AU718314B2 - Method for tin-plating and tin-plating bath having wide optimum current density range - Google Patents

Description

NSC-E819-PCT 1 METHOD FOR TIN-PLATING AND TIN-PLATING BATH HAVING WIDE OPTIMUM CURRENT DENSITY RANGE FIELD OF THE INVENTION The present invention provides a tin-plating bath and a method for tin-plating capable of plating a steel sheet with tin at a high current density to produce mainly a tin-plated steel sheet (a so-called tinplate) and having a wide optimum current density range.

BACKGROUND OF THE INVENTION Tinplate was invented during the period from the latter half of the 13th century to the half of the 16th century, and was produced principally by hot dipping.

At the beginning of the 20th century, a process capable of continuously electroplating a steel sheet with tin was completed. In the process, a steel sheet in a coil is continuously degreased, pickled, electroplated with tin, and subjected to melting treatment, chemical treatment and oil coating. Degreasing is conducted usually by exposure to an alkaline solution, electrolysis and mechanical treatment using a brush to remove cold rolling oil, etc. from the steel sheet.

Pickling is carried out by immersion or electrolysis of the steel sheet in an aqueous solution of sulfuric acid, etc. to reduce and remove oxides formed thereon. Tinplating is conducted by electroplating in a plating bath containing Sn ions. Melting treatment is performed for the purpose of ensuring brightness and corrosion resistance of the plated steel sheet. The treatment is conducted by induction heating or electric heating to heat the tin coating to temperature above the melting point of tin and immediately quenching the tin in warmed water. Chemical treatment is conducted for the purpose of preventing oxidation of the tin coating. In the treatment, the tin-plated steel sheet is subjected to immersion or electrolysis to form a chromate film thereon. Oil coating is carried out for the purpose of 2 imparting scratch resistance and rust preventive properties to the tin-plated steel sheet. The steel sheet is coated with oil such as ATBC (acetyl tributyl citrate) or DOS (dioctyl sebacate). Moreover, in some applications, the steel sheet may not be subjected to hot dip coating and chemical treatment. Although the continuous treatments as mentioned above are usually conducted by passing the steel sheet in coil having a weight of several tens of ton at a line speed of 300 to 400 m/min, they may be conducted by passing the sheet at a line speed of 100 m/min owing to operating conditions such as the connection of a new coil.

The step of tin-plating which is the most important step among the tinplate production steps described above will be explained below in detail.

A phenolsulfonic acid bath and a halogen bath have been used in the industry as plating baths for tinplating The Technology of Tinplate, London Edward Arnold Ltd., p213 (1965)), and the phenolsufonic acid bath is employed in about 80% of tinplate production lines in the world. The use of a methanesulfonic acid bath (Metal Finishing, January, AESF, p17 (1990)) has been examined in recent years to protect the environment, and the bath has been put into practical use in some of lines in the world.

Using such plating baths, a steel sheet is electroplated with tin while the steel sheet is being used as a cathode. Although the current density of tinplating varies depending on the variation of the tinplate production line speed (high current density at the time of a high line speed, low current density at the time of a low line speed), the variation width must be within the optimum current density range determined by the quality of the tinplate to be produced. The quality of the tinplate herein designates K-plate conditions (see ASTM A632, for example, an ATC current 2(alloy tin couple current) up to 0.12 A/cm 2 an ISV (alloy tin couple current) up to 0.12 pA/ CM an ISV -3 (iron solution value) up to 6.9 mg/51 ml and a TC (tin crystal) up to 9, appearance being included sometimes depending on the application). Moreover, when the plating current density is too low, a so-called "low current phenomenon" in which plating defects are formed to impair the appearance and corrosion resistance takes place. Moreover, when the current density is too high, the current efficiency quickly decreases, and so-called "burnt plating" in which tin plating becomes powdery and plating defects are formed to impair the appearance and the corrosion resistance of the tin-plated steel sheet takes place. Accordingly tin-plating must be conducted in the optimum current density range in which the low current phenomenon and burnt plating do not take place, that is, plating defects are not formed substantially.

In conventionally industrialized tinplate production lines, the lower limit of the optimum current density 2 range is from 5 to 10 A/ dm 2 and the upper limit thereof is from 20 to 30 A/dm 2 As described above, there is a close relationship between the line speed and the current density range of tin-plating. For example, for the purpose of improving the productivity of tinplates, it is satisfactory to increase the line speed. However, a tin-plating method is not satisfactory when tin-plating can be carried out only at a high current density. The tin-plating method cannot be applied to industrial use unless a tinplate of high quality can be produced by the method even at a low current density which is within the optimum current density range in the method because the method cannot correspond to the acceleration or deceleration of the tinplate line at the time of connecting a new coil.

The cost competition between the tinplate products and other products such as aluminum, bottles and paper containers has become fierce in recent years. For the purpose of economically producing products of high quality, it has become necessary to improve the -4productivity by operating the tinplate line at high speed and to maintain the product quality. When conventional techniques are applied to the high speed operation of the tinplate production line, longer tinplating tanks must be installed in accordance with a decrease in the plating time due to the high speed operation. Since the installation requires an enormous amount of investment, high speed operation by the conventional techniques is not suited to an industrial tinplate production line.

On the other hand, it has been generally known that increasing the current in tin-plating can be achieved by increasing the amount of material transfer in the boundary layer near the steel sheet to be plated, namely by increasing the concentration of Sn ions or the flow speed of the plating solution. However, the optimum current density range mentioned above is not widened substantially by the procedure described above. As a result, the conventional techniques cannot correspond to the acceleration and deceleration of the tinplate production line at the time of connecting a new coil, and cannot be suited to the line.

