GB2180257A - Thin tin and nickel plated steel sheet for welded can material - Google Patents

Description

SPECIFICATION Method for producing a thin tin and nickel plated steel sheet for welded can material Field of theinvention The present invention relates to a method for producing a thin tin and nickel plated steel sheet having excellent corrosion resistance after lacquering and excellent weldability. In detail, the invention relates to a method for producing a thin tin and nickel plated steel sheet which is characterized by an anodictreatmentof apickledsteelsheetinanalkalineelectrolytewithapHofabove10beforeplatingwithasmallamountof nickel, plating with a sma 11 amou nt of ti n on the nickel plated steel sheet, reflowing, quenching, and chromate treating the tin and nickel plated steel sheet.

By using this tin and nickel plated steel sheet, a welded can body can be easily produced at high speed in spiteofthesmallamountoftheplatedtinandnickelwithoutremovingtheplatedlayerintheweldedpart.

Background and objective Recently, electric welding has beenwidely usedfortheseaming oftinplatecan bodies in the field of food cans, aerosol cans, and miscellaneous cans, instead ofsoldering. In the seaming ofthetinplate can body, it is desirable to decrease the tin coating weight in thetinplate becausetin used for electrotinplate is very expensive and there is concern overthe exhaustion of tin resources. However, the weldability of thetinplate becomes gradually poorwith a decrease ofthe tin coating weight.

From the background described above, the development of a welded can material which is cheaperthan conventional electrotinplate, is easily welded at high speed withoutthe removal of the plated layer and is excellent in corrosion resistance after lacquering, has been required in the field of food cans. Overthe past fewyears, varioussurfacetreated steel sheets have been proposedforwelded can materialswhich can be easilywelded at high speedwithoutthe removal ofthe plated layerand ischeaperthantinplate. Forinstance, the following surface treated steel sheets have been propsed : (a) Lightlytin plated steel sheet (LTS) with below about 1. 0 g/m2 of tin which is reflowed or unreflowed aftertinplating (Japanese Patent Publication Nos.

Sho 56-3440, Sho 56-54070, Sho 57-55800, and Laid-Open Japanese PatentApplication Nos. Sho 56-75589, Sho 56-130487, Sho 56-156788, Sho 57-101694, Sho 57-185997, Sho 57-192294, Sho 57-192295, and Sho 5569297). (b) Nickel preplated LTS with below about 1. 0 g/m2 of tin (Laid-Open Japanese PatentApplication Nos.

Sho 57-23091, Sho 57-67196, Sho 57-110685, Sho 57-177991, Sho 57-200592, Sho 57-203797, Sho 60-33362, and Sho 60-56074). (c) Nickel plated steel sheetwith chromatefilm or phosphate film (Laid-Open Japanese PatentApplication Nos. 56-116885, Sho 56-169788, Sho 57-2892, Sho 57-2895, Sho 57-2896, Sho 57-2897, Sho 57-35697, Sho 57-35698).

However, LTS and nickel preplated LTS above identified as (a) and (b) have a narrowercurrent rangefor sound welding than that in tinplate, although these can bewelded withoutthe removal ofthe plated layer.

The reason whythe current rangeforsound welding in LTS and nickel preplated LTS is narrowerthan in tinplate is considered to be that the amount of free metallictin in these is smallerthan that in tinplate and also further decreases because of the change of plated free metallictin to iron-tin alloy or iron-tin-nickel alloy by heating forlacquercuring or reflowing aftertinplating.

An increase ofthetin coating weight in LTS and nickel plated LTS is contrary to the development of cheaper welded can material than tinplate, although the weldability and the corrosion resistance after lacquering are improved with an increase of it. An increase in the amount of nickel in nickel preplated LTS improvesthe corrosion resistance after lacquering, butdoes not improvetheweldability, becausethe amount offree metallictindecreasesbytheformationoftin-nickelalloyduringagingatroomtemperatureorbytheform- ation of iron tin-nickel alloy during reflowing nickel preplated LTS.

Nickel plated steel sheet with chromate film or phosphate film identified above as (c) also has a narrower current range forsound welding than that in tinplate, LTS, or nickel preplated LTS.

Furthermore, the corrosion resistance of nickel plated steel sheet is poor, although the lacquer adhesion is good. Particularly, pitting corrosion in the defective part of the lacquered nickel plated steel sheet may occur easily in acidic foods such astomato juice because the electric potential of nickel is more noble than that of steel sheet.

