CA1179629A - Process for producing a chromium-plated steel strip having enhanced weldability - Google Patents

Abstract

PROCESS FOR PRODUCING A CHROMIUM-PLATED STEEL
STRIP HAVING ENHANCED WELDABILITY

ABSTRACT OF THE DISCLOSURE
A chromium-plated steel strip having an enhanced weldability is produced by a process comprising (1) electro-plating both surfaces of a descaled steel strip with 2.8 g or less of tin per m2 of each surface of the steel strip, and (2) electrolytically chromic acid-treating the tin--plated steel strip to form a plated chromium layer on each surface of the tin-plated steel strip, the plated chromium layer preferably comprising 5 to 150 mg of metallic chromium and 3 to 30 mg, in terms of metallic chromium, of a non--metallic, chromium-containing substance, per m2 of each surface of the steel strip.

Description

PROCESS FOR PRODUCING A CHROMIU~q--PLATED STEEL
STRlP HAVING ENHAMC~D WELDABILITY

FIELD OF THE INVENTION
The present invention relates to a process for pro- .
ducing a chromium-plated steel strip having an enhanced - weldability, espec~ially regardlng s~arn weldability.
BACKGROUND OF THE INVENTION
The term "chromium-plated steel strip" used herein is synonymous with the term ~electrolytically chromic acid--treated steel strip".
The chromium-plated steel strip has an excellent lacquer-bonding property, a superior resistance to corrosion under the lacquer-coating and a beautiful plated appearance, and therefore, is useful for producing various steel pro-ducts, for example: sanitary cans, such as beverage cans and marine product cansf 18 liter cans, pails, motor oil cans and other cans, in general; horne electric appliances;
and, steel made furn.iture. One type of chromium-plated steel strip i5 commercially produced and sold under the tradcmark, Can Super, by Nippon Steel Corporation.
However, despite its excellent properties, the con-ventional chrornium-plated steel strip is limited in its use, because i~ exhibits a relatively poor weldability.
For example, in the usual production of cans to be used for beverayes, such as fruit juices, by using a soldering method or an adhesive-bonding method, it is possible to .~

produce 400 to 600 cans per minute. However, i~ the bodies of the cans are produced from the conventional chromium--plated steel strip by using a welding method, it is possible to weld only 150 to 200 cans per minute, and the reliability in airtightness of the welded cans is poorer ; than that of the soldered or adhesive-bonded cans~
- The poor weldability of the conventional chromium--plated steel strip and the inferior reliablity in airtightness of the welded can made from the conventional chromium~plated steel strip are due to the following facts.
That is, in the conventional chromium-plated steel strip, the plated coating layers comprise a metallic chromium and a non-metallic chromium substance consisting essentially of chromium oxides. Such plated coating layers exhibit poor electric and thermal conductivities. Also, the plated coating layers tend to generate heat locally and to frequently produce splashes during the welding procedure.
Accordingly, in order to eliminate the above-mentioned disadvantayes of the conventional chromium-platcd steel strip, it is necessary -to enhance the electric and thermal condllctivities o~ the plated surface layer~ th~reof. For the purpose of eliminating the disadvantages, it was attempted to form scratches on the plated coating layers, so as to make the plated coating layers porous, or to partially make the plated coating layers defective by subjecting the chromium-plated steel strip to a temper rolling procedure~ The above-mentioned attempts succeeded in the elimination of the above-mentioned disadvantages.

~ 3 --The scratches, pores or defects formed in the plated coa ing layers are effective for enhancing the weldability of the conventional chromium-plated steel strip. However, another disadvantage resulted from these attempts. That is, the above attempts all need an additional process for forming the scratches or pores in the plated coating layer~ of the conventional chromium-plated steel strip. This additional process causes the-cost o~ the products made from the conventional chromium-plated steel strip to be high.

