JPH0121234B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an electrically coated Zn or Zn-based alloy plated steel sheet. In general, when actually using steel sheets for automobiles, there are cases where they are used as is after undergoing processing such as press forming, and cases where they are subjected to phosphate treatment and then painted before being used for the final purpose. In the former case, bare corrosion resistance is an extremely important issue, while in the latter case, phosphate treatment properties and corrosion resistance after painting are extremely important issues. An object of the present invention is to provide a surface-treated steel sheet that is particularly excellent in the latter case. As a recent trend, for example, in regions with heavy snowfall such as North America and Canada, rock salt has been sprinkled on roads to melt snow, but as a result, car bodies are exposed to an extremely corrosive environment. Summer. In contrast, electrogalvanized and hot-dip galvanized steel sheets have been mainly used as materials that are resistant to such corrosive environments, but the corrosion resistance after painting is not always sufficient, and Various safety problems have arisen, including severe corrosion and holes. Under these circumstances, the development of surface-treated steel sheets with excellent corrosion resistance after painting that can withstand salt damage is strongly discouraged, and each steel manufacturer has made great efforts to develop many new surface-treated steel sheets. It is commercially available. For example, these include Zn-Ni, Zn
-Ni-Co series, Fe-Zn series, Fe-Zn-Ni series, Zn-
There are various surface-treated steel sheets, such as electroplated steel sheets such as Al-based, Zn-Mn-based, and Zn-Ti-based steel sheets, zinc chromate steel sheets, and steel sheets with chromate treatment applied thereon. When a surface-treated steel sheet is used as a steel sheet for automobiles, it goes through the steps of press forming, degreasing, phosphate treatment, and painting, but even in the above-mentioned new surface-treated steel sheets, the corrosion resistance after painting is often insufficient. Among them,
Although all types of alloy electroplated steel sheets have considerably improved bare corrosion resistance, in most cases phosphatability and post-coating corrosion resistance are often poor. As a method for improving phosphate treatment properties, a method of spraying a sodium phosphate suspension before phosphate treatment has been known. Furthermore, a method of adding heavy metal salts to increase the reactivity of the phosphate treatment solution itself is also known. However, for example, the method of spraying a suspension onto a molded product after processing requires an additional step to be added to the series of phosphate treatment steps, which is a heavy burden in terms of equipment and cost, and does not require the specifications of the existing line. In some cases it may not be possible. The method of adding heavy metal salts to the treatment liquid itself may cause excessive reaction in some areas when treating articles made of parts with various surface textures, such as automobiles, as it also accelerates reactions in other parts. There is a concern that film deposition may occur, and the cost of the chemical conversion treatment solution itself increases considerably. Although all of the above measures improve the phosphate treatment properties of the surface-treated steel sheet to some extent, they are not particularly excellent and cannot be said to be sufficient. On the other hand, as a result of various studies, the present inventors have discovered a technique that can be easily and reliably applied to the manufacturing process of electrically coated Zn or Zn-based alloy plated steel sheets, and that has an extremely large processing effect. It has been found that this method can significantly improve the phosphate treatment properties and post-painting corrosion resistance of general electrical Zn or Zn-based alloy coated steel sheets. The present invention will be explained in detail below. As a result of various studies, the inventors of the present invention found that electric Zn or
It was discovered that the interfaces of Zn-based alloy coated steel sheets are all in a special state, and the internal plating components do not match. Figure 1 shows the results of Auger measurements of how each component is distributed from the interface to the interior of a Zn-Ni alloy coated steel sheet. The horizontal axis shows the distance from the plating surface Ar-Sputtering Time (×10 ² scc), and the horizontal axis shows the contained element concentration Cx (at ratio). Although the components are constant inside the plated steel sheet, Zn is rich near the interface, O shows a fairly high peak, and the Ni concentration decreases rapidly. This phenomenon occurs for the following reasons. Generally, a vertical cell or a horizontal cell is used as a plating cell for producing electroplated steel sheets. When applying Zn--Ni alloy plating using the well-known vertical cell 4 shown in FIG. ,
There is a region where a weak current flows (hereinafter referred to as the weak current region) (dotted line range in the figure) between the steel strip and the surplus plating solution generated by pumping out the plating solution heated outside the plating bath 3. . The length of the weak current area is from the bath surface to the notification roll 5.