As described above, the various surfacetreated steel sheets proposed in (a), (b) and (c) have various prob- lems in production costand characteristics as a welded can material which can be easilywelded withoutthe removal of the plated layer at highspeed.

Accordingly, it isthe first objective of the present invention to provide a thin tin and nickel plated steel sheet having excellent corrosion resistance after lacquering and excellent weldability.

It isthe second objective of the present invention to provide a method forthe continuous production of a thin tin and nickel plated steel sheet having excellent characteristics as described above.

Brief description of the invention The first objective of the present invention can be accomplished by providing a thin tin and nickel plated steel sheet having a surface structure in which the distribution of numerous nodules of metallictin is observed by using an electron microscope on the iron-tin-nickel alloy layerformed on the steel sheet as shown in Figure 1.

The second objective of the present invention can be accomplished byan electroplating with asmall amountofnickel onthesteel sheetwhich is anodically treated in an alkaline electrolytewith a pH of above 10 followed by an electroplating with a small amount of tin onthe nickel plated steel sheet, reflowing, quenching, and then chromate treating thetin and nickel plated steel sheet.

Thetin and nickel plated steel sheetaccordingtothe methodofthe presentinvention is clearlydifferent from the nickel preplated LTS, which has already been disclosed in various Laid-Open Japanese PatentApp lications, in thesurface structure, particularlyin theform of metallictin onthe iron-tin-nickel alloy layer formed on the steel sheet, although it nevertheless is a nickel preplated LTS. Namely, in the tin and nickel plated steel sheetaccording to the present invention, numerous nodules of metallictin are presentonthe iron-tin-nickel alloy layerformed on the steel sheet. On the otherhand, itisconsidered that a uniform andthin metallictin layerisformed on the iron-tin-nickel alloy layer orthetin-nickel alloy layerformed onthesteel sheet in the known nickel preplated LTS.

Detailed description of the in vention The steel sheet used forthe production of the tin and nickel plated steel sheetaccording tothe present invention can be any cold rolled steel sheet customarily used in manufacturing electro-tin plated and tin-free steel. Preferablythethickness of the steel sheet is from about 0. 1 to about0. 35mm.

Thetinand nickel platedsteel sheetaccordingtothepresentinvention isproduced bythefollowing process : degreasingwithanalkaliandpicklingwithanacidowaterrinsingoananodictreatmentinanalkaline electroXyteo water rinsingo nickel platinge water rinsinge tinplating < water rinsing < dryingo reflow- ing quenching- > chromate treatment-water rinsing-drying.

In this process, it is possiblethatthe anodicallytreated steel sheet is plated with tin-nickel alloy, nickel-iron alloy, nickel-zinc alloy, or nickel containing boron and phosphorus instead of plating with nickel. In the present invention, an anodic treatment of a pickled steel sheet in an alkaline electrolytewith a pH of above 10 is indispensable in orderto obtain the thin tin and nickel plated steel sheet having a surface structure in which the distribution of numerous nodules of metallictin is observed by using an electron microscope on the iron-tin-nickel alloylayerformed onthesteel sheetafterreflowing ofthetin and nickel plated steel sheet. The alkaline electrolyte having a pH of above 10 used for the anodic treatment of the pickled steel sheet is made up bythe dissolution of at least one alkaline compound selected from the group consisting of a hydroxide, a carbonate, a bicarbonate, a silicate, a phosphate, and a borate of an alkali metal and ammonium compounds in water. Furthermore, the effect of the anodictreatment ofthe pickled steel sheet in the alkaline electrolyte is not reduced as long asthe pH ofthe alkaline electrolyte is maintained above 10, even if at least one compound such as an acidic phosphate, an oxalate, a citrate, and an acetate of an alkali metal, and an ammonium compound is added, the surface active agentwhich is usually added to the alkaline solution for degreasing of the steel sheet is added or a small amount of sulfuric acid or hydrochloric acid is brought into the alkaline electrolyte ofthe present invention because of insufficient rinsing after pickling.