SUMMARY OF THE INVENTION
An object ofthe present invention isto provide aprocess for producing achromium-plated steel strip having anenhanced weldability which can beused forproduciny various welded cans.
Another object ofthe present invention isto provide a process for producing achromium-plated steel strip having an enhanced weldability with a high producti~ity and low cost.
The above-mentioned objects can be attained by the process of the present invention which comprises the steps of (1) electroplating both the surfaces of a descaled steel strip with 2.8 g or less of tin per m2 of each surface of the steel strip, and; (2~ subjecting the tln-plated steel strip to an electrolytic chromic acid-treatment to form a plated chromium layer comprising 5 to 150 mg of metallic chromium and 3 to 3~ mg, in terms of metallic chromium, of a non-metallic, chromium-containing substance, per m2 of each surface of the tin-plated steel strip.
In theprocess ofthe present invention, theelectrolytic chromic acid-treatment iscarried outto anextent that each z~
~ ~ --surface of the tin-plated steel strip iscoated with aplated chromium layer comprislny 5to 150mg ofmetallic chromium and 3 to 30 mg, in terms ofmetallic chromium, o a non-metallic chromium-containing substance, per m2 of each surface, BRIEF DESCRIPTION OF THE INVENTION
Fig. lA shows an explanatory surface view of a welded portion of a conventional chromium-plated steel strip;
Fig. lB shows an ex~lanatory cross-sectional view of the welded portion shown in Fig. lA;
Fig. 2A shows an explana~ory surface view of a welded portion of a chromium-plated steel strip produced in ac-cordance with the process of the present invention Fig. 2B shows an explanatory cross-sectional view of the welded portion shown in Fig. 2A;
Fig. 3 is a microscopic photograph of a welded portion of a chromiu~-plated steel strip of the present invention as described in the sole Example;
Fig. 4 is a microscopic photograph of a welded portion of a conventional chromium-plated steel strip as described in Comparative Example l;
Fig. 5 is a m.icroscopic photograph of another con-ventional chromium-plated steel strip, as described in Comparison ~xample 2;
Fig~ 6 is a graph showing a relationship between the lacquer-bonding intensity of a chromium-plated steel strip and the amount of the tin layer plated on the steel strip surface; and Fig. 7 is a microscopic photograph of still another conventional chromium-plated steel strip as described in Comparison Example 3.

DETAILED DESCRIPTION OF THE INVENTION
The chromium-plated steel strip of the present in-vention is useful for producing various cans, for example,sanitary cans such as beverage cans, 18 liter cans, pails and other cans, in general, by means of weldiny. It is necessary for the resultant cans to exhibit a satislactory airtightness in the welded portion thereof. Therefore, the welding operation is usually carried out by an electric resistance welding method, such as seam welding. However, the welding operation may be carxied out by another method, for example, spot welding. Therefore, the weldability of the chromium-plated steel strip of the present invention should cover not only the seam weldability, but also, other weldabilities, for example, spot weldability.
In order to apply the process of the present invention, a steel strip suitable for the plating process is descaled by degreasing it with an alkali solution and, then, by pickling it in an ordinary manner. The thus descaled steel strip is termed "loam plate" hereinafter.
The loam plate is subjected to an ordinary electro-plating procedure with tin. In this case, it is necessary that both the surfaces of the loam plate are plated with

2.8 g or less of tin per m of each surface of the loam plate. It is preferable that no reflow (xe-melting) treatment is applied to the tin-plated steel strip, because the reflow tr~atment, sometimes, causes the weldability of ~ 6 --the finally resultant chromium-plated steel ~trip to decrease. However, i~ the reflow treatment is carried out to an extent that an alloy layer formed on the steel strip is not exposed to the outside of the plated tin layer on the steel strip, the reflow treatment does not affect the weldability of the resultant chomium-plated steel strip.
- As sta-~ed above, the amount of the plated tin layer - should be 2.8 9 or less, preferably, not less than 0.1 g, per m of each surface of the loam plate. More preferably, the amount of the plated tin layer is in the range of from 0.5 to 1.0 g per m2 of each surface of the loam plate.
Even if the amount of the plated tin layer is increased to more than 2.8 g per m of each surface of the steel strip, -the weldability of the resultant chromium-plated steel strip does not increase with the increase in the amount of the plated tin layer, and the welding current necessary for welding the steel strip undesirably increases with the increase in the amount of the plated tin layer.
Also, in -the welding procedure, the amount o;~ tin kha-t flows out from the welded portion of the steeL strip in-creases with the increa~e in the amount of the plated tin layer, and causes the appearance of the resultant welded product -to be unsatisfactory.
Further~ore, an excessively large amount of the plated tin layer causes the following disadvantage. Usually, the welded portion of the chromium-plated steel strip is spray--coated with a lacqu~r, so as to enhance the resistance or the welded portion to corrosion. When the amount of the ~J ~l'79~

