DESCRIPTION TINNED STEEL SHEET EXCELLENT IN CORROSION RESISTANCE

[0001] This Application claims priority to Japanese Application No. 2006-031911 filed in Japan ^' on February 9, 2006, which is herein incorporated by reference in its entirety.

Background of the Invention

[Field of Technology]

[0002] The present invention relates to a tinned steel sheet excellent in corrosion resistance, in particular, suitable for an uncoated can accommodating an acidic food such as bamboo shoot, pineapple, cherry, etc. [Background of Technology]

[0003] As for an uncoated can accommodating an acidic food, it has been demanded to reduce the amount of tin which elutes into the contents of the can as much as possible. In the present invention, "corrosion" is defined as tin eluting out into the contents of the can. As shown in FIG.3, a currently available tinned steel sheet is constituted with a steel sheet 4, an alloy layer (FeSn ₂ ) 1 and a tin layer 3, which is manufactured by electroplating a tin on the steel sheet and then is subjected to a melting treatment. [0004] Conventionally, the ATC (Alloy Tin Couple) value associated with the alloy layer (FeSn ₂ ) has been used as an evaluation index for evaluating the corrosion resistance of tinned steel sheets for uncoated cans. When the ATC value becomes higher, tin elutes out into the contents of the can and vice versa. For example, in the case of tinned steel sheet constituted with a steel sheet 4, an alloy layer 1 and a tin layer 3 as shown in FIG.3, the tin of the uppermost layer elutes into the contents of the can. The tin of the uppermost layer is converted into Sn ²⁺ ions and begins to elute by

receiving electrons from the alloy layer immediately beneath the tin layer. Therefore, if the relationship between the alloy layer 3 and the tin layer 1 allows for easy transfer of the electrons, the tin layer 3 can easily elute out into the contents of the can. In other words, the ATC value represents how easily electrons transfer from the alloy layer 1 to the tin layer 3.

[0005] As a way for lowering the ATC value, a method for controlling the alloy layer (FeSn ₂ ) constituted with tin and iron of a base steel sheet, which is formed in a melting treatment after tin plating, has been proposed. This proposed method tried to prevent the uneven formation of the tin layer and the base steel sheet, since the uneven formation of the tin layer and the base steel sheet could result in the direct contact between the tin layer and the base steel sheet. Such direct contact leads to an increase in the ATC value.

[0006] This method is, however, not sufficient to have a good result since it is difficult to avoid the formation of pinholes in the alloy layer which provide direct contact between the tin layer and the steel sheet base when the tinned steel sheet is manufactured through the normal process. In order to inhibit the formation of pinholes, for example, a method where a high amount of tin-iron alloy layer is formed to improve the continuity of the alloy layer, has been proposed. However, the formation of a high amount of alloy layer may reduce the tin metal plated layer from providing good corrosion resistance and the formed alloy layer is usually hard, which may provide poor workability.

[0007] In view of this, JP-A62-284086 and JP-A57-108291 proposes a method where a pre-plating by Ni plating or Fe-Ni alloy plating is performed first, then tin plating and melting treatment are carried out to form a densified alloy layer for inhibiting pinhole formation to improve corrosion resistance.

[0008] As for a conventional tinned steel sheet, the corrosion resistance can be evaluated by the ATC value. However, after a variety of investigations, it was found that even if the ATC value was lowered, there was still tinned steel sheet with insufficient corrosion resistance in the case where the tinned steel sheet was manufactured by tin plating and melting treatment after pre-plating of Ni plating or Fe-Ni alloy plating. JP-A04-221096 discloses a steel sheet comprising a Ni diffusing layer on the base steel sheet, a Sn-Ni-Fe alloy layer thereon and a Sn layer further thereon. This reference shows only one alloy layer. JP-A63-247393 discloses a steel sheet comprising an alloy layer having formations of island-studs in the Ni-Fe-Sn alloy layer and a Sn-Fe alloy layer on the base steel sheet, a Sn layer having formations of island-studs formed on the alloy layer, on top of which is a chromium metal layer, and a chromium hydroxide layer further thereon. In this case, only one alloy layer exists. JP-A60-089595 discloses a steel sheet comprising a diffusion layer of Ni-Sn-Fe in the base steel sheet, and an alloy layer of Ni-Sn thereon. In this case also only one alloy layer is described. [0009] On pages 81-84 of the section titled "K Plate for heavily coated electrolytic tin plate applications", (Mechariical Working and Steel Processing, written by G.G. Kamm and J.K.Krickl published in 1969 by Gordon and Breach Sci. Publ., Inc., New York) it is disclosed that the addition of an appropriate amount of Ni makes the grain size of alloy layer finer and the finer grain size makes the ATC value lower. However, this description mentions only the case where a tin steel sheet has only one alloy layer as shown in Fig.3.