For example, plating baths and plating methods in which sulfuric acid as a principal component of the baths are used at a high current density are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6- 346272 ("Sulfuric acid bath for tin-plating at a high current density and a tin-plating method"), Japanese Unexamined Patent Publication (Kokai) No. 7-207489 ("Tin plating bath"), and Japanese Unexamined Patent Publication (Kokai) No. 8-260183 ("Sulfuric acid bath having a high electric conductivity, a good sludge inhibiting ability and tin-dissolution function").

However, these patent publications provide only methods by which plating can be conducted at an increased tin ion concentration and a high plating current density.

However, the techniques in the publications do not widen 5 the optimum current density range.

Accordingly, there is a strong desire for a tinplating bath and a tin-plating method with which tinplating can be conducted at a high current density and in a wide optimum current density range so that the acceleration and deceleration of the plating line speed ranging from a high speed to a low speed at which a new coil is connected can be performed.

DISCLOSURE OF THE INVENTION The present inventors have, therefore, intensively investigated the relationship between a tin-plating current density and a plating quality, and an optimum current density range while changing the concentrations of Sn ions, Fe ions and organic additives in a tinplating bath, the relative speed between a plating solution and a steel sheet to be plated, etc. As a result, they have discovered that the combined effect of an increase in the tin concentration and the solution flow speed not only improves the threshold current density but also widens the optimum current density range.

The mechanism of this discovery is considered to be as described below. In general, when the current density is low, electrodeposition nucleus growth takes place predominantly in electrodepsition in plating.

Electrodeposition nucleus generation becomes predominant and the tin plating becomes denser as the current density increases. When the current density is increased further, hydrogen is be generated, and the plating becomes powdery at such a current density, to cause a problem with regard to the adhesion. Although the phenomenon seems to depend on a current density, it actually depends on a potential. That is, a low current density results when the potential is low, and a high current density results when the potential is high.

Accordingly, it is considered that there exists an optimum potential range as there exists an optimum 6 current density range. On the other hand, the potential or optimum potential range of a steel sheet during plating is considered to be influenced by the electric capacitance of an electric double layer at the interface of the steel sheet to be plated and the plating solution. Although the electric capacitance of an electrical double layer is strongly influenced by the thickness of the electric double layer and the ionic strength, it is significantly changed by the combined effect of a decrease in the boundary layer thickness caused by an increase in the flow speed of the plating solution, and an increase in the ionic strength caused by an increase in the concentration of Sn ions. As a result, the dependence of the current density on the potential is greatly changed, and the current density is greatly changed by a potential change smaller than before. The optimum current density range is, therefore, widened.

The present invention is based on the discovery as mentioned above, and provides what is described below.

A method for continuously plating tin on a steel sheet, comprising: providing a tin-plating bath containing 40 to 100g/L of Sn ions and 20 to 400g/L of base acid; feeding a first steel sheet having first and second ends in the length direction of the sheet from S- s the first to second ends thereof into said tin-plating bath at a first feeding speed with the relative speed difference between the steel sheet and the plating solution being held at 2 to 20m/sec, to plate tin on the surface of the first steel sheet with a first optimum current density; a:joining said second end of said first steel sheet with the first end of a second steel sheet having first and second ends in the length direction of the sheet outside said tin-plating bath while said first 2o steel sheet is being fed into said tin-plating bath at a second feeding speed with the relative speed difference between the steel sheet and the plating solution being held at 2 to 20m/sec, to plate tin on the surface of the first steel sheet at a second optimum current density, the difference between said first and second optimum current densities being at least 80A/dm2; and feeding said second steel sheet from the first to second ends thereof into said tin-plating bath at 2 a third feed speed with the relative speed difference between the steel sheet and the plating solution being held at 2 to 20m/sec to plate tin on the surface of the second steel sheet at a third optimum current density.

The method for tin-plating according to wherein the plating bath comprises 40 to 100g/L of Sn ions and 20 to 400g/L of phenolsulfonic acid.

The method for tin-plating according to wherein the plating bath comprises 40 to 100 g/L of Sn ions and 20 to 400g/L of phenolsulfonic acid, and further a brighter and/or antioxidant.

The method for tin-plating according to wherein the plating bath comprises 40 to 100g/L of Sn ions, 0.1 to 15g/L of Fe ions and 20 to 400g/L of [R:\LIBZ]05139.doc:SAK phenolsulfonic acid.

The method for tin-plating according to wherein the plating bath comprises 40 to 100 g/l of Sn ions, 0.1 to 15 g/l of Fe ions and 20 to 400 g/l of phenolsulfonic acid, and further a brightener and/or antioxidant.

The method for tin-plating according to wherein the plating bath comprises as the brightener 0.1 to 10 g/l of ethoxylated a-naphtholsulfonic acid and/or 0.1 to 10 g/l of ethoxylated a-naphthol.

The method for tin-plating according to wherein the plating bath comprises as the brightener 0.1 to 10 g/1 of ethoxylated a-naphtholsulfonic acid and/or 0.1 to 10 g/l of ethoxylated a-naphthol.

The method for tin-plating according to wherein the plating bath comprises 40 to 100 g/l of Sn ions and 20 to 400 g/l of methanesulfonic acid.

The method for tin-plating according to wherein the plating bath comprises 40 to 100 g/l of Sn ions and 20 to 400 g/l of methanesulfonic acid, and further 0.1 to 10 g/l of a brightener and/or 0.1 to g/l of antioxidant.

The method for tin-plating according to wherein the plating bath comprises 40 to 100 g/1 of Sn ions, 40 to 300 g/1 of 3-alkanolslulfonic acid which has a hydroxyl group at the P-position and which is typically represented by 2-hyroxyethan-l-sulfonic acid, and a brightener.

(11) The method for tin-plating according to wherein the variation width of the optimum current density is at least 250 A/dm 2 (12) The method for tin-plating according to wherein the variation width of the optimum current density is at least 350 A/dm 2 (13) A tin-plating bath comprising 40 to 100 g/l of Sn ions and 20 to 400 g/l of phenolsulfonic acid.

