Surface-Treated Steel Sheet, Organic Resin Coated Metal Container, and Method for Producing Surface-Treated Steel Sheet

Abstract

Provided is a surface-treated steel sheet with a compound layer containing F and composed essentially or Zr at least on one surface thereof, wherein the Zr amount is 80 to 350 mg/m.sup.2 and the F amount is 0.5 to 10 mg/m.sup.2 within the layer, and an organic resin coated metal container manufactured using the surface-treated steel sheet. The surface-treated steel sheet of the present invention is manufactured through forming a layer having the Zr amount of 80 to 350 mg/m.sup.2 at least on one surface of a steel sheet by cathode electrolytic treatment in an aqueous solution containing a Zr ion and F ion, and subsequently adjusting the surface to control the F amount to 0.5 to 10 mg/m.sup.2 by one or more treatments selected from immersion and spraying with an ion-containing aqueous solution and cathode electrolytic treatment in the ion-containing aqueous solution.

Claims

1. A method for producing a surface-treated steel sheet having a compound layer containing F and composed essentially of Zr at least on one surface of a steel sheet, the method comprising: forming a layer where a Zr amount within the layer is 80 to 350 mg/m.sup.2 by performing a cathode electrolytic treatment to the steel sheet in an aqueous solution containing a Zr ion and F ion; and adjusting a surface to control the F amount within the layer to 0.5 to 10 mg/m.sup.2 by performing any one or more treatments selected from immersion to an ion-containing aqueous solution, spraying of the ion-containing aqueous solution, and cathode electrolytic treatment in the ion-containing aqueous solution.

2. The method for producing a surface-treated steel sheet according to claim 1, wherein the ion-containing aqueous solution in adjusting the surface is an alkaline aqueous solution containing one or more types of ions selected from a sodium ion, ammonium ion, and potassium ion.

3. The method for producing a surface-treated steel sheet according to claim 1, wherein a pH of the ion-containing aqueous solution in adjusting the surface is 9 or more.

4. A method for producing a surface-treated steel sheet for forming a compound layer containing F and composed essentially of Zr at least on one surface of a steel sheet, the method comprising: forming a layer where a Zr amount within the layer is 80 to 350 mg/m.sup.2 by performing a cathode electrolytic treatment to the steel sheet in an aqueous solution containing a Zr ion and F ion; and adjusting the surface to control a F amount within the layer to 0.5 to 10 mg/m.sup.2 by performing spraying and/or immersion with water of 90° C. or more to the steel sheet.

Description

EXAMPLES

[0080] Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples. A treating material, a degreasing agent, and an organic resin layer used in the examples are arbitrarily selected from those available on the market, and they are not intended to limit the method for producing the surface-treated steel sheet of the present invention.

[0081] Further, the method for producing a surface-treated sheet and the evaluation method of each characteristic were as follows.

<Step of Forming a Layer>

[0082] As for the base sheet, a low-carbon steel sheet having a thickness of 0.225 mm and a width of 200 mm was used. Then, the steel sheet was subjected to alkaline electrolytic degreasing as the pretreatment and then pickling was performed by sulfuric acid immersion. Next, the steel sheet was immersed into an electrolytic treatment liquid and subjected to a cathode electrolytic treatment to form a compound layer containing F and composed essentially of Zr to both steel sheet surfaces. The steel sheet was then squeezed with rolls, washed with water, and further the wash water was squeezed out.

[0083] As for the electrolytic treatment liquid, an aqueous solution, of a composition where the Zr concentration is 6000 ppm and the F concentration is 7000 ppm prepared by dissolving ammonium zirconium fluoride as a Zr compound was used.

[0084] pH of electrolytic treatment liquid: 3.0 (pH adjustment was performed with nitric acid and/or ammonia)

[0085] Temperature of electrolytic treatment liquid: 40° C.

[0086] Current density during cathode electrolytic treatment: 10 A/dm.sup.2

[0087] Energization method during cathode electrolytic treatment: A cycle of “0.15-second energization and 0.1-second stop of energization” was performed for several times (hereafter called as “the number of cycles”).