This corresponds to the point of contact between the plate and the presser roll 6, that is, the range where excess plating liquid exists on the steel strip. In this weak current region, like the normal plating region mentioned above, the current density is outside the optimum current density range, so elements that are easy to electrodeposit in that current density region are preferentially electrodeposited, and the outermost layer has a balance of alloy components. My body collapses. In the case of Zn-Ni alloy plating, Zn is preferentially electrodeposited, and Ni is not very much electrodeposited. On the other hand, in electrodeposition in such a weak current region, elements are generally electrodeposited not in a completely crystalline state but in an amorphous or semi-amorphous state. These electrodeposited materials in an amorphous or semi-amorphous state tend to become oxides or hydroxides because they are active. That is, when a cross section of a plating layer of an electroplated steel sheet is schematically shown in FIG. This oxide layer 9 is covered with an amorphous or semi-amorphous substance such as an oxide or hydroxide of a specific element. The above phenomenon has been explained for Zn-Ni alloy plated steel sheets, but it is similar for other Zn-based alloy coated steel sheets, such as Fe-Zn system, Fe-Zn-Ni system, Fe-Zn system.
-Ni-Co series, Zn-Mn series, Zn-Al series, Zn-Ni-
Even in Co-based, Zn-Ni-Cr-based alloy-plated steel sheets, or Zn-plated steel sheets, the interface is covered with amorphous or semi-amorphous oxides and hydroxides of Zn. In the case of alloy plating, as described above, specific elements that are easy to electrodeposit in a weak current region are preferentially electrodeposited, and are covered with amorphous or semi-amorphous oxides or hydroxides of these elements. In any case, when producing electrical Zn or Zn-based alloy plated steel sheets, the interface is more or less covered with amorphous or semi-amorphous oxides or monohydroxides of specific elements. As a result of various studies, the present inventors have found that these amorphous or semi-amorphous oxides and hydroxides significantly reduce the surface properties of galvanized steel sheets, such as their phosphate treatment properties and paint corrosion resistance. I found it. Furthermore, as a result of many studies, the present inventors found that
For any type of galvanized steel sheet, the interface of the electroplated steel sheet can be easily modified by performing the treatments shown below. It was found that evaluation values such as cross-cut long-term atmospheric exposure, cross-cut salt water spray atmospheric exposure, post-processing cross-cut salt spray test, and water-resistant adhesion (secondary adhesion) can be significantly improved. That is, the feature of the present invention is that in the production of electrical Zn or Zn-based alloy plated steel sheets, after electroplating, C treatment (cathode electrolytic treatment) in a phosphoric acid compound bath,
C-A treatment (cathode-anode electrolysis treatment), A
- By performing C treatment (anode-cathode electrolysis treatment), A treatment (anode electrolysis treatment), or immersion treatment, amorphous or semi-amorphous oxides at the interface,
Modify or remove hydroxide by reduction or dissolution,
Its purpose is to improve surface properties. For example, in a Zn-Ni alloy plated steel plate,
When alloy electroplating was carried out so that the basis weight was 20 g/m ² and the Ni% was 13%, the distribution of elements at the plating interface was approximately as shown in FIG.
If this is directly treated with phosphate, the phosphate crystals will be small and will not grow to an appropriate size. Furthermore, after the ED coating, an intermediate coat and a top coat were applied, and a paint corrosion resistance test was conducted. In the cross-cut salt spray test, after two weeks (14 days), paint film blisters were observed in a width of 2 to 3 mm on one side, mainly at the cross-cut portion. After 6 months of long-term exposure of the crosscut to the atmosphere, blistering of the paint film was observed in a width of 2 to 3 mm on one side centering on the crosscut. In cross-cut salt water spray atmospheric exposure (spraying 3% NaCl solution once daily), 6
After several months, blistering of the coating film was observed in a width of 4 to 6 mm on one side centering on the cross-cut area. In the post-processing cross-cut salt water spray test, two weeks later, paint film blistering was observed in a width of 3 to 4 mm on one side centering on the cross-cut area in the processed area. Water resistant adhesion (peel off immediately after soaking at 50℃ for 10 days)
almost completely peeled off, and the rating was 1 point (out of 10, 10
(highest score). On the other hand, as shown in FIG. 4, the same alloy-plated steel sheet is provided with a treatment layer 10 following the plating cell 4, and this treatment layer is coated with NaH ₂ PO ₄ .2H ₂ O with a pH of 4 to 5.