(14) The tin-plating bath according to (13), 8 wherein the tin-plating bath comprises 40 to 100 g/l of Sn ions and 20 to 400 g/l of phenolsulfonic acid, and further a brightener and/or antioxidant.

The tin-plating bath according to (13), wherein the tin-plating bath comprises 40 to 100 g/l of Sn ions, 0.1 to 15 g/l of Fe ions and 20 to 400 g/l of phenolsulfonic acid.

(16) The tin-plating bath according to (13), wherein the tin-plating bath comprises 40 to 100 g/l of Sn ions, 0.1 to 15 g/l of Fe ions and 20 to 400 g/l of phenolsulfonic acid, and further a brightener and/or antioxidant.

(17) The tin-plating bath according to (14), wherein the tin-plating bath comprises as the brightener 0.1 to 10 g/l of ethoxylated a-naphtholsulfonic acid and/or 0.1 to 10 g/l of ethoxylated a-naphthol.

(18) The tin-plating bath according to (16), wherein the tin-plating bath comprises as the brightener 0.1 to 10 g/l of ethoxylated a-naphtholsulfonic acid and/or 0.1 to 10 g/l of ethoxylated a-naphthol.

(19) A tin-plating bath comprising 40 to 100 g/l of Sn ions and 20 to 400 g/l of methanesulfonic acid.

The tin-plating bath according to (19), wherein the tin-plating bath comprises 40 to 100 g/l of Sn ions and 20 to 400 g/l of methanesulfonic acid, and further 0.1 to 10 g/1 of a brightener and/or 0.1 to g/l of an antioxidant.

(21) A tin-plating bath comprising 40 to 100 g/l of Sn ions, 40 to 300 g/l of P-alkanolslulfonic acid having a hydroxyl group at the 3-position typically represented by 2-hyroxyethane-l-sulfonic acid, and a brightener.

Accordingly, when a tinplate product of high quality is to be produced efficiently in a high speed tin-plating line at a line speed of 700 m/min), prior tinning technology requires from 10 to 20 plating cells. However, according to the present invention, the production can be performed with fewer plating cells -9 (several), and therefore an extremely high economic efficiency can be achieved.

BEST MODE FOR CARRYING OUT THE INVENTION The mode of operation of the present invention will be explained below in detail.

In the present invention, the Sn ion concentration in a tin-plating bath not only improves the threshold current density but also plays a role in widening the optimum current density range by the combined effects achieved by the Sn ion concentration and the high flow rate. Accordingly, when the Sn concentration in a plating bath is too low, the combined effects cannot be achieved sufficiently. Accordingly, the Sn concentration in the plating bath must be at least g/l. The effects of improving the threshold current density and widening the range of the optimum current density are enhanced as the Sn ion concentration increases. However, since Sn at high cost dissipates owing to dragging out and splashing when the Sn ion concentration exceeds 100 g/l, the concentration becomes industrially disadvantageous. Accordingly, the Sn ion concentration in the plating bath in the present invention is desirably up to 100 g/l, and it must be from 40 to 100 g/l.

The tin-plating bath used in the present invention contains a base acid, such as phenolsulfonic acid, methanesulfonic acid and alkanolsulfonic acid, which are used in conventional tin-plating baths.

The base acid plays a role in improving the electric conductivity or making the electrodeposition form dense in addition to a role in stabilizing the Sn ions in the plating bath. When the concentration of the base acid is too low, stabilized Sn ions cannot exist and ordinary tin-plating becomes difficult owing to a decrease in the electric conductivity and so on. The concentration of the base acid must, therefore, be at least 20 g/l. Although the effects of stabilizing Sn ions and improving the electric conductivity are enhanced as the base acid concentration increases, the effects begin to saturate when the base concentration exceeds 4 0 0g/l, and the concentration is economically disadvantageous. The base acid concentration must, therefore, be up to 400g/l. Accordingly, the base acid concentration in the tin-plating bath must be from 20 to 400g/l.

The base acids used in the present invention may be obtained from industrial products produced by general industrial production processes. Moreover, it does not matter even when the industrial products contain unavoidable impurities such as unreacted products and coloured oxides mixed in the products during synthesis of the base acids. For example, phenolsulfonic acid produced as an industrial product by a general industrial production process such as the cumene process may be used. It does not matter even when the industrially produced phenolsulfonic acid contains unavoidable impurities such as unreacted phenol and coloured oxides mixed therein during synthesis thereof.

Although it is possible to conduct the industrial production using a tin-plating bath containing base acid ions and Sn ions, a tin-plating bath containing further a brightener such as ethoxylated a-naphtholsulfonic acid (ENSA) or ethoxylated a-naphthol (EN) may be used for obtaining a tin-plated steel sheet having a gloss appearance when the tin-plated steel sheet is used in a field where the appearance is required. In order to exhibit a gloss appearance imparted by the brightener, the steel sheet must be plated in a tin-plating bath 20 containing at least 0.1g/1 of ethoxylated a-naphtholsulfonic acid and/or at least 0.1g/l of ethoxylated a-naphthol. Although the effect of improving the gloss is enhanced in accordance with the addition amount of each brightener, the brightener adheres to the tinplated steel sheet and cannot be [/libz]00992:MEF 11 removed even in the step of washing the plating solution after plating when the addition amount exceeds 10 g/l, and a defective quality such as a poor appearance is caused. The addition amount of the brightener must, therefore, be up to 10 g/l. Accordingly, brighteners such as a-naphtholsulfonic acid and ethoxylated onaphthol are each added in an amount of 0.1 to 10 g/l.

Furthermore, for example, a-naphtholsulfonic acid and ethoxylated o-naphthol produced as general industrial chemicals may be used as the brighteners in the present invention, and the effect of the present invention is not lost even when these compounds contain unavoidable impurities therein from the synthesis thereof.