It is an essential condition thatthe alkaline electrolyte used forthe anodictreatment of the pickled steel sheetshould be maintained above a pH of 10, although it is not necessaryto strictly control the concentration of hydroxide, carbonate, etc., salts of an alkali metal and ammonium compound. If the concentration of the alkaline electrolyte is restricted, it is preferably in the range of 10 to 100 g/I. At below 10 9/l, a waste of electric power results because of the higher electric resistance of the alkaline electrolyte. The concentration is limited to 100 g/l from theviewpoint of economy, although the effect of the anodictreatment in the alkaline electro- lyte is notincreased even if the concentration is above 100 g/L Although it is not necessarythatthetemperature ofthe alkaline electrolyte be strictly controlled, it is preferably below90 Cfrom theviewpointof energy savings.

In orderto obtain the effectofthe anodictreatment in the alkaline electrolyte, it is necessarythatthe quantity of electricity forthe anodictreatment be in the range of3to 50 coulombs/dm2, more preferably 5to 30 coulombs/dm2. If the quantity of electricity is below 3 coulombs/dm2, the effect of the anodic treatment is not obtained because thin and dense iron oxidefilm is not uniformlyformed on the surface of the pickled steel steel. The quantity of electricity is limited to 50 coulombs/dm2fromtheviewpointofthe high speed production ofthetin and nickel plated steel sheet and from the viewpoint of energy savings, although the effect of the anodictreatment in the alkaline electrolyte is not reduced.

In the case of the alkaline electrolyte having a pH of below 10, a large quantity of electricity is necessary in orderto obtain the effect of the anodictreatment in the present invention. Therefore, a large quantity of electricity for the anodictreatment is not desirable from the viewpoint of the high speed production of thetin and nickel plated steel sheet according tothe present invention.

Forthe high speed production of tin and nickel plated steel sheet according to the present invention, it is reasonablethatthe electrolytictime be 0. 1 to 5 seconds, and the range of current density be 1 to 50A/dm2in the anodictreatmentofthe pickled steel sheet.

The amountof nickel plated on the steel sheetwhich is anodically treated beforehand in the alkaline electro- lyte with a pH of above 10 is one ofthe importantfactors in the present invention. The amountof plated nickel should be controlled in the range of 5 to20 mg/m2. The reason whythe amount of plated nickel should be controlled in the range of 5to 20 mg/m2 in the present invention is explained by Figure 2to Figure4.

Figure 2 showsthe effect of the amount of plated nickel on the distribution of nodules of metallictin on the iron-tin-nickelalloylayerformedonthesteelsheetaftertinplatingandthenreflowingofthetinandnickel plated steel sheet.

Figure 3 shows the effect of the amount of plated nickel on the electric contact resistance ofthetin and nickel plated steel sheet after heating at 210 C for 20 minutes.

Figure 4 shows the effect of the amount of plated nickel on the filiform corrosion resistance after lacquering ofthetin and nickel plated steel sheet.

In Figure 2 to Figure 4, Curve A shows examples wherein nickel plating, tinplating, and then reflowing were carried outafterthe anodictreatmentofthe pickled steel sheet in an alkaline electrolyte, and Curve B shows examples withoutthe anodictreatment before nickel plating. Samples used for Example A and B in Figure 2 to Figure4wereproducedbythefollowingsameconditionsineachstepexceptfortheanodictreatmentof samples of Example A. Furthermore, samples shown in Figure 4were cathodically treated in chromic acid electrolyte after reflowing.

Degreasing... Cathodic electrolysis in 70 g/l of NaOH under 5 A/dm2 for 2 seconds at 70 C.

Pickling... Immersioninto100g/lofH2SO4for2secondsat25 C.

Anodictreatment... Anodictreatmentin 70 g/l of NaOH (pH 14) at5A/dm2for2secondsat70 C. (Oniy samples for Example A) Nickel plating.. Plating withvariousamountsof nickel under3A/dm2at40 C by using Watts bath.

Tinplating... Plating with 700 mg/m2 of tin under 10 A/dm2 at 40 C by using phenolsulfonic acid bath (Ferrostan bath).

Reflowing... Raisingthetemperatureofthetinandnickelplatedsteelsheetupto280 Cduring1. 6 seconds by using resistance heating.

Quenching... Rapid immersion into water after reflowing Chromate treatment.. Cathodictreatment in chromic acid electrolyte containing 30 9/l of CrO3 and 0. 3 g/l of H2SO4 under 10 A/dm2 for 0. 5 seconds at 50 C.

Water rinsing was carried out between each step.