Cathode electrolytic treatment was performed at Dk = 10 A/dm ² ×1.5 sec in a bath with a concentration of 50 g _/ NaH ₂ PO ₄ .2H 2 O. The distribution of elements in the depth direction of the plating layer in these tests is shown in FIG. No amorphous or semi-amorphous oxide or hydroxide was observed near the interface. As a result of phosphate treatment of these plated steel plates, uniform and dense crystals were obtained. In addition, coating was performed under the same conditions to conduct a coating corrosion resistance test. Cross-cut salt spray test (2)
(after 2 weeks), long-term atmospheric exposure of cross-cuts (after 6 months), exposure of cross-cuts to salt water spraying in the atmosphere (after 6 months), salt spray test of cross-cuts after processing (after 2 weeks), there was no evidence of blistering of the paint film around the cross-cuts. It wasn't recognized. In addition, the results of a water resistant adhesion test (the test method is the same as above),
No evidence of peeling was observed and the score was 10 points. These results show that regardless of whether C-A treatment (cathode-anode electrolysis treatment), A-C treatment (anode-cathode electrolysis treatment), A treatment (anode electrolysis treatment) or immersion treatment is performed under specific conditions, Almost similar results were obtained. Similar results were also obtained when two or more of these treatments were combined. That is, the oxide or hydroxide layer of the amorphous or semi-amorphous substance at the interface of the electroplated steel sheet is reduced by C treatment, or a portion is reduced by C-A treatment or A-C treatment, and 1
Electroplated steel sheets can be produced by partially dissolving or by dissolving and removing them by A treatment or immersion (in any case, eliminating the oxide or hydroxide layer of amorphous or semi-amorphous substances at the interface). This shows that the phosphate treatment properties and paint corrosion resistance of the paint are significantly improved. The treatment bath used in the present invention is NaH ₂ PO ₄ 2H ₂ O,
It is a phosphoric acid compound bath such as H ₃ PO ₄ , K ₂ HPO ₄ ·2H ₂ O, Na ₂ HPO ₄ ·2H ₂ O, etc. Here, the phosphoric acid compound is used because ions (Zn-
In the case of Ni-based alloy plating, Zn, Ni) combine with phosphate ions, become phosphate compounds, and precipitate.
This is because dissolved (removed) Zn and Ni can be prevented from redepositing on the surface (if Zn and Ni accumulate in the bath and redeposit with a weak current, they will be in an amorphous or semi-amorphous state. , they immediately turn into oxides, hydroxides, return to their original state, and have no effect). Further, the C treatment, CA treatment, AC treatment, A treatment, and dipping treatment conditions may be appropriately selected depending on the manufacturing conditions of each plated steel sheet. Note that the treated layer used in the present invention is not limited to those arranged adjacently after the plating treatment as in the above-mentioned embodiments, but as long as it is placed after the plating treatment and before the pre-painting treatment. The method of the present invention can be applied even outside the plating line. As a mating system suitable for the present invention, the above-mentioned
Not only Zn-Ni system, but also Fe-Zn system, Fe-Zn-Ni system
system, Fe-Zn-Ni-Co system, Zn-Mn system, Zn-Al
The same was true for Zn-Ni-Co, Zn-Ni-Cr, and Cu-Zn galvanized steel sheets. Examples will be described in detail below. The treatment conditions of Examples 1 to 13 will be explained for each example, and Table 1 shows the evaluation results of phosphate treatability and paint corrosion resistance of these examples and comparative examples that are not subjected to the treatment of the present invention. Summarized. Although the treatment bath temperature is not particularly specified under the conditions of each example, it is usually 50°C.