Furthermore, for use in a field requiring more excellent corrosion resistance, a tin-plated steel sheet obtained by the use of a tin-plating bath which contains Fe ions and having a trace amount of Fe in the tin plating layer is used. Such a tin-plated steel sheet is used because when the tin-plated steel sheet develops a defect reaching the base steel, the corrosion current at the defective portion is mainly produced by a potential difference between the tin plating layer and the base steel. However, when a trace amount of Fe exists in the tin plating layer, the potential difference is decreased, and the corrosion current can be lowered. In order to produce such a tin-plated steel sheet having a corrosion resistance, the tin-plating bath must contain at least 0.1 g/l of Fe ions. When the concentration of the Fe ions therein is increased, the amount of Fe in the tin plating layer tends to increase, and the effect of improving the corrosion resistance is also enhanced.

However, when the Fe concentration in the plating bath exceeds 15 g/l, oxidation of Sn ions with Fe ions becomes excessive. The plating bath must, therefore, contain up to 15 g/l of Fe ions. Accordingly, the concentration of Fe ions in the plating bath must be 12 defined to be from 0.1 to 15 g/l.

The tin-plating bath of the present invention has been described above. In order to conduct tin-plating at a high threshold current density and in a wide optimum current density range, the flow of the metal bath is indispensable. As described above, since the flow of the plating solution has the effect of decreasing the boundary layer thickness, it may be said that the flow depends on a relative speed difference between the steel sheet to be plated and that of the plating solution. The speed of the steel sheet to be plated herein designates a transfer speed of the steel sheet in a so-called continuous plating line, and the moving speed of the plating solution herein designates a generally measured average bulk moving speed. When the relative speed difference therebetween is too small, the effect of decreasing the boundary layer thickness is not sufficient, and conducting tin-plating at a high threshold current density and in a wide optimum current density range becomes difficult. Accordingly, the relative speed difference between the steel sheet to be plated and the plating solution must be at least 2 m/sec. Moreover, according to the discovery of the present inventors, in order to promote removal of bubbles which are included between the electrode and the steel sheet and which impair the quality stability of the tinplate products and bubbles generated during plating, the relative speed difference between the steel sheet to be plated and the plating solution is preferably at least 4 m/sec.

In order to set the relative speed difference between the steel sheet to be plated and the plating solution to more than a predetermined value, for example, the steel sheet may be passed through a stationary plating bath at a speed higher than a predetermined value 2 m/sec or more, or the plating solution may be forcibly moved in the same 13 direction as or in a direction opposite to the transfer direction of the steel sheet. The effect of improving the threshold current density and that of widening the optimum current density range are enhanced as the relative speed difference therebetween increases.

However, when the relative speed difference therebetween exceeds 20 m/sec, there arise problems of fluttering of the steel sheet and a nonuniform flow of the plating solution in the width direction. In addition to the problems mentioned above, the high relative speed difference is economically disadvantageous because the energy (mainly an electrical energy for driving pumps, motors, etc.) consumed for generating the relative speed difference becomes excessively high. Accordingly, the relative speed difference therebetween must be from 2 to m/sec.

Furthermore, the bath temperature during plating is desirably from 30 to 60 0 C. When the bath temperature is low, the bath has a high viscosity, and the plating solution on the tin-plated steel sheet cannot be satisfactorily separated. Accordingly, the bath temperature is desirably at least 30 0 C. The viscosity of the plating bath lowers and the separation of the tinning solution is improved as the bath temperature is raised. However, when the bath temperature becomes higher than 600C, fumes are drastically generated to pollute the operating environment, and holding the bath concentration constant becomes difficult. Accordingly, the bath temperature is desirably held at up to 600C.

The tin-plating bath according to the present invention is prepared by a procedure as described below.

Water to a volume of about half the final desired volume of the tin-plating bath is charged in vessel equipped with a stirring apparatus. A base acid such as phenolsulfonic acid in a predetermined amount is subsequently charged into the vessel, and the contents are stirred. A predetermined amount of Sn ions are 14 subsequently dissolved by adding tin oxide or by electrochemically dissolving metallic tin. Furthermore, a brightener such as ethoxylated x-naphtholsulfonic acid or ethoxylated c-naphthol and Fe ions are added if necessary. A predetermined amount of Fe ions can be dissolved by adding iron oxide or by electrochemically dissolving metallic Fe.

The tin-plating bath is introduced in a vertical or horizontal tin-plating tank used in a conventional continuous steel sheet plating line, and the relative speed difference between a steel sheet to be plated and the plating bath is set to 2 to 20 m/sec. When the plating bath is a stationary one where the plating solution is not flowing substantially, the speed difference is adjusted by controlling the traveling speed of the steel sheet. Alternately, the relative speed difference therebetween may be set to 2 to m/sec by controlling the traveling speed of the steel sheet and the flow speed of the plating solution while the plating solution is forcibly flown in the direction opposite to or with the traveling direction thereof.

As described above, in order to operate the tinplate production line at high speed, it is necessary that the line be operated at a high current density corresponding to the high speed of the line and that the line can be operated at a low current density when the line speed is slowed down for connecting a new coil, or the like procedure. Moreover, these current densities must be within the optimum current density range.

According to the present invention, the tinplate production line is operated with the variation width of the current density within the optimum current density 2 range being at least 80 A/dm 2 preferably at least 250 2 A/dm 2 Furthermore, the tinplate production line may also be operated, if necessary, with the variation width thereof being at least 350 A/dm 2 particularly at least 450 A/dm 2 A continuous tinplate production line can be 450 A/din A continuous tinplate production line can be 15 in practice operated at a high speed, only when the optimum current density range is wide and the line is actually operated with such a wide variation width of the current density within the wide optimum current density range. It has heretofore been unknown that such a line operation is possible, and such a line operation has not been conducted. In conventional industrial tinplate production lines, only an optimum current density of about 5 to 30 A/dm 2 (variation width: up to 2 25 A/dm 2 has been adopted.