<Step of Adjusting the Surface>

[0088] The steel sheets obtained after following the step of forming a layer, were processed in an ion-containing aqueous solution for a predetermined time, then the steel sheets were squeezed with rolls, washed with water, further squeezed with rolls, and dried with hot air.

[0089] Among these, some steel sheets were further subjected to warm water washing by immersion or spraying after immersion with warm water of 40° C. or more after having processed with the ion-containing aqueous solution and then dried with hot air.

[0090] Further, for other steel sheets, treatment with the ion-containing aqueous solution was omitted and a process of warm water washing, where immersion or spraying after immersion with warm water of 90 to 95° C., was performed. Subsequently, the warm water was squeezed out from the steel sheet with rolls and then the steel sheet was dried with hot air.

<Measurement of the Zr Amount>

[0091] For the surface-treated sheet obtained in each example and comparative example, the Zr amount contained in the metal compound layer was measured using an X-ray fluorescence spectrometer (available from Rigaku Corporation, model number: ZSX100e).

<Measurement of the F Amount>

[0092] In a fluorescence X-ray analysis, there is a limit in microanalysis of the F amount in the light of quantitative accuracy. Particularly, it is difficult to directly quantify F from a surface-treated sheet where the F amount is 1.5 mg/m.sup.2 or less. After various investigations, we have selected the following measurement method. That is, using a special cell which can be subjected to retort pressurization, retort treatment was performed for 30 minutes at 130° C. under a condition where a surface-treated sheet with a certain area is in contact with a certain amount of ultrapure water. The fluorine ions extracted to the ultrapure water by the treatment was measured with an ion chromatograph (available from Dionex, DX-320). From the obtained F concentration, the weight of F in the ultrapure water was determined and the value was converted into the weight of F per unit area of the surface-treated-sheet and the value was defined as the F amount within the layer.

<Evaluation of Adhesiveness for a Can (Inner-Bottom Part), Cross-Cut Resistance, and Resistance to F Elution>

1. Manufacturing of an Organic-Resin-Coated Surface-Treated Steel Sheet

[0093] To the obtained surface-treated steel sheet, a 19 μm-thick stretched film including polyethylene terephthalate/isophthalate copolymerized composition, wherein 11 mol. % of isophthalic acid component is contained, was bonded by thermocompression via laminating rolls to one surface of the metal sheet that becomes the inner can-surface, and a 13 μm-thick stretched film including polyethylene terephthalate/isophthalate copolymerized composition, where 12 mol. % of isophthalic acid component is contained, and colored to white by containing titanium oxide, to the other surface that becomes the outer can-surface. Then, the obtained steel sheet was immediately cooled with water to obtain an organic-resin-coated surface-treated steel sheet while paying attention to maintain a certain amount of alignment to the film. The manufactured organic-resin-coated surface-treated steel sheet was used in manufacture of a metal can except that a part thereof was used for cross-cut evaluation.

2. Evaluation of Cross-Cut Resistance

[0094] To the part of the produced organic-resin-coated surface-treated sheet that corresponds to the inner can-surface, a cross-cut scratch of 4 cm in length that reaches up to the base was made with a cutter. Then, the scratched steel sheet was immersed into a model liquid (an aqueous solution wherein the weight concentration is 1.5% for both sodium chloride and citric acid) for one week at 37° C. and the corrosion state was evaluated. Next, the test piece was taken out from the model liquid and the state of peeling in the organic resin layer at the cross-cut section and its surroundings, and the state of color changes due to generation of corrosion products were observed and evaluated by sight. For the surroundings of the cross-cut section, “C” is indicated for those with a color-change width or a maximum film peeling width of 2 mm or more in one side, “B” for those with 1 mm or more and less than 2 mm, and “A” for those with less than 1 mm.

3. Manufacturing of a Metal Can

[0095] To both surfaces of the obtained organic-resin-coated surface-treated sheet, a paraffin wax was applied by electrostatic oiling, punched out to a 143 mm-diameter disk shape, and a drawn cup having a diameter of 91 mm and a height of 36 mm was manufactured according to a conventional method. Following this, a simultaneous drawing and ironing process was repeated twice to fabricate a cup having a small diameter and tall height. The characteristics of the cup thus obtained were as follows.