As shown in Figure 2, in the case of Example A, numerous nodules of metallictin were observed by an electron microscope on the iron-tin-nickel alloy layerformed on the steel sheet inthe rangeof5to20mg/m2 of plated nickel, but nodular metallictin is almost not present at above 20 mg/m2 and at below5 mg/m2of plated nickel.

Onthe other hand, in the case of Example B, nodular metallictin is also almost not observed withoutthe dependence on the amount of plated nickel.

Generally, 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 widerthe secondary current range in welding, the bettertheweldability. The upper limit in the available secondary current range correspondsto the welding conditions in which some defect such as splashing isfound andthe lower limit correspondsto the welding conditions in which the breakage occurs in the welded part bythe tearing test. However, in orderto obtain data wherein the available range of secondary current in welding is decided for each sample, a large amount of samples are necessary. Therefore, theweldability is evaluated by electric contact resistance, because electric contact resistance has an apparent correlation with the 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, September 1983, page 181), that is, the lowerthe electric contact resistance, the widerthe secondary current range in welding. Accordingly, if the electric contact resistance is lower, the weldability is better.

It isfound from Figure 3 thatthe electric contact resistance of samples for Example A is lowerthan in the sample for Example B atbelow20 mg/m2 of plated nickelwherein nodular metallictin is observed onthe iron-tin-nickel alloy layerformed on the steel sheet in the samplefor Example A. At below5 mg/m2 plated nickel, the electric contact resistance becomes slightly high because of the decrease in the amount of metallic tin bytheformation of an alloy consisting mainly of iron-tin alloy. The electric contact resistance increases with an increase in the amount of plated nickel because of the decrease in the amount of metallictin caused by the formation of an alloy consisting mainly oftin-nickel alloy during aging at ordinarytemperature.

As shown in Figure 4, thefiliform corrosion resistance is improved with an increase in the amount of plated nickel. In the range of 5to 30 mg/m2 of plated nickel, thefiliform corrosion resistance after lacquering of samples for Example A is excellent compared with that of Example B because the iron-tin-nickel alloy having excellent lacqueradhesion is presentwith numerous nodules of metallictin on the surface of samplesfor Example A, but the surface of the samples for Example B is covered with uniform metallictin layer having poor lacquer adhesion.

Asdescribed above, theanodictreatmentofthe pickledsteel sheetin an alkaline electrolyte and the plating with the limited amount of nickel are indispensible factors in the present invention.

The reason whythe anodic treatment of the pickled steel sheet in an alkaline electrolyte and the plating with thelimitedamountofnickelisnecessaryinordertoproducethetinandnickelplatesteelsheethaving numerous nodulesof metallictin onthe iron-tin-nickel alloylayerformed onthesteelsheetis notclear.

However, it is assumed that the reason is the formation of numerous nodulesof metallictin bydewetting of metallictin electro deposited in the part wherein iron oxide is formed on the pickled steel sheet by the anodic treatment in an alkaline electrolyte and the concentration of metallictin tothe partwherein nickel is electrodeposited during the formation of an iron-tin-nickel alloy layer by reflowing aftertinplating. Furthermore, it is assumed that the reasonwhythetinandnickelplatedsteelsheethavingnumerousnodulesofmetallictinontheiron-tin-nickel alloy layerformed on the steel sheet is excellent in weldability and corrosion resistance after lacquering, particularly filiform corrosion resistance, is that excellent weldabil ity is maintained in the area having a large amountof metallictin in nodularform compared with an averageamountof plated metallictin andthat excellentcorrosionresistanceafterlacqueringismaintainedintheareawhereiron-tin-nickelalloyhaving excellent lacquer adhesion is exposed on the surface without being covered by a uniform metallictin layer.

Therefore, theamountof nickel above20 mg/m2 is not desirable in the presentivention because thin iron oxideformed bythe anodictreatment inthe alkaline electrolyte is almost removed during nickel plating. In the present invention, a known nickel plating electrolyte such as a Watts bath containing 200 to 300g/l of nickel sulfate, 20 to 50 g/t of nickel chloride, and 20 to 40 g/i of boric acid ora nickel sulfamate bath containing 300to 500 g/i of nickel sulfamate, and 20to40g/i of boric acid is usedfor nickel plating ofthe steel sheet treated anodically in an alkaline electrolyte. The plating 5 to 20 mg/m which is required in the present invention is carried out by using these electrolytes described above under 2 to 30A/d M2 30 to 70'C of the electrolyte temperature, and 1 to 10 coulombs/dm2 of a quantity of electricity.

