GB1594896A - Tinplating - Google Patents

Description

(54) TINPLATING (71) We, TOYO KORAN CO., LTD., a Japanese body corporate of 4-3 Kasumigaseki 1-chome, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to a method of producing tinplate and to the production of containers therefrom.

In the manufacture of seamless containers, the drawing and ironing process has been used. As materials used for seamless containers, electrolytic tinplates have also been used with economic advantages. A commercial electrolytic tinplate is made by continuously electrolytically plating tin onto steel and then the tin coating is normally melted and flow-brightened. Such commercial flow-melted (reflowed) electrolytic tinplate material is found to need a slightly greater load in the drawing and ironing operation than that of a non-flow-brightened electrolytic tinplate. This is because the iron-tin alloy formed by flow-melting is hard and brittle and results in an increase of friction between the steel substrate and the die during the drawing and ironing steps.Accordingly, a matte electrolytic tinplate material produced without a flow-melting operation is widely used for producing seamless containers because it does not require such a load in the drawing and ironing steps.

The reduction of the drawing and ironing load serves to extend the life of the tools used in such drawing and ironing operation and therefore, it is important to reduce the drawing and ironing load.

In producing the tin cans or containers on a commercial basis, the speed in the ironing operation today is about 120 cans or containers per minute and in some cases, the rate increases because of the productivity demands. However, as the ironing speed increases, certain disadvantages occur such as "frosting" and scratching on the exterior surface of the containers in the ironing step. This frosting phenomenon occurs because the temperature of the exterior surface of the container rises during the ironing operation and as a result, that portion of the container subjected to elevated temperatures becomes dim and lusterless.

Presumably, this is due to the melting of the tin layer on the exterior surface of the containers during the ironing step at the increased temperature. At any rate, regardless of the cause of the phenomenon, frosting does occur at such elevated temperatures and spoils the appearance of the exterior surface of the container.

From the above, it will be apparent that there is a distinct need for improving the production of matte electrolytic tinplates by decreasing the drawing and ironing loads at high speeds and to prevent frosting and scratching of the surface of the containers during the ironing step at speeds of over 120 cans/min.

We have now found a method of producing seamless containers which uses an electrolytic tinplate having an iron-tin alloy formed electrolytically on a low carbon steel sheet or strip.

This method, in our tests, has been found capable of producing seamless containers readily and easily at reduced loads during the drawing and ironing steps. We have also found that our method extends the life of the tools used for drawing and ironing the containers and that the containers produced at high speeds are free or only suffer slightly from the aforementioned frosting and scratches on the exterior surface of the containers.

In its broadest aspect, the invention provides a method of producing tinplate without flow-melting which comprises electrolytically depositing a coating 0.005 to 0.2 g/m2 of an iron-tin alloy (calculated as tin) on a low carbon steel sheet or sttrip, and then electroplating tin over the iron-tin alloy. The invention also includes cans when wholly or partially made of tinplate produced by our method, and in particular includes a method of producing a seamless container which comprises drawing and/or ironing (usually a combination of both operations) tinplate produced by our method.

In the accompanying drawings, to which reference is made below, Figure 1 is an electron microphotograph of an iron-tin alloy of a commercially flow-melted electrolytic tinplate, said microphotograph being magnified 5,000 times.

Figure 2 is an electron microphotograph magnified 5,000 times of an iron-tin alloy formed electrolytically in accordance with the present invention.

Figure 3 is a pattern of electron diffraction for an iron-tin alloy formed electrolytically in accordance with the present invention.

Figure 4 is a potential-time curve of an electrolytic tinplate obtained in accordance with the present invention.

The iron-tin alloy formed electrolytically according to the present invention may be obtained by electrolytically tinplating a steel strip or sheet in the presence of a small amount of iron oxide on the steel surface with generation of hydrogen during the electro-plating.

The iron oxide is conveniently formed by exposing the steel to air after degreasing and rinsing.

In this step, we believe that tin ions formed in the tin electroplating step combine with iron ions generated by reduction of the iron oxide, and the electrochemical formation of an iron-tin alloy takes place.

