodic current density of 30 A/dm2, because the electrode potential of the steel sheet is kept to about -1.5 V vs SCE but that of the tin sheet moves to -0.8 to -1.0 vs SCE. The behavior of the tin plated steel sheet is the same as the tin sheet. There-fore, the tin plated steel sheet should be electrolyzed under a cathodic current density above 30 A/dm2 for the deposition of metallic chromium on the tin plated steel sheet.
Generally, current density for the reduction of chromic acid at -0.8 to -1.0 V vs SCE increases with increases in the concentration of chromic acid, temper-ature of chromic acid solution and the flow speed of the solution. For instance, the tin plated steel sheet should be electrolyzed under a current density above 50 A/dm2 for the deposition of metallic chromium on the tin plated steel sheet, if the concentration of chromic acid in the electrolyte increases to 250 g/l at the same temper-ature and the same flow speed of the electrolyte as shown in Figure 1.
Therefore, it is indispensable in the present invention that the tin plated steel sheet in which the exposed steel s~rface lies scattered after tinplating be galvanostatically electrolyzed under a higher current density than that encountered when the electrode potential of the tin plated steel sheet is kept to -0.8 to -1.0 V vs SCE in the chromic acid electrolyte for the deposition of metallic chromium.
Figure 2 shows the state wherein the surfaces of plated tin 3 and the expose~ steel base 5 are covered wlth metallic chromium 2 and hydrated chromium oxide 1. Figure 3 shows the state wherein iron-tin alloy 4 is formed in the intersurface between plated tin 3 and steel base 5 by reflowing after tinplating and the surfaces of plated tin and the exposea steel lZ80085 base are covered with metallic chromium 2 and hydrated chromium oxide 1 the same as shown in Figure 2.
Figure 4 shows a magnified photograph by a scanning electronmicorscope and the intensities of Sn.K~ and Cr.K~ of a surface treated steel sheet according to the present invention which is produced by the following conditions: A steel sheet was plated with 470 mg/m2 of tin under the conditions of the present invention and then it was covered with metallic chromium of 45 mg/m2 and hydrated chromium oxide of 13 mg/m2 as chromium by a potentiostatic electrolysis under the conditions wherein the electrode potential of the tin plated steel sheet was kept to -1.5 V vs SCE in the chromic acid electrolyte containing 50 g/1 of chromic acid, 0.5 g/l of sulfuric acid and 5 g/l of sodium fluoride at 50C and 120 m/min of flow speed of the electrolyte. In this case, the measured cathodic current density was 30 to 32 A/dm2.
Figure 5 also shows a magnified photograph by a scanning electronmicroscope and the intensity of Sn.K~ and Cr.K~ of a surface treated steel sheet which is produced by the following conditions: A steel sheet was plated with 470 mg/m2 of tin under the conditions of the present invention and then it was treated galvanostatically in the chromic acid electrolyte used for the sample of Figure 4 under 28 A/dm2 of cathodic current density. In this case, the measured electrode potential of the tin plated steel sheet was -0.9 to -1.0 V vs SCE and the tin plated steel sheet was covered with the film having 60 mg/m2 of total chromium.
In Figure 4 and Figure 5, a white straight line shows the scanned position for the measurement of the intensities of Sn.K~ and Cr.K~.
- 13a -The changes in the intensities of Sn Ka and Cr Ka were shown in the upper part and lower part of the scanning line, respectively.
In Figure 4 and Figure 5, it was found from the change of the intensity of Sn Ka that the white part was the tin plated part and the black part was the exposed steel surface after tinplating.
Furthermore, it was found from the change of the intensity of Cr~Ka that a film containing chromium was uniformly formed on the plated tin and the exposed steel surface in Figure 4 and a thicker film containing chromium was formed only on the plated tin in Figure 5.
By using a known chemical method in which hydrated chromium oxide was dissolved in an alkali hydroxide solution, it was found the film containing chromium consists of metallic chromium and hydrated chromium oxide in Figure 4 and only hydrated chromium oxide in Figure 5.
EXAMPLES OF THE PRESENT INVENTION
The present invention is illustrated by the following Examples.
In Example 1 to Example 7 and Comparative example 1 to Comparative example 7, a cold rolled steel sheet having a thickness of 0.21 mm was basically treated by the following process after electrolytically degreasing in a solution of 70 g/1 of sodium hydroxide under 10 A/dm2 of cathodic current density for 2 seconds at 70 C, water rinsing, pickling by an immersion into 70 g/l of sulfuric acid for 3 seconds at 25 C and then water rinsing.
In Example 1 to Example 5 and Comparative example 1 to Comparative example 5, a steel sheet pretreated under the conditions described above was treated by the following process.
Tinplating ~ water rinsing , formation of metallic chromium and hydrated chromium oxide ~ water rinsing ~ drying.
In Example 6 and Example 7, chromium plating was carried out after tinplating and then hydrated chromium oxide was formed.