In the present invention, the amount of tin plated on the nickel plated steel sheet is also one o the important factors. The amount of plated tin should be controlled in the range of 400 to 900 mg/m2.

If the amount of plated tin is below400 mg/m2, numerous nodulesof metallictin are notformed onthe iron-tin-nickel alloy layerformed on the steel sheet by reflowing because a large part of electrodeposited metallictin changesto iron-tin-nickel alloy by reflowing. Atabove900 mg/m2of platedtin, numerousnodules of metallictin are not obtained by reflowing because the iron-tin-nickel alloy layerformed by reflowing is uniformly covered with a largeamountof metallictin.

Tinplating on the nickel plated steel sheet in the present invention is carried out by using a knowntinplating electrolyte used forthe production of electrotinplate. For instance, a phenolsulfonic acid bath (Ferrostan bath) containing 10 to 30 g/l of phenolsulfonic acid as sulfuric acid, 10 to 40 g/l of stannous sulfate or stannous phenolsulfonate and 0. 5 to 10 g/l of ethoxylated &alpha;-naphthol sulfonic acid, halogenide bath containing stan- nous chloride, alkali halogenide, and additives or an alkaline bath containing alkali stannate and alkali hydroxide is used in the present invention.

The conditionsfortinplating in the present invention are also almostthe same asthatforthe production of conventional electrotinplate. For instance, tinplating by using a Ferrostan bath is carried out under 5 to 50 A/dm2 of cu rrent density at 30 to 55 C of electrolyte temperatu re.

Reflowing, that isflow melting of electrodeposited metallictin afternickel and tin plating, is also indispensible in ordertoform numerous nodulesof metallictin onthe iron-tin-nickel alloylayerwhich isaninventive feature in the present invention.

The known method in which a temperature above the melting point of tin is maintained for a shorttime by resistance heating and induction heating can be usedforreflowing ofthetin and nickel plated steel sheetin the present invention.

It is suitable in the present invention that the tin and nickel plated steel sheet is heated from 235to 350 Cfor 0. 5 to 3 seconds and then it is immediately quenched into water.

Reflowing ata highertemperaturefora longertime is notdesirable because ofthe poorweldabilitycaused bythe change of a large part of plated metallictin to iron-tin-nickel alloy, particularly in the case of a lower amountofplated metallictin. Furthermore, reflowing at lowertemperaturefora shorttime is not alsodesir- able because of the poor corrosion resistance after lacquering caused by insufficientformation oftheiron-tin- nickel alloy layer, particularly in the case of a higher amount of plated metallictin.

After reflowing, the tin and nickel plated steel sheet according to the present invention is cathodically treated in a known electrolyte such as a sodium dichromate solution which is used for conventional posttreatment of an electrotinplated, ora chromic acid solution containing a small amount of sulfuric acid, fluoric acid, fluoboric acid, fluosilicic acid, an alkali saltthereof, and a combination thereof which is used forthe production of conventional tin free steel having a upper layer of hydrated chromium oxide and a lower layer of metallic chromium, in order to ensure excellent characteristics in lacquer adhesion, corrosion resistance before or after lacquering. For instance, the tin and nickel plated steel sheet according to the present invention is cathodicallytreated in 20to 100 g/l of a dichromate of an alkali metal or ammonium or chromic acid solution containing 0. 01 to 5% of sulfuric acid, fluoric acid, fluoboric acid, fluosilicicacid, an alkali metal saltthereof, or a combination thereof based on the amount of chromic acid under 5 to 40 A/dm2 of a current density of 0. 1 to 5 seconds of a treating time at 30 to 70 C of electrolyte temperature.

The amount of total chromium in the fim formed on the tin and nickel plated steel sheet by cathodic treatment in dichromate or chromic acid solutoin described above should be limited to 3to 20 Mg/M2, prefer- ably 5to 15 mg/2. If the amount of total chromium is below 3 mg/m2, the excellent corrosion resistance before or after lacquering is not obtained, althoughthe weldability does not change.