In forming the iron-tin alloy according to the present invention, either an acid or alkaline electrolyte can be used, an alkaline electrolyte being preferred. When an acid electrolyte is used, hydrogen is generated in a low(dilute) stannous tin solution during electrolysis and thus a stannous tin solution containing less stannous than 15 g/l is suitable. The morphology of the iron-tin alloy formed electrolytically according to the present invention differs from the iron-tin alloy formed by flow-melting as shown in Figures 1 and 2 of the drawings. It is apparent that the iron-tin alloy formed electrolytically is very fine in structure.Indeed, it has been found that the friction between the steel substrate and the die used for the drawing and ironing operations is reduced by forming the fine iron-tin alloy and this fact is corroborated by the results of testing the material to evaluate the drawability and ironability thereof.

To reduce the load and to prevent frosting and scratching of the surface of the tinplate, the amount of the iron-tin alloy formed electrolytically on the surface of the steel sheet should be at least 0.005 g/m2, calculated as tin. Also, to facilitate the drawing and ironing operation, the amount of alloy formed electrolytically should not exceed 0.2 g/m2, again calculated as tin.

The degreasing of the steel prior to plating may be effected by conventional methods, for example electrolytically, using an acid or alkaline electrolyte. The use of an alkaline electrolyte in this step is preferred.

The tinplating step carried out after the formation of the iron-tin alloy may also be effected by conventional methods, using conventional additives if desired. The use of an acid electrolyte in this step is preferred. The amount of tin deposited will be the same as in normal practice. The steels used in our method may also be the same as those generally used in conventional tinplating.

The following examples illustrate the invention. The additive agent used in the tinplating step in the examples was "ENSA-5", an ethoxylated a-naphthol sulphonic acid containing an average of 5 ethylene oxide units per a-naphthol group.

Example 1 The composition and mechanical properties of a cold rolled low carbon steel sheet used is indicated in Table 1.

TABLE 1 The Composition of the Steel (percent by weight) C ..................... 0.05 Mn .. ......... . 0.30 S . ....... ..... 0.015 P . ....... . . 0.014 Si . . . 0.02 Al .............. . 0.054 Cu . . . 0.007 Cr ........... 0.056 Fe . the balance The Mechanical Properties of the Steel Ultimate tensile strength . . 37.8 kg/mm2 Yield strength .. . 26.1 kg/mm2 Tensile elongation . ....... . 39.0% Hardness ....... . 49 Rockwell 30T The steel sheet of thickness 0.32 mm was electrolytically degreased in a 7% by weight solution of sodium hydroxide and rinsed in water.The sheet was then exposed in air for one second to form iron oxide on the steel sheet, then coated with tin an an amount of 0.05 g/m2 in an acid electrolyte of low stannous tin content containing stannous tin in a concentration of 2.0 g/l and sulfuric acid in an amount of 5 g/l. In this electrolysis, the current density was 30 A/dm2 and the current efficiency was 5%. The amount of an iron-tin alloy formed by the electrolysis was 0.01 g/m2. The iron-tin alloy was identified as FeSn2 from an analysis of the electron diffraction. The pattern of electron diffraction is shown in Figure 3 and the result of electron diffraction analysis is indicated in Table 2.

The steel sheet was then electro-plated with tin in an amount of 5.6 g/m2 in an acid electrolyte containing stannous tin in an amount of 30 g/l, sulfuric acid in an amount of 20 g/l and an additive agent in an amount of 5 g/l. In the electrolysis, the current density was 30 A/dm2, and the current efficiency was 99%.

The electro-plated tinplate was then passivated in dilute sodium dichromate and rinsed and di-octyl sebacate (DOS) oil was then applied. The amount of the iron-tin alloy was measured by a coulometric method. The iron-tin alloy was determined by measuring the time (L in Figure 4) corresponding to the dissolution of an iron-tin alloy in a recorded potential-time curve. The potential-time curve is shown in Figure 4.