plated tin layer is excessively large, the surface of the welded portion is covered wi~h a relati~ely thick layer of a tin-iron alloy which is fragile. r~en the alloy layer is coated with a lacquer, the alloy layer is frequently broken while the welded portion of the steel strip is worked, and this breakaye of the alloy layer results in a separation of the lacquer coating from the surface of the base steel.
Next, the tin plated steel strip is subjected to an electrolytic chromic acid~treatment to form a plated lQ chromium layer on each surface of the tin-plated steel strip. The resultant plated chromium layer preferably comprises 5 to 150 mg of metallic chromium and 3 to 30 mg, in terms of metallic chromium, of a non-metallic, chromium--containing substance, per m2 of each surface of the tin--plated steel strip. The non-metallic, chromium-contalning substance consists essen-tially of chromium oxides.
In the electrolytically plated chromium layer, ik is preferable that the amount of -the metallic chromium is in the range o~ from 5 to 150 mg, more preferably, ~rom 10 to ~0 mg, per m2 o each surEace oE the tin-platecl steel strip. When the amount o~ the metallic chromium is less than 5 mg per m oE each steel strip surface, sometimes, the resultant chromium-plated steel strip may exhibit unsatisfactory qualities, for example, an inferior resistance to corrosion under a coating of lacquer and a poor close~onding property to the coa-ting of lacquer.
Also, when the amount o the metallic chromium is more than 150 mg per m of each surface of the steel strip, the - 8 ~

resultant pla-ted chromium layer, sometimes, may exhibit a poor electric conductivity in the seam welding procedure.
Also, the excessively large amount of the metallic chromium sometimes may cause the resultant plated chrmium layer to exhbit an enhanced tendency of generating heat locally and forming splashes in the welding operation, in spite o~ the fact that the plated tin layer is located - under the plated chromi~ layer.
In the plated chromium layer, it is preferable that the amount of the non-metallic, chromium-containing sub-stance is in the range of from 3 to 30 mg, more preferably, from 7 to 12 mg, per m2 of each surface of the tin-plated steel strip. If the amount of the non-metallic, chromium--containing substance is less than 3 mg per m2 of each lS surface, sometimes the resultant chromium-plated steel strip may exhibit an unsatisfactory close-bonding property to the lacquer coating, and a poor resistance to corrosion unless the chromium plated steel strip is coaked with lacquer. Also, usually, it i~ very dlf~iclllt to produce the plated chromlum layer containing less than 3 mg of the non-metallic chromium-containincJ substance unless an excessive amount of the substance is remo~ed ~rom the plated chromium layer. The removal of the substance can be effected only by using a special removing device or the substanceO
Also, when the amount of the non-metallic chromium--containing substance i5 more than 30 mg per m of each surface, the resultant plated chromium layer, sometimes,