degree is usually used. In both cases, it can be seen that when the method of the present invention is used, the phosphate treatability and paintability are significantly improved compared to the conventional method without post-treatment. Therefore, in the present invention, in the production of electrical Zn or Zn-based alloy plated steel sheets, after electroplating, the plated steel sheets are subjected to C treatment (cathode electrolytic treatment) and C-A treatment (cathode-anode treatment) in a phosphoric acid compound bath. Electroplated steel sheets with excellent phosphate treatment properties and paint corrosion resistance can be obtained by performing electrolytic treatment), AC treatment (anode-cathode electrolysis treatment), A treatment (anodic electrolysis treatment), and immersion treatment. This is a useful method. Example 1 Surface-treated steel sheets with Zn-Ni alloy plating of 20g/ ^m2 and these surface-treated steel sheets further
NaH ₂ PO ₄・2H ₂ O 50g/in bath, Dk=10A/
A surface-treated steel sheet that had been subjected to dm ² ×2 sec cathodic electrolytic treatment was investigated for its phosphate treatment properties and paint corrosion resistance. Example 2 A surface-treated steel plate plated with Zn-Al alloy at 30g/ ^m2 and further plated with NaH ₂ PO ₄・2H ₂ O 10g/m2
A surface-treated steel sheet that had been subjected to cathodic electrolytic treatment in a bath with Dk = 10 A/dm ² ×1.5 sec was investigated for phosphate treatment properties and paint corrosion resistance. Example 3 A surface-treated steel plate plated with Fe-Zn-Ni alloy ^at 45g/m2 and further coated with NaH ₂ PO ₄ 2H ₂ O30
A surface-treated steel sheet that had been subjected to CA treatment (cathode-anode electrolytic treatment: 5 A/dm ² ×2 sec) in a bath of 200 ml was investigated for its phosphate treatment properties and paint corrosion resistance. Example 4 Surface-treated steel sheet coated with Zn-Ni-Cr alloy ^at 30 g/m2 and further coated with NaH ₂ PO ₄ 2H ₂ O20
A surface-treated steel sheet that had been subjected to AC treatment (anode-cathode electrolytic treatment: 5 A/dm ² ×3 sec) in a bath of 300 ml was investigated for its phosphate treatment properties and paint corrosion resistance. Example 5 A surface-treated steel plate plated with Fe-Zn alloy at 60 g/ ^m2 and further plated with NaH ₂ PO ₄ 2H ₂ O 70 g/m2
The phosphate treatment properties and paint corrosion resistance of surface-treated steel sheets immersed in a bath at 60°C for 3 seconds were investigated. Example 6 A surface treated steel plate coated with 20g/ ^m2 of Zn-Mn alloy and further plated with 200g/m of NaH ₂ PO ₄・2H ₂ O
C treatment in bath (cathode electrolysis treatment: Dk=15A/
^Regarding surface-treated steel sheets subjected to
We investigated phosphate treatment properties and paint corrosion resistance. Example 7 A surface-treated steel plate coated with Fe-Ni-Zn-Co alloy at 30 g/ ^m2 and further coated with H ₃ PO ₄ 200 g/m2.
A surface-treated steel sheet that had been subjected to CA treatment (cathode-anode electrolysis treatment: 10 A/dm ² ×1.5 sec) in a bath was investigated for phosphate treatment properties and paint corrosion resistance. Example 8 A surface-treated steel plate plated with Fe-Zn alloy at 40g/ ^m2 and further coated with _Na2HPO4・_2H2O20g / _m2 .
The phosphate treatment properties and coating corrosion resistance of surface-treated steel sheets that had been subjected to A-C treatment (anode-cathode electrolysis treatment: 15 A/dm ² × 2 sec) in a bath were investigated. Example 9 A surface-treated steel plate coated with Zn-Ni alloy at 30g/ ^m2 and further coated with KH ₂ PO ₄・2H ₂ O 50g/m2
C treatment (cathode electrolytic treatment: 15A/dm ²
The phosphate treatment properties and paint corrosion resistance of the surface-treated steel sheets that had been subjected to a 1.5 sce coating were investigated. Example 10 Surface-treated steel sheet with Zn plating of 40 g/ ^m2 and these further coated with Na ₂ HPO ₄・2H ₂ O 150 g/m2
C treatment (cathode electrolytic treatment: 10A/dm ²
The phosphate treatment properties and paint corrosion resistance of the surface-treated steel sheets that had been subjected to a 1.5 sce coating were investigated. Example 11 A surface-treated steel sheet plated with Zn-Ni-Co alloy at a rate of 20g/ ^m2 and further coated _{with K2HPO4} .
Phosphate treatment properties and paint corrosion resistance were investigated on surface-treated steel sheets that had been subjected to anodic electrolytic treatment at 10 A/dm ² ×2 seconds in a 200 g 2H ₂ O/bath. Example 12 A surface-treated steel sheet plated with Cu-Zn alloy at a rate of 30 g/ ^m2 and further coated with Na ₂ HPO ₄ .