According to the present invention, an actually high tinplate production line speed is realized by a combination of a high tin ion concentration, a high relative speed difference between a steel sheet to be plated and a plating solution, a wide variation width of a current density within a wide optimum current density range and, if necessary, a specific base acid at a high concentration. The tinplate production line is also operated at a low speed in accordance with connection of a new coil or the like procedure, by widely varying the current density within the optimum current density range. The tinplate production line may thus be continuously operated.

In order to ensure a stabilized product quality in conducting tin-plating by the method of the present invention, a uniform flow rate of the plating solution must be maintained in the width and longitudinal directions. In order to realize the uniform flow rate, it is important that the spacing between the anode and the steel sheet be always held constant, and it is desirable that an insoluble anode be used as the anode.

The plating solution may be satisfactorily made to flow by a conventional water-jet pump.

The amount of tin plating is adjusted by the amount of a current. The steel sheet thus plated is washed with water, and sent to the next steps such as reflowing and chemical treatments.

16

EXAMPLES

Tin-plating baths were prepared by the procedures as mentioned above, and steel sheets 0.22 mm thick for tinplate were plated with tin and the optimum current density range was measured. Measurements of the optimum current density range were made on corrosion-resistant tinplates and matte tinplates.

Samples of corrosion-resistant tinplates were prepared by plating steel sheets with 11.2 g/m 2 of Sn at various current densities, and subjected to melting treatment by electrical heating at a rate of 30 0 C/sec.

These samples were subjected to a K-plate adaptability test, a gloss appearance test and a corrosion resistance test.

The K-plate adaptability test was conducted by measuring an ATC current (alloy tin couple current), an ISV (iron solution value), the TC (tin crystal), described in ASTM A632, and judging whether or not the tin-plated steel sheets are adapted to K-plate.

The gloss appearance test was conducted by visually evaluating the appearance of the samples, and it was judged whether or not the samples had a particularly excellent brightness.

The corrosion resistance test was conducted by immersing the samples in 5% citric acid at 30 0 C for a month, and evaluating the corrosion resistance by visually judging the corrosion of the steel sheets.

The optimum current density range is defined as a current density range where samples satisfying the Kplate conditions in the K-plate adaptability test can be produced. In addition, a current density range where samples having a particularly excellent gloss can be produced is defined as a gloss optimum current density range, and a current density range where samples having a particularly excellent corrosion resistance can be produced is defined as a high corrosion resistance optimum current density range.

17 On the other hand, samples of matte tinplates were prepared by plating steel sheets with 2.8g/m 2 of Sn at various current densities.

These samples were subjected to a plate adhesion test, a gloss appearance test and a corrosion resistance test.

In the plate adhesion test, an adhesive tape was applied to a plated steel sheet, and peeled off the steel sheet. The plate adhesion was evaluated by visually judging the amount of Sn adhering to the tape.

The gloss test was conducted by visually evaluating the appearance of the samples, and judged whether or not the samples had a particularly excellent gloss.

The corrosion resistance test was conducted by immersing the samples in 5% citric acid at 30 0 C for a month, and evaluating the corrosion resistance by visually judging the corrosion of the steel sheets.

A current density range where samples having an excellent gloss can be produced is defined as a gloss optimum current density range and a current density range where samples having an excellent corrosion resistance can be produced is defined as a high corrosion resistance optimum current density range.

Table 1 shows the results of examples. In the table, phenolsulfonic acid was used as the base acid in Examples 1 to 36, and also in Comparative Examples 1 to 4.

I Methanesulfonic acid was used as the base acid in Examples 37 and 38. P-Alkanolsulfonic 2o acid was used as the base acid in Example 39. In addition, ENSA and EN in the table represent ethoxylated ax-naphtholsulfonic acid and ethoxylated a-naphthol, respectively.

As shown in Tables 1 and 2, although the various types optimum current density ranges were only about 20A/dm 2 in prior art as shown in Comparative Examples, the 2 various types of optimum current density ranges in the present invention were as wide as S 25 from lA/dm 2 to 300 to 500A/dm 2 Moreover, the current density range tended ooo* *ooo *o [/libz]00992:MEF 18 to be widened as the Sn concentration in a tin-plating bath increased, and as the relative speed difference between the steel sheet to be plated and the plating solution increased.

Accordingly, when a tinplate product of high quality is to be produced efficiently in a high speed tin-plating line at a line speed of 700 m/min), the prior art requires from 10 to 20 plating cells.

However, according to the present invention, the production can be performed with fewer plating cells (few or several), and therefore an economically extremely high efficiency can be achieved.

FIELD OF UTILIZATION IN INDUSTRY The present invention is useful for mass-producing tin-plated steel sheets (tinplate products).

19 Table 1-1 Examples and Comparative Examples of phenolsulfonic acid bath Tin-plating bath composition Bath Plat- Corrosion-resistant temp. ing tinplate (A/dm 2 solution flow rate Sn Base ENSA EN Fe (oC) O.C.D G.O.C H.C.R ions acid ions sec) .O.C.

D.R.