Summary of the Invention

[0010] The present invention has been conceived based on consideration of the above-described problems. An object of the present invention is therefore to provide a

tinned steel sheet excellent in corrosion resistance by controlling the formation of an alloy layer with respect to a tinned steel sheet manufactured through the process of pre-plating of Ni or Fe-Ni alloy and then tin plating followed by a melting treatment.

[0011] The inventors have diligently researched, and it was found that, for lowering

ATC value, although it is important to increase the covering rate of the alloy layer (i.e., reducing the uncovered portion of the base steel sheet), in addition, it is also very effective not to allow Ni to diffuse into the tin layer in order to prevent the tin from eluting out of the tin layer.

[0012] That is, a tinned steel sheet of the present invention comprises an alloy layer

2 on an alloy layer 1 and Ni concentration of the alloy layer 2 is defined in order to prevent Ni from diffusing from the alloy layer 1 to the tin layer and reduce the difference in the rest potential between the alloy layer and Sn. The rest potential is defined as an electrical potential of each layer of a tinned steel sheet soaked in a liquid, and is measured as a difference in electrical potential between each alloy layer of a tin steel sheet and a hydrogen electrode soaked in a liquid.

[0013] In other words, by setting the Ni concentration of alloy layer 2 to an appropriate value, Ni diffusion into the adjacent Sn layer can be inhibited and also the rest potential of Sn and the alloy layer 2 becomes close to each other, which leads to lowering the ATC value.

[0014] A tinned steel sheet of the present invention excellent in corrosion resistance comprises: an alloy layer (1) on a surface of steel sheet (4) to be an inner wall of can (after being formed into the can), wherein the alloy layer (1) contains, by mass % (based on the mass of the layer (I)), Sn of 50-75%, Fe of 20-40% and Ni of 7-20%; ■ an alloy layer (2) on the alloy layer (1), wherein the alloy layer (2) contains, by mass %

(based on the mass of the layer (2)), Sn of 70-80%, Fe of 15-25% and Ni of 0.1-7%; and a non-alloyed Sn layer (3) of which coating weight is 1.5 - 15.0g/m are formed in the named order; wherein a total amount of Sn per unit area in the alloy layers (1) and (2) is 0.2 - 2.0g/m ² and a total amount of Ni per unit area in the alloy layers (1) and (2) is 0.001 - 0.3g/m ² .

[0015] In the tinned steel sheet of the present invention, Sn elution is inhibited, which can provide excellent corrosion resistance.

Brief Description of the Drawings

[0016] FIG.1 shows a structure of tinned steel sheet of the present invention. [0017] FIG.2 shows an embodiment of the present invention where a chemical treatment layer is formed on an unalloyed Sn layer. [0018] FIG.3 shows an example of conventional tinned steel sheet. [0019] FIG.4 is an embodiment of the invention showing the rest potential of each of steel sheet, alloy layer and Sn layer when they are exposed to the contents inside the can.

Detailed Description of the Invention

[0020] Preferred embodiments to implement the present invention are described below.

[0021] As shown in FIG.1, a tinned steel sheet of the present invention comprises a steel sheet 4 to act as an inner wall of a can, and on a surface thereof, in order, an alloy layer 1, an alloy layer 2 on the alloy layer 1, and a non-alloyed Sn layer 3 on the alloy layer 2.

[0022] FIG 2 is similar to FIG. 1, except FIG. 2 also shows chemical treatment layer

5 on the non-alloyed Sn layer 3.