3~

may exhibit poor electric and thermal conductivities and an enhanced tendency of generating heat locally~ The local heat generation results in the formation of splashes. This phenomenon causes the workability and weldability of the chromium-plated steel strip to be decreased. Furthermore, the excessively large amount of the non-metallic, chromium--containiny substance sometimes may result in the forma-tion ~f undesirable pores or hollows in nugget portions and in the vinicity of the nugget portions in the welded product.
Therefore, the stability and reliability of the welded portion of the chromium-plated steel strip may become unsatisfactory.
The tin-electroplating procedure and the electrolytic chromic acid-treatment can be carried out in the usual manner. That is, the tin-electroplatiny procedure may be carried out using a usual ferro-stannous platiny bath containing stannous sulfate and phenolsulfonic acid, at a temperature of from 35 to 55C at a curren-t density of from 10 to 50 A/dm2 for 0.03 to 10 seconcls. The ~erro-stannous platin~ bath preferably cvntains 10 to 30 g/Q of stannous ions and 4 to 16 y/~, in terms of sulfaric acid, Oe a free acid. Also, the electrolytlc chromic acid-treatment may be carried out by using a usual chromic acid electrolytic bath containing 30 to 200 g/Q of CrO3 , 0 to 6 g/Q of a fluoride type additive, for example, Na2SiF6 and 0 to 2 g/Q of sulfuric acid, at a temperature of 35 to 55C at a current density of from 10 to 100 A/dm2 for 0.01 to 100 seconds~
In -the resultant chromium-plated steel strip of the present invention, the plated chromium layer which comprises a metallic chromium having a melting point of 1905C and a non-metallic chromium-containing substance that consists essentially of chromium oxides, and.has a low electric conductivity, is formed as an upper plated layer on an under plated layer which consists of tin having a melting point of 231.9C and a high electric conductivity.
Therefore, when the chromium-plated steel strip of the present invention is welded, a portion of the upper plated chromium layer is pressed by a welder ~lectrode and a portion of the under plated tin layer located under the pressed portion of the upper layer is molten in the initial stage of the welding procedure. Therefore, in the initial stage of the~ welding procedure, the portion of the upper plated chromium layer, which is located on ~he molten portion of the under plated tin layer, is broken and removed ~rom the plated steel strip, and in the subsequent staye, the steel strip is firmly welded.
That is, the under plated tin layer can eliminate all the disadvantac3es o~ the chromium layer, such as low electri.c conductivity, local generation of heat and poor thermal conductivity thereof, and allo~ the steel strip to be firmly welded.
In this connection, it should be noted that the ~ntire 25 amount of tin in the undex layer should not be converted into a tin-iron alloy when heating the under layer -to a temperature above the melting point of th~ tinl because the tin-iron alloy has a high melting point of about 1130C and 6~9 therefore, cannot be molten in the initial stage of the welding procedure. The tin-iron alloy layer cannot cause the upper plated chromium layer to be partially eliminated, Since the upper plated chromium layer is not partially eliminated, the welding procedure for the steel strip is carried out in the presence of the upper plated chromium layer which has a high melting point and low electric and thermal conductivities. Therefore, heat is generated locally in the welded portion and the local generation of heat results in the formation of splashes. The resultant nuggets are small and discon-tinuous. Accordingly, the conversion of the entire amount of tin in the under layer would result in a poor seam weldability of the steel ~trip and, therefore, should be avoided.
The features and advantages of the process of the present invention will be further described with refer-ence to the accompanying drawings.
In Figs, lA and lB, two pieces 1 and 2 oE a con-ventional chromium-plated steel strip are welded to each other by an ordinary seam welding method. The plated chromium layers on the steel strip surfaces contain 97.8 mg/m2 of metallic chromium and 12.5 mg/m of a non-metallic chromium-containing substance. As a result of the welding procedure, a p]urality of spots 3, each con-sisting of a residue of molten metal discharged from the welded portions of the pieces 1 and 2, are formed in the - lla -portion la of the surface of the piece 1~ The slze and configuration of the spots 3 are di~erent ~rom each other. The configuration of the spot is termed "flow line". That _ ,t3~

is, the flow lines of the spots are unstable.
Also, a number of marks 4 were foxmed on the surface 2a of the piece 2 when a welding current was applied to the surface 2a using a welder electrode consisting of a copper wire. This welding procedure caused the surface 2a to be very slightly fused locally to make the marks 4.
In view of Figs. lA and lB, it is clear that the - surface 2a, which contacted the welder electrodes in the welding procedure, was substantially not fused, and ~he pieces l and 2 were fuse-bonded by the heat generated in the interface between the pieces l and 2. Also, the heat--affected zone formed by the welding procedure was very small and there was no fuse-bonding between nugget portions.
Therefore, a portion of the molten metal, consisting es-lS sentially of iron in the nuggets, flowed out from thenon-fuse bonded nugget portions. The ilowed out metal formed the spots 3 on the surface la. The flow lines of the spots are different from each other.
In E'igs. 2~ ancl 2B, two pieces 21 and 22 o~ a chromium--plated steel strip of the present invention are welded to each other by means of a seam welding method by using a copper wire electrode. The chromium-plated steel strip has under layers each consisting of l.l g/m of tin and upper layers ~ach consisting of 92.5 mg/m2 of metallic chromium and 8.9 mg/m of a non-metallic, chromium-containing substance~