Ex. 1 42 138 0.00 0.00 0.00 50 5.5 1-300 non non Ex. 2 80 389 0.00 0.00 0.00 45 2.2 1-300 non non Ex. 3 65 22 0.00 0.00 0.01 30 15.7 1-400 non non Ex. 4 97 85 0.00 0.01 0.00 35 18.9 1-500 non non Ex. 5 55 289 0.00 0.00 0.07 60 8.7 1-300 non non Ex. 6 41 128 0.13 0.00 0.00 35 5.0 1-300 1-300 non Ex. 7 78 391 5.46 0.00 0.00 32 3.5 1-300 1-300 non Ex. 8 55 24 9.80 0.02 0.00 41 9.0 1-400 1-400 non Ex. 9 97 106 2.54 0.00 0.03 54 15.7 1-500 1-500 non 51 256 3.00 0.00 0.00 45 2.1 1-300 1-300 non Ex.11 45 80 0.00 9.94 0.08 41 12.9 1-300 1-300 non Ex.12 65 356 0.00 5.42 0.00 58 14.0 1-400 1-400 non Ex.13 78 321 0.00 0.16 0.01 60 6.0 1-400 1-400 non Ex.14 42 264 0.00 2.25 0.00 55 8.6 1-300 1-300 non 52 180 3.40 0.40 0.00 58 4.0 1-300 1-300 non Ex.16 63 76 2.80 5.90 0.02 47 2.3 1-300 1-300 non Ex.17 68 195 8.70 6.00 0.00 51 18.9 1-500 1-500 non Ex.18 42 138 0.00 0.10 8.40 50 5.5 1-300 non 1-300 Ex.19 80 389 0.03 0.00 12.60 45 2.2 1-300 non 1-300 65 22 0.00 0.04 6.00 30 15.7 1-500 non 1-500 Note: O.C.D.R. Optimum current density range G.O.C.D.R. Gloss optimum current density H.C.R.O.C.D.R. High corrosion resistance density range range optimum current 20 Table 1-2 Examples and Comparative Examples of phenolsulfonic acid bath Tin-plating bath composition Bath Plat- Matte tinplate temp. ing (A/dm 2 solution flow rate Sn Base ENSA EN Fe O.C.D G.O.C H.C.R ions acid ions sec) .O.C.

S# D.R.

Ex. 1 42 138 0.00 0.00 0.00 50 5.5 1-300 non non Ex. 2 80 389 0.00 0.00 0.00 45 2.2 1-300 non non Ex. 3 65 22 0.00 0.00 0.01 30 15.7 1-400 non non Ex. 4 97 85 0.00 0.01 0.00 35 18.9 1-500 non non Ex. 5 55 289 0.00 0.00 0.07 60 8.7 1-300 non non Ex. 6 41 128 0.13 0.00 0.00 35 5.0 1-300 1-300 non Ex. 7 78 391 5.46 0.00 0.00 32 3.5 1-300 1-300 non Ex. 8 55 24 9.80 0.02 0.00 41 9.0 1-400 1-400 non Ex. 9 97 106 2.54 0.00 0.03 54 15.7 1-500 1-500 non 51 256 3.00 0.00 0.00 45 2.1 1-300 1-300 non Ex.ll 45 80 0.00 9.94 0.08 41 12.9 1-300 1-300 non Ex.12 65 356 0.00 5.42 0.00 58 14.0 1-400 1-400 non Ex.13 78 321 0.00 0.16 0.01 60 6.0 1-400 1-400 non Ex.14 42 264 0.00 2.25 0.00 55 8.6 1-300 1-300 non 52 180 3.40 0.40 0.00 58 4.0 1-300 1-300 non Ex.16 63 76 2.80 5.90 0.02 47 2.3 1-300 1-300 non Ex.17 68 195 8.70 6.00 0.00 51 18.9 1-500 1-500 non Ex.18 42 138 0.00 0.10 8.40 50 5.5 1-300 non 1-300 Ex.19 80 389 0.03 0.00 12.60 45 2.2 1-300 non 1-300 65 22 0.00 0.04 6.00 30 15.7 1-500 non 1-500 Note: O.C.D.R. Optimum current density range G.O.C.D.R. Gloss optimum current density H.C.R.O.C.D.R. High corrosion resistance density range range optimum current 21 Table 1-3 Examples and Comparative Examples of phenolsulfonic acid -bath Tin-plating bath composition (g/l) Bath PlaLtemp. ing solution f low _____rate 0 (m/ sec) Corros ion-resistarit tinpia!te (A /dn 2 Sof Base ions I acid a a. a a a p p.

p 0 a a. a S a.

a Ex. 2 1 ET 23 Ex. 24 Ex. 2 5 Ex.26 Ex. 27 ET.28 77 3 0 31 Ex 33 Ex. 34 Ex.3 5 Ex. 376 Ex.38 Ex. 39 7. E.1I 7.E,.2 F. E. 3 C.E.4 97 41 78 __5 97 _5 78 42 52 6;3 68 68 49 80 63 45L 35 24 19 85 289 128 391 24 106 256 8 0 321 264 180 76 195 195 84 389 80 65 44 68J i

ENSA

0.0 0.00 0.13 5.46 9.80 2.54 3.00 0.-0 0 0.07 0.00 0.00 3.40 2.80 8.70 8.70 EN Fe ions 0.07 0.40 0.00 0. 13 0.00 3.45 0 .00 7.54 0. -08 6.12 0.00 0.86 0.00 14.70 9.94 11. 40 5.42 8. 59 0.16 2.45 2.25 7. 2 6 0.40 0.47 5.90 3.00 6.00 7.00 6.00 7.00 35 60o- 35 32 41 54 45 41 60 55 58 47 51 51 45- 45 47 41- 41 58 55 59 18.9 8.7 5. 0 3.5 9.0 15.7 2.1 12.9 14.0 6.0 8.6 4.0 2.3 18.9 1.8 1.0- 2.2 2.T 3 12.9 12.9 14.0 1.3 0.7

.R.

1-5 50 1-400 1-3 00 1-300 1- 400 1-500 1-3 00 1-300 1-500 1-400 1-300 1-300 1-300 1-500 10- 150 10-90 1-300 1-300 1T-3 00 100-o 130 on* 5-20 1-10 G.O.c

R,

non non0 1-300 1-300 1-400 1-500 1T-3 00 1-30 1-500 1-400 1-300 7-500 10- 150 10-90 non 1-300 1-300 1-To 130 non** 5-20 H. C. R .o.C.