In Comparative example 6 which shows an example of electrotinplate, tin plated steel sheet was treated by using sodium dichromate solution. In Comparative example 7 which , - 14 -1~800~35 shows an example of tin free steel, a steel sheet was directly treated by chromic acid solution containing additives.
In Example 2, Example 4, Example 7, Comparative example 2, Comparative example 4 and Comparative example 6, the tin plated steel sheet was immediately quenched in water after raising the temperature of the tin plated steel sheet to 280 ~C during 1.6 seconds before chromium plating or electro-lytic chromic acid treatment. In Comparative example 5, nickel is plated on a steel sheet by using a Watt's bath containing 250 g/l of NiSO4-6H20, 30 g/l of NiC12-6H2O and 40 g/l of H3BO3 under 5 A/dm2 of cathodic current density at Furthermore, phenolsulfonic acid as the acid and ethoxylated a-naphthol as the additive in the tinplating electrolyte are respectively used in Example 1 to Example 7 and Comparative example 1 to Comparative example 6.
In each Example and Comparative example, the conditions for tinplating, chromium plating and electrolytic chromic acid treatment are shown in detail in the Table.
The weldability and lacquer adhesion of the thus treated steel sheet in the above described Examples and Comparative examples were evaluated by the following testing methods after the measurement of the amounts of plated nickel, plated tin, metallic chromium and chromium in hydrated chromium oxide by the fluorescent X-ray method, the results of which are shown in the attached Table.
(1) Weldability The weldability is evaluated by an available range of secondary current in welding as shown in the report by N.T.
Williams (Metal Construction, April 1977, pages 157-160), that is to say, the wider the secondary current range in welding, the better the weldability. The upper limit in the available secondary current range corresponds to the welding conditions in which some defect such as splashing is found and the lower limit corresponds to the welding conditions in which the breakage occurs in the welded part tearing tests.
In order to obtain data wherein the available range of secondary current in welding is decided in each sample, large amounts of samples are necessary.
1~80S)85 Therefore, the weldability was evalua-ted by an electric contact resistance according to the following method, because an electric contact resistance has an apparent correlation with an available range of secondary current in welding as shown in the report by T. Fujimura (Journal of the Iron and Steel Institute of Japan, Vol. 69, No. 13, Sept. 1983, page 181), that is, the lower the electric contact resistance, the wider the secondary current range in welding. Accordingly, if the electric contact resistance is lower, the weldability is better.
At first, the sample was treated on both sides cut to a size of 20 mm x 100 mm after baking at 210 C for 20 minutes. The electric contact resistance of the sample was calculated from the change of voltage in a pair of copper disk electrodes (diameter: 65 mm, thickness: 2 mm) to which 5 amperes of direct current were supplied and 50 kg of load was added, when two sample pieces were inserted between a pair of the copper disk electrodes rotating at Sm/min.
(2) Lacguer adhesion The sample was baked at 210 C for 12 minutes after coating with 50 mg/dm2 of an epoxy-phenolic type of lacquer.
The two coated sample pieces, which were each cut to a size of 5 mm x 150 mm, were bonded together using a nylon adhesive having a thickness of 100 ~m at 200 C for 30 seconds under 3 kg/cm2 of pressure by a Hot Press.
The bonding strength of the assembly, which is shown as kg/5mm, was measured by a conventional tensile testing machine.
~8008S
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Claims (12)
1. A surface treated steel sheet having double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide on a low tin plated steel sheet, wherein the amount of plated tin is 50 to 900 mg/m2, the amount of metallic chromium in said lower layer is 7 to 100 mg/m2 and the amount of hydrated chromium oxide in said upper layer is 5 to 50 mg/m2 as chromium, and wherein 30 to 80% of the surface of the steel sheet is covered with plated tin and the size of the exposed steel surface areas, after tinplating, is 0.5 to 20 µm in diameter when expressing the irregularly exposed areas as circles.
2. A surface treated steel sheet having double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide on a low tin plated and reflowed steel sheet, wherein the amount of plated tin is 50 to 900 mg/m2, the amount of metallic chromium in said lower layer is 7 to 100 mg/m2 and the amount of hydrated chromium oxide in said upper layer is 5 to 50 mg/m2 as chromium, and wherein 30 to 80%
of the surface of the steel sheet is covered with plated tin and the size of the exposed steel surface areas, after tinplating, is 0.5 to 20 µm in diameter when expressing the irregularly exposed areas as circles.
of the surface of the steel sheet is covered with plated tin and the size of the exposed steel surface areas, after tinplating, is 0.5 to 20 µm in diameter when expressing the irregularly exposed areas as circles.
3. A surface treated steel sheet according -to claims 1 or 2, wherein the amount of the plated tin is 100 to 600 mg/m2, the amount of metallic chromium in said lower layer is 20 to 70 mg/m2 and the amount of hydrated chromium oxide in said upper layer is 7 to 30 mg/m2 as chromium.