2H ₂ O150g/C-A treatment in bath (cathode)
The phosphate treatment properties and coating corrosion resistance of the surface-treated steel sheets subjected to anode electrolytic treatment (5 A/dm ² ×2 sec) were investigated. Example 13 A surface-treated steel sheet plated with Sn--Zn alloy at a rate of 20g/ ^m2 and further coated with NaH ₂ PO ₄ .
2H ₂ O250g/A-C treatment in bath (anode-
A surface-treated steel sheet that had been subjected to cathodic electrolytic treatment (5 A/dm ² ×3 sec) was investigated for its phosphate treatment properties and paint corrosion resistance. The test methods and evaluation methods used in the above examples are as follows. 1) Phosphate treatability Evaluation: ◎: Uniform, dense crystals, small crystals ○: 〃, crystals are small and large △: 〃, crystals are large ×: Partly spotted, crystals are coarse XX: Spotted, crystals are quite coarse 2) Cross-cut salt spray test Cross-cut after phosphate treatment - ED coating - intermediate coating - top coating, conduct salt water spray test for 2 weeks, and measure the blistering width of the paint centering on the cross-cut. Evaluation: ◎: No blistering width (zero) ○: One side blistering width within 0.5mm △: 〃 0.5-2mm ×: 〃 2-5mm ××: 〃 5mm or more3) Cross-cut long-term atmospheric exposure Phosphate treatment - ED painting - Intermediate coating - After applying the top coat, cross-cut it, expose it to the atmosphere for 6 months, and measure the blistering width of the paint around the cross-cut. Evaluation: Same as 2) 4) Cross-cut salt water spraying atmospheric exposure Phosphate treatment - ED coating - Intermediate coating - After coating, cross-cut and expose to the atmosphere, once a day.
Sprinkle the sample with 3% NaCl solution. After 6 months, measure the blistering width of the paint centering on the cross cut. Evaluation: Same as 2) 5) Cross-cut salt spray test after processing After drawing, phosphate treatment - ED coating - Intermediate coating - Top coating After cross-cutting on the drawing part, 1
A salt water spray test is conducted for several months, and the blistering width of the paint is measured centering on the cross cut. Evaluation: Same as 2) 6) Water resistant adhesion After phosphate treatment - ED coating - Intermediate coating - Top coating
Immerse in warm water at 50℃ for 10 days, and immediately perform a cross-cut peel test. Evaluation: 10 points: Zero paint film peeling 9 points: 〃 0-1% 8 points: 〃 1-3% 7 points: 〃 3-5% 6 points: 〃 5-10% 5 points: 〃 10-20% 4 points: 〃 20-30% 3 points: 〃 30-50% 2 points: 〃 50-90% 1 point: 〃 90-100% [Table]

[Brief explanation of drawings]

Figure 1 is a diagram showing the distribution of elements contained in the plating layer of a Zn-Ni alloy coated steel sheet, Figure 2 is an explanatory diagram of the plating area when electroplating is performed using a vertical plating layer, and Figure 3 The figure is a schematic diagram of a cross section of the plating layer of an electroplated steel sheet, Figure 4 is an explanatory diagram of the treated layer used in the present invention adjacent to the plated layer, and Figure 5 is a diagram showing the treated layer used in the present invention.
FIG. 3 is a distribution diagram of each component of a Zn--Ni based alloy plating layer. DESCRIPTION OF SYMBOLS 1... Steel strip, 2... Electrode, 3... Plating bath, 4... Plating cell, 5... Current roll, 6... Presser roll, 7...
Base iron, 8... Metal plating layer, 9... Oxide layer, 10...
Processing layer, normal plating area, weak current area.

Claims (1)

[Claims] 1. In the production of electrical Zn or Zn-based alloy plated steel sheets, after electrolytic Zn or Zn-based alloy plating, the plated steel sheets are subjected to C treatment (cathode electrolytic treatment) and C-A treatment (cathode electrolytic treatment) in a phosphoric acid compound bath. -Anodic electrolytic treatment), A-C treatment (anode-cathode electrolytic treatment), A treatment (anodic electrolytic treatment), and immersion treatment.
1. A method for producing an electroplated steel sheet with Zn or Zn-based alloy, which comprises removing oxides from the electroplated surface by performing one or more of the following steps.