D.R.'

1-550 1-400 1-300 1-3 00 1-400 1-500 1-300 1-300 1-500 1-400 1-300 1-300 1-300 1-500 150 10-90 non non non 100- 130 non** non 2.65 2.87 32 0.00 0.00 0.00 2.80 5.0 0.02 0.00 9.94 0.08 3.45 9.94 11.40 2.54 5.42 8.59 2.40 8.40 2.70 0.07 00 Note O.CD.R Otimm curentdenitynong Noe:O.C.D.R. =Ol ptimum current density range H.C.R.O.C.D.R_ High corrosion resistance optimum current density range *Plating operation became difficult.

22 Table 1-4 Examples and Comparative Examples of phenolsulfonic acid bath Tin-plating bath Compositio _(il) Sn Base i o sj Iac id- ENSAI 1 EN a a. a a a a 0 a a. a a .a a Ex. 21 EX. 22 Ex. .23 Ex. 24 Ex 25 Ex.2 6 Ex. 27 Ex. 28 Ex.2 9 Ex.3 0 Ex. 31 Ex. 32 Ex. 33 Ex.3 4 Ex. 3 5 Ex.3 6 Ex. 37 Ex.3 8 Ex. 39 C.E. 1 C. E. 2 C. E. 3 C. E. 4 L 7 ~78 51 45 65 78 42 52 63 68 68 49 80 6_3 45 35 875 289 128 391 24 106 256 80o 3 56 321 264 180 76 195 195 84 389 76 80 65 0.00 0.13 5.46 9.80 2 .54 3 .00 0 .00 0 .07 0.00 0.00 3.40 2.80 8.70 870 2.65 0.00 27.80 0.00 3.45 2 .54 2.40 0.07 0.07 0.00o C00 0.00 0.08 0.00 0.00 9.94 57.4 2 0.16 2.25 0.40 5.90 6.00 6.00 Fe ions 0.40 0_13 3 .45 7.54 6.12 0.86 14.7 0 11.40 8.59 2.45 7.26 0 .47 300 7.00 7 .00 Bath temp.

(IC)

35 60o 35 32 41 54 45 41 58 60 55 58 47 51 ing solution f low rate (m/ sec) 18.9 8.7 5 .0 3.5 9.0 15.7 2.1 12. 9 14.0 6. 0 8.6 4.0 2.3 18.9 1. 8 1.0 2.2 2.3 12.9 12.9 14. 0 1.3 077 Pa-1Matte tinplate 0. C. D 1-550 1-400 1-300 1-300 1-400 1-500 1-300 1-3 00 1-500 1-400 1-3 00 1-3 00 1-3 00 1-500 150 10-90 1-300 1-3 00 17-300 130 non* 5-20

D.R.

non 1-300 1-300 1-400 1-_5 0 0 1-300 1-300 1-500 1T-400 1-300 1-300 1-300 1-500 10-_ 150 10-90 n;on.

1-300 1-300 100- 130 non** 5-20 C. R D. R.

1-550 1-400 1-300 1-300 1-40-0 1-500 1-300 1-300 1-500 1-400 1-300 1-300 1-300 1-500 150 10-90 non non non 100- 130 non*n- 5720 non I(A/dn 2 2.87 3.26 45 0.00 0.00 45 59 0 0.02 47 9.94 0.08 41 994 11.40 41 42 8.59 58 8.40 2.70 []5 0.2 0.07 159 165 __17 24 44 19 68 Note: 11 non.-. piu cretdnst ag Noe O.C.D.R.=Gl Optimum current density range H.C.R.O.C.D.R High corrosion resistance Optimum current density range *Plating operation became difficult.

23 i Table 2-1 Examples and Comparative Examples of methanesulfonic acid bath Tin-plating bath composition Bath Plat- Corrosion-resistant temp. ing tinplate (A/dm 2 solution flow rate Sn Base Brigh- Anti- Fe (oC) O.C.D G.O.C H.C.R ions acid tener oxida ions sec) .O.C.

nt

D.R.

Ex. 1 45 125 0.00 0.00 0.00 45 4.5 5-270 non non Ex. 2 75 389 0.00 0.00 4.50 45 2.3 5-270 non non Ex. 3 60 22 0.00 0.00 0.01 30 15.1 5-270 non non Ex. 4 98 85 3.00 0.02 0.00 34 18.8 5-450 non non Ex. 5 42 289 0.3 9.21 0.07 65 8.5 5-270 non non Ex. 6 41 128 8.99 5.22 8.20 35 4.9 5-300 5-300 non Ex. 7 85 391 5.62 1.26 0.00 32 3.7 5-300 5-300 non C.E.1 34 66 3.85 9.84 0.00 45 11.9 100- 100- 100- 120 120 C.E.2 64 15 2.59 5.62 9.49 55 14.0 non** non** non** C.E.3 22 44 3.40 7.40 2.50 55 1.6 10-20 10-20 10-20 C.E.4 18 66 0.06 0.1 0.06 55 0.4 3-10 non non Note: O.C.D.R. Optimum current density range G.O.C.D.R. Gloss optimum current density range H.C.R.O.C.D.R. High corrosion resistance optimum current density range Plating operation became difficult to practice.

24 Table 2-2 Examples and Comparative Examples of methanesulfonic acid bath Tin-plating bath composition Bath Plat- Matte tinplate temp. ing (A/dm 2 solution flow rate Sn Base Brigh- Anti- Fe O.C.D G.O.C H.C.R ions acid tener oxida ions sec) .O.C.

nt

D.R.