Referring to Figs. 2A and 2B, a portion of molten metal in the nugget portions flows out from the nugget portions and forms a continuous molten metal layer 23 having a stable flow line. Also, the contac-t of the wire electrodes with the surface 22a causes the contacting portion of the surface 22a to be well fused to make clear marks 24~ The heat-affected zone formed on the welding portions of ~he pieces 21 and 22 is relatively large, and the interface portions of the pieces 21 and 22 are well - fused. The stable flow line o the molten metal layer 23 shows that the interface between the nuggets is satis-factorily heated to the melting temperature so as to fuse bond firmly the pieces 21 and 22 to each other.
Referring to Fig. 3, showing a microscopic view of a welded portion o a chromium-plated steel strip of the presen~ invention, the nuggets formed in the welded portion are remarkably large and firmly fuse-bonded to each other, and therefore, the welded portion is extremely airtight.
Referring to Fig. 4, showin~ a rnicroscopic view of a welded portion of a conventional chromium-plated steel strip having no under plated tin layer, the nu~ets ~ormed in the welded portion ar~ small and the fuse-bondiny of the steel strip sur~aces i5 effected only just under the wire electrodes brought into contact with the steel strip surface.
Refexring to Fig. 5 showing a microscopic view of a welded portion of another chromium-plated steel strip having excessively thick under plated tin layers, the nuggets formed in the welded portions are unsatisfactorily fuse-bonded~ Therefore, the airtightness of the welded llt7g~z9 portion is, sometimes, unsatisfactory.
Fig. 6 sho~7s a relationship between the bonding strength of the lacquer coating to a chromium-plated steel strip of the present invention having under plated tin layers on both surfaces thereof, and the total amount of the under plated tin layers.
The total amount of the under plated tin layers was varied from 0 to 1~.0 g/m2 as indicated in Fig-. 6, and the upper plated chromium layers consisted of 20 mg/m2 of metallic chromium and 10 mg/m of a non-metallic, chromium--containing substance. The chromium-plated steel strip was shaped into a can having a diameter o~ 65 mm and an overlap width of 0.8 mm, by means of an electric resistance seam welding method using wire electrodes, at a welding current of 21A (in the primary side) under a pressure of 50 kg.
The resultant can was subjected to a flanging procedure. The 1anged can was coated by sprayincJ a paint in an amount of 80 mg/dm onto the surfaces of the can.
The coated can was heated at a temperature o 170C for 5 minutes to cure the lacquer coating.
In order to -test the bonding strength of the lacquer coating to the surface of the can, an adhesive tape was adhered onto the inside surface of the welded, flanged portion of the coated can, and, then, the adhesive tape was peeled off from the inside surface. The area of a portion of the lacquer coating separated from the inside surface of the can, together with the adhesive tape, was measured.
The bonding strength was represented by -the ratio (O~ of .

~ ~t~9 ~

the measured area o the separated portion of the lacquer coating to the entire area of the inside sur~ace of the welded, flanged portion. The larger the ratio (~), the lower the bonding strength of the lacquer coating to the welded chromium-plated steel strip surface.
From Fig. 6, it is evident that in order to obtain a practical satisfactory bonding strength, the total amount of the plated tin ~ayers on both surfaces should be 5.6 g/m or less. Also, the excessively large amount of plated tin layers causes the resultant product to be expensive. Therefore, on each of the surfaces of the chromium-plated steel strip of the present invention, the amount of the plated tin layer should be 2.8 mg or less per m2 of each surface.
lS In Fig. 7, showing a microscopic view of the welded portion of still another chromium-plated steel strip in which the tin layers are entirely converted into tin-iron alloy layers, the nuggets formed in -the w~lded portion ~re small and splashes are formed in the welded portion. That is, this type oE chromiur~-plated steel strip exhibits an unsatisfactory weldability.
The following example illustrates, but does not limit, the present invention.

A loam plate, having a thickness of 0.23 mm which had been degreased and pickled in a usual ~anner, was subjected to an electroplating procedure by using a ferro-stannous plating bath containing 30 g/Q of stannous ions and 14 to ~t71~

15 g/~, in terms of sulfuric acid, of a free acidj at a temperature of 43C at a current density o 25 A/dm2 for O.9 seconds, whexeby 1.2 g/m2 of tin were plated on both the surfaces of the loam plate.
The tin-plated loaJn plate was subjected to an elec-trolytic chromic acid treatment by using a chromic acid--treating bath containing 150 g/Q of CrO3 , 4.5 g/Q of Na2SiF6 and 0.3 g/~ of sulfuric acid, at a tempçrature of 43~ at a current density of 70 A/dm for 0.4 seconds. The resultant plated chromium layers on both the surfaces of the loam plate consisted of 41.8 mg/m of metallic chromium and 12.4 mg/m2 of a non-metallic, chromium-containing substance.
The plated loam plate was coated with 3.0 to

4.0 mg/m of DOS, and, then, subjected to an electric resistance welding procedure at a frequency of 180 Hz at a welding speed of 23~5 m/min under a pressure of 5 Kg at a weldiny current ~primary side) of 19 to 21h~ The overlap width of the welded portion was O.B mm.
The microscopic photograph (magnification:25) of a cross-section of the resultant welded portlon along the seam portion thereof, is shown in Fig. 3.
Since the under tin layer was molten in the initi~1 stage of the welding procedure, the electric and thermal conductivities of the welded portion were enhanced, the nuggets formed in the welded portion grew satisfactorily and the resultant welded portion exhibited a satisfactory airtightness.