[0023] Hereinafter "mass %" for composition is represented simply as" %" and is based on the total mass of the particular layer.

[0024] The alloy layer 1 contains Sn of 50-75%, Fe of 20-40% and Ni of 7-20%.

The alloy layer 2 contains Sn of 70-80%, Fe of 15-25% and Ni of 0.1-7%. The non-alloyed Sn layer 3, called "free Sn layer", having a coating weight is 1.5 -

15.0g/m ² is formed thereon. A total amount of Sn per unit area in the alloy layers (1) and (2) is 0.2 - 2.0g/m ² and a total amount of Ni per unit area in the alloy layers (1) and

(2) is 0.001 - 0.3 g/m ² .

[0025] The total amount of Sn per unit area as described above refers to the sum of the amount of Sn per unit area of alloy layer 1 and the amount of Sn per unit area of alloy layer 2.

[0026] The alloy layer 2 plays an important role in the tinned steel sheet of the present invention, i.e., the role of inhibiting diffusion of Ni from the alloy layer 1 to the

Sn layer.

[0027] Further, a particularly important factor is the Ni percentage in the alloy layer

2. The upper limit of the Ni percentage in the alloy layer 2 is 7% in the present invention. It was found after a variety of investigations that if the percentage exceeds

7%, the amount of Sn elution increases even if the ATC value is low. The inventors theorize that the heat from the processes of bake printing, welding and pasteurizing including retort, Ni can diffuse easily into the Sn layer since the affinity between Ni and

Sn is very high, which causes corrosion resistance of the Sn layer to lower with increasing amounts of Ni in the Sn layer. That is, it is presumed that as Ni diffuses into the Sn layer, the Sn layer begins to have a tendency of self-corrosion (chemical

dissolution). The lower limit of the Ni percentage in the alloy layer 2 is 0.1%. The ATC value becomes large if the Ni percentage becomes less than 0.1%. It is presumed that when the Ni percentage is 0.1% or more, the rest potential of the alloy layer becomes close to that of Sn. Whereas when the Ni percentage is less than 0.1%, the rest potential of the alloy layer starts to differ from that of Sn, i.e. the difference in rest potentials becomes large, which leads to a large ATC value.

[0028] FIG.4 shows an embodiment of the invention, and is a graph of the rest potentials of each of the steel sheet, alloy layer and Sn layer when they are exposed to the contents inside the can. As there exists an organic acid without an oxygen inside the can, the rest potential is higher in the order of Sn, alloy layer, Fe and Ni. There is a significant difference in the rest potential between the conventional alloy layer and Sn layer, which results in Sn easily eluting out as Sn ions thereby giving a higher value of ATC. A newly developed alloy of this invention has a rest potential which is closer to that of Sn, i.e., lower than that of a conventional alloy layer. As a result, elution of Sn is inhibited, i.e., ATC value is lowered. As for alloy layer 1 of the invention, the lower limit of the Ni percentage is 7%. Tf the Ni percentage is less than 7%, a fine and dense alloy layer cannot be formed, which leads to insufficient ATC value and corrosion resistance. The upper limit of Ni percentage in the alloy layer 1 is 20%. When the Ni percentage exceeds 20%, the alloy layer cannot further be refined and the cost becomes high.

[0029] In the normal tinned steel sheet, an FeSn ₂ layer is formed between an unalloyed Sn layer (free Sn layer) and a base steel sheet. Fe accounts for 19% and Sn accounts for 81% in the FeSn ₂ layer. When Ni is added so as to be less than 20%, an alloy layer is formed where ratio of Fe to Sn in the composition starts to vary and does not remain as the desired exact ratio of 19:81. From the standpoint of productivity, it is