At above 20 mg/m2 oftotal chromium in the film formed by cathodic treatment in dichromate orchromic acid solution, the current range for sound welding becomes narrow because of the formation ofthicker chromium oxide having high electricresistancebythedehydration oftheformed hydratedchromiumoxide ortheoxidation of metallicchromium during heating forlacquercuring.

Inthe presentinvention, the presence of hydrated chromium oxide is indispensable in orderto obtainthe excellent corrosion resistance before or after lacquering. Moreover, the presence of metallic chromium is desirableforthe improvement offiliform corrosion resistance after lacquering. Therefore, in the casewhere thefilm formed by chromate treatment consists of an upper layerof hydrated chromium oxide and a lower layer of metallic chromium, the amount of metallic chromium should be limited to 2 to 17 mg/m2 and the amount of hydrated chromium oxide should be limited to 3 to 18 mg/m2 as chromium.

Brief description of the drawings Figure 1 shows the magnified photograph wherein numerous nodulesof metallictin 1 aredistributedon theuniform iron-tin-nickel alloylayer2inthesurfaceofthetin and nickel plated steel sheet according tothe present invention.

Figure 2shows the effect of the amount of plated nickel on the distribution of nudular metallic tin on the iron-tin-nickel alloy layer formed on the steel sheet after tinplating and then reflowing.

Figure 3shows the effect of the amount of plated nickel on the electric contact resistance of the tin and nickel plated steel sheet after heating at 210 C for 20 minutes.

Figure 4shows the effect of the amount of plated nickel on the filiform corrosion resistance after lacquering of the tin and nickel plated steel sheet.

In Figures 2 to figure 4, curve A shows examples wherein nickel plating, tinplating, and then reflowing were carried out after the anodic treatment of the pickled steel sheet in an alkaline electrolyte according tothe present invention and Curve B shows examples without anodictreatment before nickel plating. Furthermore, samplesshowninFigure4werecathodicallytreatedinchromicacidelectrolytecontainingasmallamountof sulfuric acid after reflowing.

Examples of the present invention The present invention is illustrated by the following examples.

In Example 1 to Example 4and Comparative Example 1 to Comparative Example 4, a cold rolled steel sheet having a thickness of 0. 2 mm was basically treated bythe following process after electrolytically degreasing in a solution of 70 g/l of sodium hydroxide, water rinsing, pickling by an immerison into 100 g/l of sulfuric acid, and then water rinsing.

Anodictreatment in an alkaline electrolyte-water rinsing nickel plating rinsingotinplatingo water rinsing drying- reflowing quenching-- > chromate treatment- water rinsing drying.

In Comparative Example 1, anodic treatment in an alkaline electrolyte was omitted in the present scheme as described above. In Comparative Example 2, reflowing aftertinplating was omitted. In Comparative Example 3, the anodically treated steel sheetwas plated with nickel abovethe upper limited amount in the present invention. In Comparative Example 4, the nickel plated steel sheetwas plated with tin belowthe lowerlimited amount in the present invention.

In Example 1 to Example4 and Comparative Example 1 to Comparative Example 4, a Watts bath containing 250 g/i of NiS04. 6H20, 30 g/l of NiC12-6H20, and 40 g/i of H3BO3 or sulfamic acid bath containing 350 g/l of nickel sulfamate and 40 g/l of H3BO3 was used for nickel plating. Furthermore, a Ferrostan bath containing 60 g/I of SnS04, 30 g/l of phenolsulfonic acid and 5 g/i of ethoxylated a-naphtol sulfonic acid or alkaline bath containing 80 g/l of Na2SnO3 and 15 g/l of NaOH was used fortinplating after nickel plating.

The tin and nickel plated steel sheetwas immediately quenched in water after reflowing, and thetem- perature of the tin and nickel plated steel sheet dropped to 280 C over 1. 6 seconds except in Comparative Example 2. The other conditions in every Example are shown in the attached table.

The weldability, and corrosion resistance after lacquering of the tin and nickel plated steel sheet in the above described Examples and Comparative Examples were evaluated by the following testing methods afterthe measurement of the amounts nickel, tin, metallic chromium, and chromium in a hydrated chromium oxide by the fluorescent X-ray method, and the results were shown in the attached Table.

(1 Weldability The weldabilitywas evaluated by electric contact resistance for the reason already described.