The results of the electron diffraction analysis for an iron-tin alloy formed by electrolysis are set forth in Table 2 below: TABLE 2 Lattice constant d (Angstroms) The results of analysis FeSn2 * 2.67 2.67 2.56 2.57 2.29 2.31 2.05 2.07 1.70 1.64 1.62 1.54 1.50 1.52 *Depends on an X-ray powder data file (ASTM) The electrolytic tinplate was cut into a circular blank having a diameter of 125.5 mm by means of a punch press. The flat circular blank was then drawn through a capping die by means of a drawing punch of 67.9 mm diameter.

After the drawing, the cup was passed through three ironing dies. The clearance between each of the dies and the punch of 52.7 mm diameter are shown in Table 3, in which case, the ironing speed was 180 cans/min.

TABLE 3 The clearance between each of the dies and the punch in the ironing operation are as follows: Clearance (mm) 1st ironing 0.29 2nd ironing 0.18 3rd ironing 0.10 As an evaluation of the formability of the tinplated containers, the area where frosting was expected to break out on the exterior surface of the seamless container was inspected and the maximum punch load at the third ironing operation was measured. The results -are shown in Table 4.

Example 2 A steel sheet of the same type as used in Example 1 was electrolytically degreased in a 5% by weight sodium hydroxide solution and rinsed in water. The sheet was then electrolytically treated by using anodic electrolysis in a 5% by weight sulfuric solution and rinsed. The existence of a small amount of iron oxide on the surface of the steel sheet was confirmed.

The steel sheet was then coated with tin in an amount of 0.1 g/m2 in an alkaline electrolyte containing stannic tin in an amount of 40 g/l and sodium hydroxide in an amount of 15 g/l. In this electrolysis, the current density was 3 A/dm2 and the current efficiency was 30%. The amount of iron-tin alloy formed by this electrolysis was 0.05 g/m2. The iron-tin alloy was identified as FeSn2 by electron diffraction analysis.

The steel sheet was then electro-plated with tin in an amount of 5.6 g/m2 in an acid electrolyte containing stannous tin in an amount of 30 g/l, sulfuric acid in an amount of 20 g/l and an additive agent in the amount of 5 g/l. The electrolytic tinplate was then passivated in dilute sodium dichromate and rinsed, and di-octyl sebacate (DOS) oil was then applied to the surface thereof.

The electrolytic tinplate was tested and evaluated by the drawing and ironing process as described in Example 1.

Example 3 A steel sheet of the same type as used in Example 1 was electrolytically degreased in a 5% by weight sodium hydroxide solution and rinsed in water.

The steel sheet was then coated with tin in an amount of 0.2 g/m2 in an alkaline electrolyte containing stannic tin in an amount of 45 g/l and sodium hydroxide in an amount of 15 g/l without pickling in sulfuric acid solution. In this electrolysis, the current density was 5 A/dm2 and the current efficiency was 33%. The amount of an iron-tin alloy formed by electrolysis was 0.11 g/m2. The iron-tin alloy was identified as FeSn2 by electron diffraction analysis.

The steel sheet was electro-plated with tin in an amount of 5.6 g/m2 in an acid electrolyte containing stannous tin in an amount of 30 g/l, sulfuric acid in an amount of 20 g/l and a conventional additive agent in an amount of 5 g/l. The electrolytic tinplate was then passivated in dilute sodium dichromate and rinsed and di-octyl sebacate (DOS) oil was then applied.

The electrolytic tinplate was tested and evaluated by the drawing and ironing process as described in Example 1.

The amount of frosting which broke out on the exterior surface of the seamless container produced according to the present invention and the maximum punch load during high speed ironing at 180 cans/min. are set forth in Table 4 below.

TABLE 4 Example No. Frosting which Maximum punch load occured during at the third the ironing step ironing (Kg) 1 slight amount 2490 2 slight amount 2470 3 slight amount 2685 Flow-melted frosting occurs tinplate * wholly on the 2950 external surface Matte heavy formation tinplate of frosting 2830 without flow-melting * *The amount of tin coating in the conventional electrolytic tinplate is 5.6 g/m2.

As shown by Table 4, it is evident that the electrolytic tinplate prepared by the method of the present invention significantly reduces the ironing punch load and prevents significant frosting on the exterior surface of seamless steel containers during the high speed ironing operation as compared with convekntional electrolytic tinplates (flow-melted tinplate and matte tinplate without flow-melting).

