JPH0210236B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a surface-treated steel sheet that is excellent in various properties required for a rust-proof steel sheet for automobiles, such as bare corrosion resistance, corrosion resistance after painting, and weldability, and a method for manufacturing the same. Zn-plated steel sheets are used as rust-prevention treated steel sheets for automobiles, but because the pure Zn layer corrodes at a high rate, they are not suitable for use in particularly harsh corrosive environments, such as road anti-icing agents such as rock salt sprayed in cold regions. If exposed, the amount of Zn plating must be considerably increased (for example, if the amount of Zn plating is 20 to 40g/ ^m2 ,
120g/ ^m2 ), it is becoming impossible to expect long-term rust prevention. However, increasing the amount of plating deposit not only increases costs but also has disadvantages in terms of performance, such as deterioration of plating adhesion, workability, and weldability. On the other hand, in order to suppress the activity of the Zn-plated layer, metals that are potentially more noble than Zn, such as Fe and Ni, are added to the Zn.
The corrosion rate of the plated layer is reduced by precipitating an alloy with
30g/m ² ) has been developed and is being used in some car bodies. Here, since the Zn-Fe alloy plated steel sheet has excellent phosphoric acid treatment properties as a pre-painting treatment, it has good corrosion resistance after painting. However, since the alloy plating layer does not have a sufficient protective and anticorrosive effect on the steel plate, the bare corrosion resistance is insufficient. In addition, since the corrosion rate of the plated layer of Zn-Ni alloy plated steel sheets is significantly reduced, the protective and anticorrosive effect on the steel sheet is reduced, and the formation of red rust from the steel sheet is significant, especially after processing.
Its corrosion resistance is not necessarily sufficient. This deterioration in corrosion resistance of Zn-Ni alloy plated steel sheets after processing becomes more pronounced as the Ni content in the plated layer increases. The reason for this is that as the Ni content increases, the protective and anticorrosion effect decreases, and many cracks that reach the steel plate occur during processing. As mentioned above, conventional Zn-plated steel sheets and Zn-based alloy-plated steel sheets each have drawbacks. The present inventors recognize that, as mentioned above, it is extremely difficult to improve all the properties required for automotive rust-proof steel sheets, such as bare corrosion resistance, corrosion resistance after coating, and weldability, with single-layer plating. In addition to this, we conducted repeated research based on the standpoint of eliminating the economic and operational disadvantages caused by including alloying additives in all the required plating thicknesses. As a result, the corrosion rate of the plating layer has been significantly reduced, and the occurrence of red rust from the steel plate (plated original sheet), which is likely to occur due to alloying of the plating layer, has been reduced. As a matting steel plate with excellent performance required for rusted steel plates,
Zn or Zn-based alloy plating is applied, and a second
It has been found that a laminated plated steel plate coated with Co-W or Co-Cr alloy as a layer satisfies the required performance. Co-W or Co-Cr alloy plating with Zn or Zn
Among the corrosion products generated in a corrosive environment, the plating applied on the alloy plating contains a single phase of zinc hydroxide, Zn(OH) ₂ , which is effective in suppressing the oxygen reduction reaction, which is a cathode reaction during corrosion. This is thought to be due to the fact that it is formed on the plated surface in a denser and more stable manner compared to the plated surface, and it showed extremely excellent corrosion resistance in corrosive environments such as cold/hot cycle corrosion tests and composite corrosion tests. The present invention has been made based on such knowledge, and according to the first aspect of the present invention, on a steel plate,
The first layer is a Zn or Zn-based alloy plating layer with a plating amount of 3 g/m ² or more, and the second layer is a plating layer with a plating amount of 0.003 to 3 g/m ² .
Co-W or Co-Cr with Cr content of 0.01~10wt%
A laminated plated steel plate having an alloy plated layer and having excellent corrosion resistance, weldability, etc. is provided. According to the second aspect of the present invention, a steel plate is plated with Zn or a Zn-based alloy according to a conventional method, and then coated with Co in a cobalt plating bath containing 10 to 100 g of Co and 0.01 to 10 g of W or Cr. A method for producing a highly corrosion-resistant surface-treated steel sheet is provided, which includes the step of electroplating -W or a Co-Cr alloy. The present invention will be explained in more detail below. First, Zn or Zn-based alloy plating is applied in advance to a steel plate manufactured through a normal plated original plate manufacturing process such as surface cleaning treatment by an electroplating method or a hot-dip plating method. Here, Zn plating means electric
Zn plating and fused Zn plating are included, and Zz alloy plating refers to alloyed fused Zn plating (Zn-Fe),
Including Zn-Fe alloy electroplating, Zn-Ni alloy electroplating, Zn-Al composite electroplating,
Ni, Cr, Al, Mg, Mn, Fe, W, V, Sn, Mo,
A total of at least one type of In, Cu, Zr, Ti, Bi, etc.