Atfirst, the sample plated on both sides was cutto a size of 20mm x 100mm after heating at2M Cfor20 minutes. The electric contact resistance was calculated from the change of voltage in a pair of copper disk electrodes (diameter : 65mm, thickness : 2mm) to which 5 amperes of direct current were supplied and 50kg of load was added, when two sample pieces were inserted between a pair of the copper disk electrodes rotating at 5m/min.

(2) Filiform corrosion resistance after lacquering (Test 1) The sample was baked at 200 C for 10 minutes after coating with 75 mg/dm2ofa vinyl organosol. Thecoated samplewas immersed into 3% sodium chloridesolutionfor 1 hourand thenwas left in a chamber having 85% of relative humidity at 45 Cfor 10 days afterthe surface of the coated sample was cross-hatched by a razorand then expanded for 5mm by an Erichsen tester.

The result offiliform rusting spread from the scratched part of the coated sample was divided into 5 ranks, namely, excellent, good, fair, poor. and bad.

(3) Undercutting corrosion resistance after lacquering (Test 2) The samplewas baked at210 Cfor 10 minutes after coating with 65 mg/m2 of an epoxy-phenolic type of lacquer. The coated sample was immersed into the deaerated solution containing 1. 5% of citric acid and 1. 5% of sodium chloridefor 15 days at 37 C afterthe surface of the coated samplewas cross-hatched by a razor.

The result of corrosion in the scratched part of the coated sample was dividied into 5 ranks, namely, excel- TABLE Comparatvie Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 example 1 example 2 example 3 exdample 4 Anodictreatment NaOH 70g/l KOH 20 g/l Na2CO310g/l Na4P2O710H2O NaOH 70g/l NaOH 70g/lo KOH 20g/l 30 C 40 C 30 C 10g/l 40 C Notratment 30 C 30 C 40 C in an alkaline 5 A/dm2, 10 Adm2, 5 Ad/m2, 10 A/dm2, 5 A dm2, 5 A/dm2, 10 A/dm2, 1 sec. 3 sec. 2 sec. 3 sec. 1 sec. 1 sec. 3 sec. electrolyte pH 14 13.5 11.5 10.5 - 14 14 13.5 Watts bath Watts bath Sulfanic sulfanic Watts bath Watts bath Watts bath Watts bath 40 C, 40 C acid bath acid bath 40 C, 40 C, 35 C, 40 C, Conditoins Ni plating 5A/dm2 5Ad/m2 40 C, 30 C, 5 A/m2 5 A/dm2 10A/dm2 5 A/dm2 10a/dm2 3 Ad/m2 in each Phenolsul- Phenolsul- phenolsul- Phenolsul- Phenolsul- Phenolsul Sn plating fonic acid fonic acid Alkali bath Alkali bath fonic acid fonic acid fonic acid fonic acid bath 35 C bath 40 C 80 C, 3A/dm2 80 C, 2A/dm2 bath 35 C bath 35 C bath 35 C bath 35 C process 10 A/dm2 10 A/dm2 10 A/dm2 10 A/dm2 10 A/dm2 10 A/dm2 Reflowing Reflow Reflow Reflow Reflow Reflow Unreflow Reflow Reflow Chromate CrO3 30g/l CrO3-40g/l Na2Cr2O7 CrO330g/l CrO330g/l CrO3 60g/l CrO330g/l CrO3 30g/l treatment H2SO4 30 gl Na2SiF60.5g/l H2SO40.3g/l H2SO40.g/l H2SO40.3g/l NaF 0.5g/l (Cathodic 0.3g/l naF0.5g/l 40 C 30 C 40 C 40 C 40 C 40 C electrolysis) 40 C, 40 C 10 A/dm2 15 A/dm2 10 A/dm2 12 A/dm2 10 A/dm2 15 A/dm2 10 A/dm2 15 A/dm2 Distriubtion of metallic Nodular Nodualr Nodular Nodular Uniform Uniform Uniform Uniform Snon the surface Ni 18 5 10 15 10 15 70 17 Coating Sn 420 870 750 720 450 700 500 220 weight metallic Cr 7 15 0 10 6 5 5 15 (mg/m2) Cr in hydrated Cr oxide 7 3 5 6 5 6 7 3 Electric contact resistance 0.4 0.2 0.2 0.3 1.1 0.3 6.2 8.3 (m#) Character- Corrosioin istics resistance Test 1 Excellent Excellent Excellent Excellent Fair Poor Excellent Fair after Test 2 excellent Excellent Excellent Excellent Good good Excellent Fair lacquering