WHAT WE CLAIM IS: 1. A method of producing tinplate without flow-melting which comprises electrolytically depositing a coating of 0.005 to 0.2 g/m2 of an iron-tin alloy (calculated as tin) on a low carbon steel sheet or strip, and then electroplating tin over the iron-tin alloy.

2. A method as claimed in claim 1 wherein the iron-tin alloy is formed by electrolytically plating the steel with tin in the presence of iron oxide.

3. A method as claimed in claim 2 wherein an alkaline or acid electrolyte is used in the formation of the iron-tin alloy.

4. A method as claimed in claim 2 or claim 3 wherein the iron oxide is formed by exposing the steel to air after degreasing and rinsing.

5. A method as claimed in any one of the preceding claims wherein tin is electroplated over the iron-tin alloy from an acid electrolyte.

6. A method of producing tinplate substantially as described herein in any one of the Examples.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. described in Example 1. Example 3 A steel sheet of the same type as used in Example 1 was electrolytically degreased in a 5% by weight sodium hydroxide solution and rinsed in water. The steel sheet was then coated with tin in an amount of 0.2 g/m2 in an alkaline electrolyte containing stannic tin in an amount of 45 g/l and sodium hydroxide in an amount of 15 g/l without pickling in sulfuric acid solution. In this electrolysis, the current density was 5 A/dm2 and the current efficiency was 33%. The amount of an iron-tin alloy formed by electrolysis was 0.11 g/m2. The iron-tin alloy was identified as FeSn2 by electron diffraction analysis. The steel sheet was electro-plated with tin in an amount of 5.6 g/m2 in an acid electrolyte containing stannous tin in an amount of 30 g/l, sulfuric acid in an amount of 20 g/l and a conventional additive agent in an amount of 5 g/l. The electrolytic tinplate was then passivated in dilute sodium dichromate and rinsed and di-octyl sebacate (DOS) oil was then applied. The electrolytic tinplate was tested and evaluated by the drawing and ironing process as described in Example 1. The amount of frosting which broke out on the exterior surface of the seamless container produced according to the present invention and the maximum punch load during high speed ironing at 180 cans/min. are set forth in Table 4 below. TABLE 4 Example No. Frosting which Maximum punch load occured during at the third the ironing step ironing (Kg)

1 slight amount 2490

2 slight amount 2470

3 slight amount 2685 Flow-melted frosting occurs tinplate * wholly on the 2950 external surface Matte heavy formation tinplate of frosting 2830 without flow-melting * *The amount of tin coating in the conventional electrolytic tinplate is 5.6 g/m2.

As shown by Table 4, it is evident that the electrolytic tinplate prepared by the method of the present invention significantly reduces the ironing punch load and prevents significant frosting on the exterior surface of seamless steel containers during the high speed ironing operation as compared with convekntional electrolytic tinplates (flow-melted tinplate and matte tinplate without flow-melting).

WHAT WE CLAIM IS: 1. A method of producing tinplate without flow-melting which comprises electrolytically depositing a coating of 0.005 to 0.2 g/m2 of an iron-tin alloy (calculated as tin) on a low carbon steel sheet or strip, and then electroplating tin over the iron-tin alloy.

2. A method as claimed in claim 1 wherein the iron-tin alloy is formed by electrolytically plating the steel with tin in the presence of iron oxide.

3. A method as claimed in claim 2 wherein an alkaline or acid electrolyte is used in the formation of the iron-tin alloy.

4. A method as claimed in claim 2 or claim 3 wherein the iron oxide is formed by exposing the steel to air after degreasing and rinsing.

5. A method as claimed in any one of the preceding claims wherein tin is electroplated over the iron-tin alloy from an acid electrolyte.

6. A method of producing tinplate substantially as described herein in any one of the Examples.

7. Tinplate when produced by a method as claimed in any one of the preceding claims.

8. A can when made wholly or partially from tinplate as claimed in claim 7.

9. A method of producing a seamless container which comprises drawing and/or ironing tinplate as claimed in claim 7.

10. A method of producing a seamless container substantially as described herein in any one of the Examples.

11. A seamless container when produced by a method as claimed in any one of claims 9 or 10.