4. A process for continuously producing a surface treated steel sheet, which comprises the steps of:
a) electroplating a steel sheet with tin in an amount of 50 to 900 mg/m2;
b) chromium plating the tin plated steel sheet obtained in step (a) in an electrolyte containing 100 to 300 g/l of chromic acid and at least one additive selected from the group consisting of fluorine and sulfur compounds to provide from 1 to
a) electroplating a steel sheet with tin in an amount of 50 to 900 mg/m2;
b) chromium plating the tin plated steel sheet obtained in step (a) in an electrolyte containing 100 to 300 g/l of chromic acid and at least one additive selected from the group consisting of fluorine and sulfur compounds to provide from 1 to
5% of fluoride, sulfite, sulfate or thiosulfate ions, based on the weight of chromic acid, thereby forming a layer of metallic chromium on the tin plated steel sheet, in which the amount of metallic chromium is 7 to 100 mg/m2; and c) forming a top layer of hydrated chromium oxide on the chromium and tin plated steel sheet obtained in step (b), the amount of hydrated chromium oxide in said top layer ranging from 5 to 50 mg/m2 as chromium.
5. A process according to claim 4, wherein the tin plated steel sheet obtained in step (a) is reflowed prior to the chromium plating of step (b).
5. A process according to claim 4, wherein the tin plated steel sheet obtained in step (a) is reflowed prior to the chromium plating of step (b).
6. A process for continuously producing a surface treated steel sheet, which comprises the steps of:
a) electroplating a steel sheet with tin in an amount of 50 to 900 mg/m2; and b) forming a double layer consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide on the tin plated steel sheet obtained in step (a), the amount of metallic chromium in said lower layer ranging from 7 to 100 mg/m2 and the amount of hydrated chromium oxide in said upper layer ranging from 5 to 50 mg/m2 as chromium, the formation of said double layer on the tin plated steel sheet being carried out in an electrolyte containing 30 to 100 g/l of chromic acid and at least one additive selected from the group consisting of fluorine and sulfur compounds to provide 1 to 5% of fluoride, sulfite, sulfate or thiosulfate ions, based on the weight of chromic acid.
a) electroplating a steel sheet with tin in an amount of 50 to 900 mg/m2; and b) forming a double layer consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide on the tin plated steel sheet obtained in step (a), the amount of metallic chromium in said lower layer ranging from 7 to 100 mg/m2 and the amount of hydrated chromium oxide in said upper layer ranging from 5 to 50 mg/m2 as chromium, the formation of said double layer on the tin plated steel sheet being carried out in an electrolyte containing 30 to 100 g/l of chromic acid and at least one additive selected from the group consisting of fluorine and sulfur compounds to provide 1 to 5% of fluoride, sulfite, sulfate or thiosulfate ions, based on the weight of chromic acid.
7. A process according to claim 6, wherein the tin plated steel sheet obtained in step (a) is reflowed prior to the formation of said double layer in step (b).
8. A process according to claims 4 or 6, wherein the tin plating of step (a) is carried out at a temperature of 40 to 60°C and under a cathodic current density of 2 to 10 A/dm2 in stannous sulfate electrolyte or stannous phenolsulfonate electrolyte containing 30 to 80 g/l of stannous ion, 15 to 60 g/l of acid and 0.2 to 2 g/l of an .alpha.-naphthol additive.
9. A process according to claim 4, wherein the chromium plating of step (b) is carried out at a temperature of 30 to 60°C and under higher current density than that shown when the electrode potential of the tin plated steel sheet is kept at -0.8 to -1.0 V vs a saturated calomel electrode.
10. A process according to claim 6, wherein the formation of said double layer in step (b) is carried out at a temperature of 30 to 60°C and under higher current density than that shown when the electrode potential of the tin plated steel sheet is kept at -0.8 to -1.0 V vs a saturated calomel electrode.
11. A process according to claims 4 or 6, wherein said fluorine compound is at least one compound selected from the group consisting of hydrofluoric acid, fluoboric acid, fluosilicic acid, ammonium bifluoride, an alkali metal bifluoride, ammonium fluoride, an akali metal fluoride, ammonium fluoborate, an alkali metal fluoborate, ammonium fluosilicate, an alkali metal fluosilicate and aluminum fluoride.
12. A process according to claims 4 or 6, wherein said sulfur compound is at least one compound selected from the group consisting of sulfuric acid, ammonium sulfate, an alkali metal sulfate, chromium sulfate, ammonium sulfite, an alkali metal sulfite, ammonium thiosulfate and an alkali metal thiosulfate.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000502984A CA1280085C (en) | 1986-02-28 | 1986-02-28 | Surface treated steel sheet for welded can material and method for its production |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000502984A CA1280085C (en) | 1986-02-28 | 1986-02-28 | Surface treated steel sheet for welded can material and method for its production |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1280085C true CA1280085C (en) | 1991-02-12 |
Family
ID=4132579
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000502984A Expired - Lifetime CA1280085C (en) | 1986-02-28 | 1986-02-28 | Surface treated steel sheet for welded can material and method for its production |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1280085C (en) |
-
1986
- 1986-02-28 CA CA000502984A patent/CA1280085C/en not_active Expired - Lifetime
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