important to keep the Sn concentration in the range of 50-75% and the Fe concentration in the range of 20-40% for the alloy layer 1, and to keep the Sn concentration in the range of 70-80% and Fe concentration in the range of 15-25% for the alloy layer 2. [0030] It is also important to have an unalloyed Sn layer formed in a coating weight of 1.5-15.0g/m ² over the alloy layer 2. Inside the can, free Sn of the inner wall reduces the oxygen trapped inside the can and the oxidizing material contained in the contents of the can. In other words, free Sn is oxidized to form Sn ²⁺ and elutes out. If the unalloyed Sn layer amount is less than the lower limit 1.5g/m , the above mentioned reducing effect cannot sufficiently be expected, and the contents inside the can would be decomposed/rotten. Also, if the unalloyed Sn layer amount is more than the upper limit of 15.0 g/m ² , the effect is saturated and further increases in amount simply adds to the cost. Therefore the amount of the unalloyed Sn layer ranges from 1.5 to 15.0 g/m ² , preferably from 3.9 to 14.0 g/m ² , more preferable from 4.5 to 12.0 g/m ² . [0031] A tinned steel sheet of the present invention is characterized by a structure of an alloy layer containing Ni located between an unalloyed Sn layer (free Sn layer) and a base steel sheet. That is, it is important that an alloy layer 1 contacting the base steel sheet has a composition containing Sn of 50-75%, Fe of 20-40% and Ni of 7-20%, and another alloy layer 2 contacting the free Sn layer has a composition containing Sn of 70-80%, Fe of 15-25% and Ni of 0.1-7%. Preferably, an alloy layer 1 contacting the base steel sheet has a composition containing Sn of 63-73%, Fe of 30-40% and Ni of 7-10%, and another alloy layer 2 contacting the free Sn layer has a composition containing Sn of 70-80%, Fe of 15-25% and Ni of 3-7%. More preferably, an alloy layer 1 contacting the base steel sheet has a composition containing Sn of 65-70%, Fe of 30-40% and Ni of 7-10%, and another alloy layer 2 contacting the free Sn layer has a composition containing Sn of 70-80%, Fe of 15-25% and Ni of 3-7%.

[0032] Further, it is also important that the total amount of Sn contained in the alloy layers 1 and 2 is 0.2-2.0g/m ² and the total amount of Ni is 0.001-0.3 g/m ² . If the total amount of Sn in the alloy layers 1 and 2 is less than 0.2g/m ² , the ATC value is not appropriate. Also in this invention, the Sn content accounts for 50% or more in the alloy layer 1 and 2. This large amount in the alloy layer suggests that the alloy layer is thick. Since the exposure of the surface of the base steel sheet is inhibited (i.e., there is a reduced uncovered portion of the base steel sheet) as the alloy layer becomes thicker, the corrosion resistance of the tinned steel sheet increases. However, when the tinned steel sheet with the thicker alloy layer is subjected to working such as bending or the like, the alloy layer tends to be cracked, i.e., an uncovered portion of the base steel sheet can be formed, which leads to lowering of the corrosion resistance. Also the thicker alloy layer may cause poor adhesiveness between the alloy layer and the upper layer of non-alloyed Sn. In view of the above problems, the upper limit of the total amount of Sn is 2.0 g/m ² . Preferably, the range of total amount of Sn in the alloy layers 1 and 2 is 0.2 - 0.7 g/ m ² in view of workability.

[0033] As for Ni, if the total amount of Ni in the alloy layers 1 and 2 is less than 0.001 g/m , it is unable to form a fine and dense alloy layer capable of avoiding formation of the uncovered portion of the base steel sheet. Whereas, when the total amount of Ni in the alloy layers 1 and 2 exceeds 0.3 g/m ² , the effect of fining the alloy layer is saturated, and further increase simply adds to cost. In view of this, the upper limit of Ni is 0.3 g/m ² . Preferably, the range of total amount of Ni in the alloy layers 1 and 2 is 0.005 - 0.040 g/m ² in view of cost.

[0034] There is no specific limitation on the method for manufacturing the tinned steel sheet of the present invention which comprises in the order upward from the surface of base steel sheet; an alloy layer (1) containing by mass %, Sn of 50-75%, Fe

of 20-40% and Ni of 7-20%; an alloy layer (2) containing by mass %, Sn of 70-80%, Fe of 15-25% and Ni of 0.1-7%; and an unalloyed Sn layer (3) of which coating weight is 1.5 - 15.0g/m ² ; wherein a total amount of Sn in the alloy layers (1) and (2) is 0.2 - 2.0g/m ² and a total content of Ni in the alloy layers (1) and (2) is 0.001 - 0.3g/m ² (wherein the mass % is based on the total mass of the particular layer). [0035] For example, there is a method of plating two layers on the base steel sheet, i.e., first, plating Fe-Ni alloy with high Ni percentage; second, plating Fe-Ni alloy with low Ni percentage or plating Fe; third, plating Sn; and then performing a melting treatment. There is another method of performing a melting treatment twice, i.e., first, plating Fe-Ni alloy, second, plating a small amount of Sn and then performing a melting treatment, third, plating Fe-Ni alloy with low Ni percentage or plating Fe, fourth, plating Sn and then performing a melting treatment.