7~3~3 --o~5Y~ L~-Læ,~ e 1 The same procedures as those described in Example 1 were carried out with the following exception. No tin--electroplating procedure was applied to the loam plate.
The plated chromium layers on both surfaces of the loam plate consisted of 43.3 mg/m2 of metallic chromium, and 11.7 mg/m , in terms of metallic chromium, of a non--metallic, chromium-containing substance. Also, the DOS
was uséd in an amount of 3.0 to 4.0 mg/m2.
During the welding procedure, the phenomena of ex-plusion and surface flash were obser~ed in the weldedportion.
Fig. 4 shows a microscopic view (magnified 25 times) of a cross-section of the welded portlon along the seam portion thereof. From Fig. 4, it is clear that the nuggets formed in the welded portion are small, and welding was effected only in the portions in which khe welding current become maximum.
~ arative Ex~ le 2 . .
The same procedures as those described in Example l were carried out, except that the plated -tin layers on both the surfaces of the loam plate were in a total amount of 9.2 g/m ; t~e electrolytic chromic acid-treating both contained 100 g/Q of CrO3 and 3 g/Q of Na2SiF~ ; the chromium plating procedure was carried out at a current density of 35 ~/dm2 at a temperature of 43C for 0.7 seco~ds; the resultant chromium layers on both the surfaces consisted of 14.h mg/m of metallic chromium and 3~$J

11.5 mg/m2, in terms o~ metallic chromium, of a non--metallic, chromium-containing su~s~ance, and; DOS was used in an amount of about 3~0 mg/m2.
The excessively large amount of the plated tin layer s caused the loss of heat during the welding procedure to be large due to the melting of the tin and, there~ore, the fuse bonding of the nuggets to each other/ in the welded portion, was unsatisfactory. Accordingly, the reliability in airtightness of the welded portion was poor.
1~ Comparative Example 3 The same procedures as those described in Example 1 were carried out, except that the tin-plated loam plate was heated up to a temperature of 240C at a heating rate of 90C/sec, for 2.4 secondss and, then, the heated loam plate was immediately cooled with cold water, to convert the entire amount of the tin into a tin-iron alloy; and the plated chromium layers on both the surfaces o~ the loam plate consisted of 38.5 mg/m2 of metallic chromium and 12.5 mg/m2, in terms of metallic chromium~ of a non-metallic chroTniuTn-containing subs-tance.
The cross-section o~ the welded portion along the seam portion thereof is indicated in Fig. 7. The -tin-iron layers having a high mel-ting point and a poor thermal conductivity caused the nugge-ts formed in the welded portion to be small and resulted in the formation of undesirable splashes. That is, the resultant chromium-plated steel strip exhibited a poor weldability.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a chromium-plated steel strip having an enhanced weldability, comprising the steps of:
(1) electroplating both the surfaces of a descaled steel strip with 2.8 g or less of tin per m2 of each surface of said steel strip, and (2) subjecting said tin-plated steel strip to an electrolytic chromic acid-treatment to form a plated chromium layer comprising 5 to 150 mg of metallic chromium and 3 to 30 mg, in terms of metallic chromium, of a non-metallic, chromium-containing sub-stance, per m2 of each surface of said tin-plated steel strip.

2. The process as claimed in Claim 1, wherein said plated tin layer is in an amount of from 0.1 g to 2.8 g per m2 of each surface of said steel strip.

3. The process as claimed in Claim 1, wherein the amount of said plated tin layer is in the range of from 0.5 to 1.0 g per m2 of each surface of said steel strip.

4. The process as claimed in Claim 1, wherein the amount of said metallic chromium in said plated chromium layer is in the range of from 10 to 40 mg per m2 of each surface of said tin-plated steel strip.

5. The process as claimed in Claim 1, wherein the amount of said non-metallic, chromium-containing substance is in the range of from 7 to 12 mg in terms of metallic chromium per m2 of each surface of said tin-plated steel strip.