[0096] Cup diameter 52.0 mm

[0097] Cup height 111.7 mm

[0098] Thickness decrease rate of can wall with respect to the original sheet 30%

[0099] After dome formation, the cup was subjected to heat treatment for 60 seconds at 220° C. to eliminate resin film distortion, which is then followed by a trimming process of the open end edge, printing to the curved surface, a process of neck-in to a diameter of 50.8 mm, and a flanging process to manufacture a 200 g seamless can.

4. Evaluation of Adhesiveness in Inner Can-Surface

[0100] Using a manufactured can, retort treatment was performed for 30 minutes at 125° C. after filling up with distilled water based on a usual method. Then, its content was removed after removing the lid from the canbody, and cut in half on the line of 45 degrees to the rolling direction of the surface-treated sheet. Next, the can cut in half was immersed to a liquid prepared by adding 0.02% by weight of surfactant to 1% by weight of sodium chloride aqueous solution for one hour. After the immersion, the can was cut further into half from the can bottom side on the line of 135 degrees to the rolling direction and adhesiveness was evaluated by observing the state of peeling in the cut surface of the inner can-surface bottom radius section which was cut last. “C” is indicated for those found with a peeling around the cut surface, “B” for those with slight peeling when the cut section was touched with a needle with a sharp end, and “A” for those where no peeling was found.

5. Evaluation of Resistance to F Elution

[0101] The manufactured can was filled with 183 g of ultrapure water and subjected to retort treatment for 30 minutes at 130° C. Then, measurement for fluorine ions extracted into the ultrapure water was performed with an ion chromatograph (available from Dionex, DX-320). When F is detected, “C” was indicated, and when F was at the detection limit (0.1 ppm) or less, “B” was indicated.

Example 1

[0102] In the step of forming a layer, cathode electrolytic treatment was performed for 7 cycles to the steel sheet surface and the electrolytic treatment liquid was squeezed out. Then, the steel sheet was washed with water at the room temperature and further, the wash water was squeezed out with rolls. Next, in the step of adjusting the surface, the steel sheet was immersed into a mixed aqueous solution at 40° C. for one second, wherein sodium carbonate and sodium bicarbonate were mixed, and the pH was adjusted to 9.5. Following this, the steel sheet was further immersed into hot water of 95° C. for one second, washed with water after the aqueous solution is squeezing out with rolls, and dried after further squeezing out the wash water with rolls to obtain a surface-treated steel sheet.

[0103] Next, the Zr amount and F amount of the sheet after the step of forming a layer but before the step of adjusting the surface, and the surface-treated steel sheet after the step of adjusting the surface were measured in accordance with the method described above. The results are shown in Table 1. Nevertheless, the Zr amount of the layer after the step of adjusting the surface was almost the same as the Zr amount after the step of forming a layer, thus it was omitted.

[0104] Using the produced organic-resin-coated surface-treated steel sheet and metal can, cross-cut resistance, adhesiveness at the inner can-bottom part, and resistance to F elution were evaluated. The results of the performance evaluation are shown in Table 1.

[0105] In Table 1 and in Table 2 described later, the ion-containing aqueous solution includes both cases where it was prepared by adding a chemical agent accordingly to obtain the target pH and where it was prepared while determining the concentration of the chemical agent in advance. Only for the latter case, the concentration of the chemical agent is indicated. Further, for the pH value, the one measured at 25° C. is used. More, as for Table 1 and Table 2, “-” mark is shown in the table for those cases where treatment with ion-containing aqueous solution was not performed, and where warm water washing with water at 40° C., or more is not performed.

Examples 2 to 22

[0106] As in Example 1, conditions and the plating amount (the Zr amount and F amount) in the step of forming a layer, conditions used in the step of adjusting the surface and the F amount of the layer after having gone through the step, and the results of the performance evaluation for the organic-resin-coated surface-treated sheet and metal can are shown in Table 1. However, in Examples 3 to 11, Example 21, and Example 22, warm water washing was performed by immersion for the first half of the treatment time shown in Table 1 and spraying for the second half. As for Examples 14 to 16, in the step of adjusting the surface, surface adjustment was performed, while using the steel sheet as the cathode, by repeating the cycle of “0.15-second energization and 0.1-second stop of energization” twice in the ion-containing aqueous solution at a current density of 10 A/dm.sup.2.