[0036] In this invention, a chemical treatment layer for preventing oxidation of the free Sn layer can be formed on the free Sn layer. For example, a metal chromium layer and a chromium hydrous oxide layer formed over the entire steel sheet by using chromium plating bath or chromate treatment bath, or a chemical treatment film formed by an electrolytic treatment using a sodium phosphate bath can be applied. As for conditions of the bath or the treatment, there is no specific limitation. In the case of cathodic treatment in a chromic acid bath, for example, a tin oxide layer formed on a surface of the tin layer can be removed in advance so that the metal chromium layer and the chromium hydrous oxide layer can be easily formed. More specifically, it is realized, for example, by doing cathodic treatment with a steel sheet after tin plating in an aqueous solution of sodium carbonate.

[0037] FIG.2 shows another embodiment of the present invention where a chemical treatment layer 5 is formed on the unalloyed Sn layer 3.

[0038] Examples of the present invention are described below. Method for measuring ATC value:

[0039] A test piece, where an alloy layer is uncovered by electrolytically separating a free Sn layer in an aqueous solution of sodium chloride, is prepared. A liquid for the test is prepared by first, boiling a mixture of 500cc of tomato juice (KAGOME™ Tomato Juice salt-free) and 500cc of distilled water, and then adding thereto one liter of water containing 0.19g of SnCl ₂ *2H ₂ O (SnCl ₂ *2H ₂ O (Sn ²⁺ lOOppm)) and 0.5g of potassium sorbate, then aging the mixture. After a test piece is coupled with Sn in the test liquid and kept for 20 hours at 27 ⁰ C in a nitrogen gas atmosphere, a coupling current is measured.

Quantification method of percentage of element in an alloy layer: [0040] Quantification of the percentage of each element in the alloy layers 1 and 2 is determined as follows. First, a thin sample of a layer cross-section is prepared by a Focused Ion Beam method using a gallium ion beam, then elemental analysis is performed with points at intervals of 5nm in the cross-section direction by TEM (transmission electron microscope) outfitted with an energy dispersive X-ray analyzer (manufactured by Hitachi, Ltd).

Measuring of the amount of Sn elution:

[0041] Pineapple with syrup is canned using a can of φ307 x 409 and kept at 28 ⁰ C for 6 months. Then the flesh of the pineapple and the syrup is mixed and filtered so that Sn elution can be measured.

[0042] Conditions for plating and chemical treatment used for the examples described below are as follows.

Ni plating condition:

[0043] A Ni plating bath composition: nickel sulfate of 240g/l, nickel chloride of

80g/l and boric acid of 30g/l

Plating temperature: 5O ⁰ C

Current density: 30A/dm ²

Fe-Ni alloy plating condition 1 :

[0044] A Fe-Ni alloy plating bath composition: nickel sulfate of 75g/l, nickel chloride of 140g/l, ferrous sulfate of llOg/1 and boric acid of 45g/l

Plating temperature: 45 ⁰ C

Current density: 25 A/dm ²

Fe-Ni alloy plating condition 2:

[0045] A Fe-Ni alloy plating bath composition: nickel sulfate of 25g/l, nickel chloride of 50g/l, ferrous sulfate of 120g/l and boric acid of 45g/l

Plating temperature: 45 ⁰ C

Current density: 20A/dm ²

Fe plating condition:

[0046] A Fe plating bath composition: ferrous sulfate heptahydrate of 250g/l and ammonium sulfate of 120g/l

Plating temperature: 5O ⁰ C

Current density: 20A/dm ²

Sn plating condition:

[0047] A Sn plating bath composition: stannous sulfate of 25g/l and phenolsulfonic acid of20g/l

Plating temperature: 5O ⁰ C

Current density: 15 A/dm ²

Chemical treatment condition:

Bath composition: sodium bichromate of 25g/l

Plating temperature: 6O ⁰ C

Current density: 5A/dm ²

[Example 1]

[0048] A tinned steel sheet was prepared for a corrosion resistance test and was prepared by the following steps (l)-(4) in order: (1) performing Fe-Ni alloy plating on a degreased and acid-pickled cold-rolled steel sheet which was 0.21mm thick using the above-mentioned Fe-Ni alloy plating condition 1 so as to have Ni of 8mg/m ² ; (2) performing Fe plating using the above-mentioned Fe plating condition so as to have Fe of 10mg/m ; (3) performing Sn plating using the above-mentioned Sn plating condition so as to have Sn of 5.7g/m and then performing a melting treatment; and (4) performing chemical treatment using the above-mentioned chemical treatment condition so as to form a chemical treatment layer of 9mg/m ² in terms of chromium metal. [Example 2]

[0049] A tinned steel sheet was prepared for a corrosion resistance test and was prepared by the following steps (l)-(5) in order: (1) performing Fe-Ni alloy plating on a

degreased and acid-pickled cold-rolled steel sheet which was 0.21mm thick using the above-mentioned Fe-Ni alloy plating condition 1 so as to have Ni of 155mg/m ² ; (2) performing Sn plating using the above-mentioned Sn plating condition so as to have Sn of 0.8g/m ² and then performing a melting treatment; (3) performing Fe-Ni alloy plating using the above-mentioned Fe-Ni alloy plating condition 2 so as to have Ni of 25mg/m ² ; (4) performing Sn plating using the above-mentioned Sn plating condition so as to have Sn of 13.5g/m ² and then performing a melting treatment; and (5) performing chemical treatment using the above-mentioned chemical treatment condition so as to form chemical treatment layer of 9mg/m ² in terms of chromium metal. [Example 3]

[0050] A tinned steel sheet was prepared for a corrosion resistance test and was prepared by the following steps (l)-(4) in order: (1) performing Fe-Ni alloy plating on a degreased and acid-pickled cold-rolled steel sheet which was 0.21mm thick using the above-mentioned Fe-Ni alloy plating condition 1 so as to have Ni of 15mg/m ² ; (2) performing Fe-Ni alloy plating using the above-mentioned Fe-Ni alloy plating condition 2 so as to have Ni of 10mg/m ; (3") performing Sn plating using the above-mentioned Sn plating condition so as to have Sn of 11.5g/m ² and then performing a melting treatment; and (4) performing chemical treatment using the above-mentioned chemical treatment condition so as to form a chemical treatment layer of 9mg/m ² in terms of chromium metal. [Example 4]

[0051] A tinned steel sheet was prepared for a corrosion resistance test and was prepared by the following steps (l)-(4) in order: (1) performing Fe-Ni alloy plating on a degreased and acid-pickled cold-rolled steel sheet which was 0.21mm thick using the above-mentioned Fe-Ni alloy plating condition 1 so as to have Ni of 15mg/m ² ; (2)

performing Fe-Ni alloy plating using the above-mentioned Fe-Ni alloy plating condition 2 so as to have Ni of lOmg/m ² ; (3) performing Sn plating using the above-mentioned Sn plating condition so as to have Sn of 4.0g/m ² and then performing melting treatment; and (4) performing chemical treatment using the above-mentioned chemical treatment condition so as to form a chemical treatment layer of 9mg/m ² in terms of chromium metal. [Example 5]

[0052] A tinned steel sheet was prepared for a corrosion resistance test and was prepared by the following steps (l)-(3) in order: (1) performing Fe-Ni alloy plating on a degreased and acid-pickled cold-rolled steel sheet which was 0.21mm thick using the above-mentioned Fe-Ni alloy plating condition 1 so as to have Ni of 8mg/m ² ; (2) performing Fe plating using the above-mentioned Fe plating condition so as to have Fe of lOmg/m ² ; and (3) performing Sn plating using the above-mentioned Sn plating condition so as to have Sn of 5.7g/m ² and then performing a melting treatment. [Example 6]