Ex. 1 45 125 0.00 0.00 0.00 45 4.5 5-270 non non Ex. 2 75 389 0.00 0.00 4.50 45 2.3 5-270 non non Ex. 3 60 22 0.00 0.00 0.01 30 15.1 5-270 non non Ex. 4 98 85 3.00 0.02 0.00 34 18.8 5-450 non non Ex. 5 42 289 0.3 9.21 0.07 65 8.5 5-270 non non Ex. 6 41 128 8.99 5.22 8.20 35 4.9 5-300 5-300 non Ex. 7 85 391 5.62 1.26 0.00 32 3.7 5-300 5-300 non C.E.1 34 66 3.85 9.84 0.00 45 11.9 100- 100- 100- 120 120 120 C.E.2 64 15 2.59 5.62 9.49 55 14.0 non** non** non** C.E.3 22 44 3.40 7.40 2.50 55 1.6 10-20 10-20 10-20 C.E.4 18 66 0.06 0.1 0.06 55 0.4 3-10 non non Note: O.C.D.R. Optimum current density range G.O.C.D.R. Gloss optimum current density range H.C.R.O.C.D.R. High corrosion resistance optimum current density range Plating operation became difficult to practice.

Claims (22)

1. A method for continuously plating tin on a steel sheet, comprising: providing a tin-plating bath containing 40 to 100g/L of Sn ions and 20 to 400g/L of base acid; feeding a first steel sheet having first and second ends in the length direction of the sheet from the first to second ends thereof into said tin-plating bath at a first feeding speed with the relative speed difference between the steel sheet and the plating solution being held at 2 to 20m/sec, to plate tin on the surface of the first steel sheet with a first optimum current density; joining said second end of said first steel sheet with the first end of a second steel sheet having first and second ends in the length direction of the sheet outside said tin-plating bath while said first o steel sheet is being fed into said tin-plating bath at a second feeding speed with the relative speed difference between the steel sheet and the plating solution being held at 2 to 20m/sec, to plate tin on the surface of the first steel sheet at a second optimum current density, the difference between said first and second optimum current densities being at least 80A/dm 2 and feeding said second steel sheet from the first to second ends thereof into said tin-plating bath at 15 a third feed speed with the relative speed difference between the steel sheet and the plating solution being held at 2 to 20m/sec to plate tin on the surface of the second steel sheet at a third optimum current density.

A method for tin-plating according to claim 1, wherein the plating bath comprises 40 to 100g/L of Sn ions and 20 to 400g/L of phenolsulfonic acid. .o 2

3. A method for tin-plating according to claim 1, wherein the plating bath comprises 40 to d 100g/L of Sn ions and 20 to 400g/L of phenolsulfonic acid and further a brightener and/or an antioxidant.

4. A method for tin-plating according to claim 3, wherein the plating bath comprises, as the brightener, 0.1 to 10g/L of ethoxylated cx-naphtholsulfonic acid and/or 0.1 to 10g/L of ethoxylated 25 ~-naphthol.

A method for tin-plating according to any one of claims 2-4, wherein the plating bath further comprises 0.1 to 15g/L of Fe ions.

6. A method for tin-plating according to claim 1, wherein the plating bath comprises 40 to 100g/L of Sn ions and 20 to 400g/L of methanesulfonic acid. o

7. A method for tin-plating according to claim 1, wherein the plating bath comprises 40 to 100g/L of Sn ions and 20 to 400g/L of methanesulfonic acid, and further 0.1 to 10g/L of a brightener and/or 0.1 to 10g/L of an antioxidant.

8. A method for tin-plating according to claim 1, wherein the plating bath comprises 40 to 100g/L of Sn ions and 40 to 300g/L of p-alkanolsulfonic acid which has a hydroxyl group at the 3s 3-position. [R\I.IBZ]05139 doc:SAK 26

9. A method for tin-plating according to claim 8, wherein the p-alkanolsulfonic acid is 2-hydroxyethane-l-sulfonic acid.

A method according to claim 8 or claim 9 wherein the plating bath further comprises a brightener.

11. A method for tin-plating according to any one of claims 1-10, wherein the variation width of the optimum current density is at least 250Aldm 2

12. A method for tin-plating according to claim 11, wherein the variation width of the optimum current density is at least 350A/dm 2

13. A tin-plating bath comprising 40 to 100g/L of Sn ions and 20 to 400g/L of phenolsulfonic io acid.

14. A tin-plating bath according to claim 13, further comprising a brightener and/or an antioxidant.

The tin-plating bath according to claim 14, wherein the tin-plating bath comprises as the brightener 0.1 to 10g/L of ethoxylated a-naphtholsulfonic acid and/or 0.1 to 10g/L of ethoxylated 15 a-naphthol.

16. The tin-plating bath according to any one of claims 13 to 15 further comprising 0.1 to 15g/L of Fe ions.

17. A tin-plating bath comprising 40 to 100g/L of Sn ions and 20 to 400g/L of methanesulfonic acid. 20

18. A tin-plating bath according to claim 17, wherein the tin-plating bath comprises 40 to 6 100g/L of Sn ions and 20 to 400g/L of methanesulfonic acid, and further 0.1 to 10g/L of a brightener and/or 0.1 to 10g/L of an antioxidant.

19. A tin-plating bath comprising 40 to 100g/L of Sn ions, and 40 to 300g/L of P-alkanolsulfonic acid having a hydroxyl group at the 3-position. 25

20. A tin-plating bath according to claim 20 wherein the p-alkanolsulfonic acid is 2-hydroxyethane-l-sulfonic acid.

21. A method for tin-plating, substantially as hereinbefore described with reference to any one of the examples but excluding the comparative examples.

22. A tin-plating bath, substantially as hereinbefore described with reference to any one of 1o the examples but excluding the comparative examples. Dated 4 February, 2000 Nippon Steel Corporation Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON [R:\LIBZ]05139,doc:SAK