TABLE-US-00001 TABLE 1 Step of adjusting the surface Step of forming a layer Amount Amount coated coated on on surface- Warm water surface- treated sheet Treatment with ion-containing washing treated Performance evaluation Number Zr F aqueous solution treatment sheet Inner can- Resistance of amount amount Chemical Temp. Time Temp. Time F amount Cross-cut surface to F cycles mg/m.sup.2 mg/m.sup.2 agent pH ° C. Second ° C. Second mg/m.sup.2 resistance adhesiveness elution Example 1 7 94 12 Na.sub.2CO.sub.3, 9.5 40 1 95 1 4.2 A A B NaHCO.sub.3 Example 2 6 80 10 Na.sub.2CO.sub.3, 10.2 40 1 — 5.5 B B B NaHCO.sub.3 Example 3 12 167 26 Na.sub.2CO.sub.3, 10.5 40 1 95 2 4.1 A A B NaHCO.sub.3 Example 4 7 95 12 Na.sub.2CO.sub.3, 10.8 40 1 95 2 0.55 B A B NaOH Example 5 11 144 21 Na.sub.2CO.sub.3, 10.8 40 1.5 95 3 1.5 A A B NaOH Example 6 8 121 17 Na.sub.2CO.sub.3, 10.8 40 1 95 2 1.1 A A B NaOH Example 7 7 94 12 Na.sub.2CO.sub.3, 10.8 40 1 40 2 2.8 A A B NaOH Example 8 12 164 24 Na.sub.2CO.sub.3, 11.3 40 1 40 2 8.5 A A B NaOH Example 9 12 165 24 Na.sub.2CO.sub.3, 11.3 40 1 65 7 0.8 A A B NaOH Example 12 166 25 Na.sub.2CO.sub.3, 11.3 40 1 95 7 3.9 A A B 10 NaOH Example 12 160 22 — 90 4 9.8 A A B 11 Example 14 216 33 Na.sub.2CO.sub.3, 11.2 45 5 — 1.7 A A B 12 0.08 mol./lit. Example 24 326 52 Na.sub.2CO.sub.3, 11.2 45 5 — 4.2 A A B 13 0.08 mol./lit. Example 8 104 14 Na.sub.2CO.sub.3, 11.2 45 0.3 sec. by — 0.7 B A B 14 0.08 electrolysis mol./lit. Example 14 203 31 Na.sub.2CO.sub.3, 11.2 45 0.3 sec. by — 1.2 A A B 15 0.08 electrolysis mol./lit. Example 24 345 55 Na.sub.2CO.sub.3, 11.2 45 0.3 sec. by — 1.7 A A B 16 0.08 electrolysis mol./lit. Example 8 115 16 Na.sub.2CO.sub.3, 10.0 30 1 — 5.3 A A B 17 HNO.sub.3 Example 12 168 25 Na.sub.2CO.sub.3, 10.0 30 2 — 7.5 A A B 18 HNO.sub.3 Example 12 152 22 Na.sub.2CO.sub.3, 9.7 30 2 — 8.2 A A B 19 HNO.sub.3 Example 12 110 15 NH.sub.4OH, 9.2 60 2 — 9.8 A A B 20 NH.sub.4Cl Example 12 155 23 NaOH, 12 60 1 60 2 3.2 A A B 21 Na.sub.2HPO.sub.4 Example 8 103 14 NaOH 10.8 60 2 40 2 6.4 A A B 22 0.001 mol./lit.

Comparative Examples 1 to 10

[0107] As in Example 1, conditions and the plating amount (the Zr amount and F amount) for the step of forming a layer, conditions used in the step of adjusting the surface and the F amount 670 of the layer after having gone through the step, and the results of the performance evaluation for the organic-resin-coated surface-treated sheet and metal can are shown in Table 2. However, in Comparative Examples 6 to 8 and Comparative Example 10, warm water washing was performed by immersion for the first half of the treatment time shown in Table 2 and spraying for the second half. As for Comparative Example 9, in the step of adjusting the surface, surface adjustment was 675 performed, while using the steel sheet as the cathode, by repeating the cycle of “0.15-second energization and 0.1-second stop of energization” for 4 times in the ion-containing aqueous solution at a current density of 10 A/dm.sup.2.