[0053] A tinned steel sheet was prepared for a corrosion resistance test and was prepared by the following steps (l)-(3) in order: (1) performing Fe-Ni alloy plating using a degreased and acid-pickled cold-rolled steel sheet which was 0.21mm thick using the above-mentioned Fe-Ni alloy plating condition 1 so as to have Ni of 15mg/m ² ; (2) performing Fe-Ni alloy plating using the above-mentioned Fe-Ni alloy plating condition 2 so as to have Ni of 10mg/m ² ; and (3) performing Sn plating using the above-mentioned Sn plating condition so as to have Sn of 11.5g/m and then performing a melting treatment.

[Comparison example 1]

[0054] A tinned steel sheet was prepared for a corrosion resistance test and was prepared by the following steps (l)-(4) in order: (1) performing Sn plating on a degreased and acid-pickled cold-rolled steel sheet which was 0.21mm thick using the above-mentioned Sn plating condition so as to have Sn of 8.7g/m and then performing a melting treatment; and (2) performing chemical treatment using the above-mentioned chemical treatment condition so as to form a chemical treatment layer of 8mg/m ² in terms of chromium metal. [Comparison example 2]

[0055] A tinned steel sheet was prepared for a corrosion resistance test and was prepared by the following steps (l)-(3) in order: (1) performing Ni plating on a degreased and acid-pickled cold-rolled steel sheet which was 0.21mm thick using the above-mentioned Ni plating condition so as to have Ni of 310mg/m ² ; (2) performing Sn plating using the above-mentioned Sn plating condition so as to have Sn of 10.0g/m ² and then performing a melting treatment; and (3) performing chemical treatment using the above-mentioned chemical treatment condition so as to form a chemical treatment layer of 9mg/m ² in terms of chromium metal. [Comparison example 3]

[0056] A tinned steel sheet was prepared for a corrosion resistance test and was prepared by the following steps (l)-(4) in order: (1) performing Fe-Ni alloy plating on a degreased and acid-pickled cold-rolled steel sheet which was 0.21mm thick using the above-mentioned Fe-Ni alloy plating condition 2 so as to have Ni of 5mg/m ² ; (2) performing Fe plating using the above-mentioned Fe plating condition so as to have Fe of 10mg/m ² ; (3) performing Sn plating using the above-mentioned Sn plating condition so as to have Sn of 7.1g/m ² and then performing a melting treatment; and (4) performing chemical treatment using the above-mentioned chemical treatment condition

so as to form a chemical treatment layer of 9mg/m ² in terms of chromium metal. [Comparison example 4]

[0057] A tinned steel sheet was prepared for a corrosion resistance test and was prepared by the following step (1): (1) performing Sn plating using the above-mentioned Sn plating condition so as to have Sn of 8.7g/m ² and then performing a melting treatment.

[0058] TABLE 1 shows the results of examples 1-6 and comparison examples 1-4, i.e., percentage (mass %) of each component of alloy layer 1, alloy layer 2 and Sn layer 3 quantified by above-mentioned element analysis; total amount (g/m ) of each component; ATC value; amount of Sn elution; and evaluation of corrosion resistance ("good" - is when the amount of Sn elution is less than 36ppm: "improvement required" - is when the amount of Sn elution is around 40ppm: "not good" - is when the amount of Sn elution is more than 40ppm).

[TABLE 1]

^■ : mprovement equ re

[0059] Examples 1-6 meet the range of percentage of component element of alloy layer 1, alloy layer 2 and Sn layer 3 required by the present invention. It was found that the amount of elution of Sn is reduced in examples 1-6, i.e., the corrosion resistance of all the examples was "good."

[0060] On the contrary, comparison examples 1-4 were prepared with the range of percentage of component element of alloy layer 1, alloy layer 2 and Sn layer 3 required by the present invention. It was found that the amount of elution of Sn increased in examples 1-4, i.e., the corrosion resistance of all the examples was evaluated as either "improvement-required" or "not-good."

[0061] AU references referred to hereinabove, are herein incorporated by reference in their entirety.