Claims (18)

1. A method for producing a tin and nickel plated steel sheet having a surface structure in which the distribution of numerous nodulesofmetallictin areobserved byusing an electron microscope (1000 magnifi- cations) on an iron-tin-nickel alloy layerformed on a steel sheetwith comprises : (1) anodically treating a substantially clean steel sheet in an alkaline electrolytewith a pH of above 10, (2) nickel electroplating the steel sheettreated by step (1), (3) electrotinplating the steel sheet plated with nickel by step (2), (4) reflowing and quenching the tin and nickel plated steel sheet by step (3), (5) chromate treatment of thetin and nickel plated steel sheet obtained by step (4)

2. The method according to claim 1, wherein the amount of plated nickel isfrom 5to 20 mg/m2, the amount of plated tin is from 400 to 900 mglm2, and the amount of hydrated chromium oxide is from 3 to 20 mg/m2 as chromium.

3. The method according to claim 1, wherein the amount of plated nickel is from 5 to 20 mg/m2, the amountof platedtin isfrom 400to 900 mg/m2, andthe amountoftotal chromium, hydrated chromium oxide as chromium and metallic chromium is from 3 to 20 mg/m2, from 3 to 18 mg/m2 and from 2 to 17 mg/m2, respectively, in the film consisting of an upper layer of hydrated chromium oxide and a lower layer of metallic chromium.

4. The method according to claim 1, wherein said anodic treatment of a substantially clean steel sheet in analkalineelectrolyteiscarriedoutunderaquantityofelectricityof3to50coulombs/dm2, acurrentdensity of 1 to 50A/dm2 and a treating time of 0. 1 to 5 seconds at a temperature of 20to 90 C in an alkaline electrolyte with a pH of above 10 containing 10 to 100 g/i of at least one alkaline compound selected from the group consisted of a hydroxide, a carbonate, a bicarbonate, a silicate, a phosphate, a pyrophosphate, and a borate of an alkali metal and ammonium compound.

5. The method according to claim 1, wherein said nickel plating is carried out under a quantity of electricity of 1 to 10 coulombs/dm2, a current density of 2 to 30 A/dm2 at a temperature of 30to 70 C in a nickel plating electrolytecontaining 200to300 g/l of nickel sulfate, 20 to 50 g/l of nickel chloride, and 20to 40 g/l of boricacid or containing 300 to 500 g/i of nickel sulfamate, and 20 to 40 g/l or boric acid.

6. The method accordingto claim 1, wherein said tinplating is carried out undera currentdensity of 5to50 A/dm2 at a temperaure of30to 55'C in a tinplating electrolyte containing 10 to 40 g/i of stannous sulfate or stannous phenolsulfonate, 10 to 30 g/l of phenolsulfonic acid as sulfuric acid, and 0. 5to 10 g/l ofethoxylated a-naphthol sulfonic acid or ethoxylated oz-naphthol.

7. The method according to claim 1, wherein said reflowing and quenching of the tin and nickel plated steel sheet is carried out by an immediate immersion into water, heating said tin and nickel plated steel sheet to 235 to 350 C during 0. 5 to 3 seconds.

8. The method according to claim 1, wherein said chromate treatment ofsaid tin and nickel plated steel sheet is carried out under a cathodic current density of 5to 40A/dm2and a treating time of 0. 1 to 5 seconds ata temperature of 30to 70 C in a solution containing 20to 100 g/l of a dichromate of an alkali metal or ammonium compound containing 20to 100 9/l of chromic acid having 0. 01 to 5% of sulfuric acid, fluoboric acid, fluosilicic acid, an alkali salt thereof, and a combination thereof based on the amount of chromic acid.

9. Thetin and nickel plated steel sheetaccordingtoclaim (1).

10. The tin and nickel plated steel sheet according to claim (2).

11. The tin and nickel plated steel sheet according to claim (3).

12. The tin and nickel plated steel sheet according to claim (4).

13. The tin and nickel plated steel sheet according to claim (5).

14. Thetin and nickel plated steel sheetaccording toclaim (6).

15. The tin and nickel plated steel sheet according to claim (7).

16. The tin and nickel plated steel sheetaccordingto claim (8).

17. Thetin and nickel plated steel sheet according to claim (9).

18. Thetin and nickel plated steel sheet according to claim (10).