TABLE-US-00002 TABLE 2 Step of adjusting the surface Step of forming a layer Amount Amount coated coated on surface- Warm water on treated sheet Treatment with ion-containing washing surface- Performance evaluation Number Zr F aqueous solution treatment F Inner can- Resistance of amount amount Chemical Temp. Time Temp. Time amount Cross-cut surface to F cycles mg/m.sup.2 mg/m.sup.2 agent pH ° C. Second ° C. Second mg/m.sup.2 resistance adhesiveness elution Comparative 2 43 3.1 — — 3.1 C C B Example 1 Comparative 4 69 9 — — 9 C C B Example 2 Comparative 8 110 15 — — 15 B C C Example 3 Comparative 14 194 29 — — 29 B C C Example 4 Comparative 24 331 37 — — 37 B C C Example 5 Comparative 12 160 22 — 90 1 17 B C C Example 6 Comparative 12 160 22 — 90 2 14 B B C Example 7 Comparative 12 160 22 — 90 3 12 B B C Example 8 Comparative 8 104 14 Na.sub.2CO.sub.3, 11 45 0.6 sec. by — — 0.41 C B B Example 9 0.08 electrolysis mol./lit. Comparative 8 105 14 NaOH 13.5 65 2 95 2 0.08 C B B Example 10

[0108] As shown in Table 1, by making the Zr amount within the layer 80 to 350 mg/m.sup.2 in the step of forming a layer, and by treating the steel sheet after the step of forming a layer in the ion-containing aqueous solution or hot water of 90° C. for a predetermined time in the step of adjusting the surface, a steel sheet with the F amount in the layer of 0.5 mg/m.sup.2 to 10 mg/m.sup.2 was manufactured. In Examples 1 to 22, not only the obtained organic-resin-coated metal sheet was excellent in cross-cut resistance, but also adhesiveness of the inner metal-can surface, resistance to F elution, and adhesiveness of the organic resin layer were excellent. Further, even after a fabricating process and retort treatment, or even when there is a crack in the resin layer, the organic resin layer exhibited excellent adhesiveness, thus it was confirmed that the container was excellent in maintaining the content quality.

[0109] On the other hand, as shown in Table 2, after having gone through the step of forming a layer, the cross-cut resistance of the organic-resin-coated metal sheet was unsatisfactory in Comparative Examples 1 and 2 where the Zr amount was less than 80 mg/m.sup.2. As for the can performance, although the can had resistance to F elution, it was confirmed that adhesiveness of the inner surface was insufficient. Though not shown in the examples, under a condition where the Zr amount was small, it was confirmed that the above performance does not improve even when the step of adjusting the surface is followed. Meanwhile, in Comparative Examples 3 to 5, where the Zr amount was 80 mg/m.sup.2 or more and where the step of adjusting the surface was not followed, the cross-cut resistance of the organic-resin-coated metal sheet improved but resistance to F elution decreased. Also, improvement was not confirmed even in Comparative Example 6, where the step of adjusting the surface was performed for one second with hot water of 90° C. Further, in Comparative Examples 7 and 8, where the step of adjusting the surface was performed for 2 to 3 seconds in hot water of 90° C., adhesiveness of the inner can-surface improved but improvement for F elution was unsatisfactory. In contrast, in Comparative Examples 9 and 10, where electrolytic treatment was excessively performed in the ion-containing aqueous solution in the step of adjusting the surface or where excess treatment was performed using high-concentration alkali, the F amount within the layer became less than 0.5 mg/m.sup.2. Accordingly, it was confirmed that, although adhesiveness of the inner can-surface and resistance to F elution were improved, cross-cut resistance was reduced.

[0110] Further, although the F amount within the layer is about 0.4 mg/m.sup.2 in the electrolytic chromic acid treated steel sheet, the F amount within the layer in each example was 0.5 mg/m.sup.2 or more even after the step of adjusting the surface. Accordingly, as for the surface-treated steel sheet, it has become apparent that in the case of a steel sheet with a surface-treated layer composed of a compound composed essentially of Zr, the F amount needs to be larger than that in the electrolytic chromic acid treated steel sheet.