It also includes Zn-based alloy plating containing 0.1 to 30 wt% (including both electroplating method and hot-dip plating method). As mentioned above, the metal used for this lower plating layer is one that has a strong protective and anti-corrosion effect on steel sheets (Zn plating), or one that has a moderately suppressed protective and anti-corrosion effect (Zn-based alloy plating).
Further, the thickness of the lower plating layer is preferably in the range of 3 to 60 g/m ² from the viewpoint of corrosion resistance, weldability, etc. If the lower plating layer is less than 3 g/m ² , the uniform coverage of the lower plating layer on the steel plate (base plate) will not be sufficient, and even if the superimposed effect with the upper plating layer is taken into account, satisfactory red rust will occur. No preventive effect can be obtained. A plating weight exceeding 60 g/m ² is undesirable because weldability and workability deteriorate. As described above, the present invention provides a method for applying Zn or Zn-based alloy plating to a steel plate, and then coating the plated layer with Co.
-W or Co-Cr alloy plating layer is formed, and as can be seen from Tables 1 and 2, the amount of plating is 0.003 to 3 g/m ² , preferably
0.01~1g/ ^m2 , and the W or Cr content in the matte film is 0.01~10wt%, preferably 0.05~
It is 5wt%. And this Co-W or Co-
The Cr alloy plating layer exhibits excellent performance with an extremely small amount of adhesion. The reason for the presumed effect is as follows. (1) Since the Co-W or Co-Cr alloy plating layer is thin and porous, it is thought that it does not completely cover the underlying Zn or Zn-based alloy plating layer, and therefore the surface is composed of Zn and Co-W. Or the Co-Cr alloy is finely and uniformly mixed and distributed. When a product with such a surface condition is exposed to a corrosive environment such as salt spray, a local battery is formed between Zn and Co-W or Co-Cr alloy, and the exposed Zn on the surface becomes an anode and is eluted. do. However, on the Co-W or Co-Cr alloy that serves as the cathode, zinc hydroxide Zn immediately reacts with hydroxyl ions ^OH
(OH) ₂ and deposits on the surface. This Zn
(OH) ₂ is dense and stable, as well as porous.
Since it is held by the Co-W or Co-Cr alloy plating layer, corrosion of the underlying Zn or Zn-based alloy plating layer is significantly suppressed. (2) Furthermore, by applying this Co-W or Co-Cr alloy plating, phosphate treatment properties and performance after painting, especially corrosion resistance after electrodeposition painting, are significantly improved. That is, since rust-proof steel sheets for automobiles, which are the main object of the laminated plated product of the present invention, are generally used in many fields after being treated with phosphoric acid and painted, they are required to have excellent performance. The surface-treated steel sheet of the present invention is effective against the formation of microcells necessary to obtain uniformly dense phosphate crystals during phosphate treatment, as described above.
Since Zn and Co-W or Co-Cr alloy are finely and uniformly mixed and distributed, the phosphate treatability is significantly improved. (3) Furthermore, in addition to the excellent phosphate treatment properties due to the effect of the upper plating layer, the performance after coating, especially the corrosion resistance after electrodeposition coating, is significantly improved. In other words, the formation of uniform and dense phosphate crystals improves the adhesion with the paint film, and the uniform and dense phosphate crystals prevent corrosive solutions from penetrating through the paint film in a corrosive environment onto the plated surface. To prevent this from occurring, improve corrosion resistance after painting. Furthermore, because the plated surface is extremely stable, even if a corrosive solution reaches the surface through the coating, for example, a Co-W or Co-Cr alloy plated film and a dense and stable corrosion product (zinc hydroxide) will form. The film prevents defects such as blistering from occurring in the electrodeposition coating, and has a remarkable effect of improving corrosion resistance. (4) On the other hand, in the steel sheet according to the present invention, Co-W or
Since the Co-Cr alloy plating layer is on the surface, it becomes difficult for the Cu chip for welding and the Zn present in the underlying layer to form an alloy, which prevents the chip tip from changing shape and significantly improves continuous weldability. Furthermore, since the steel sheet according to the present invention has excellent corrosion resistance, it is possible to reduce the amount of the underlying Zn or Zn-based alloy plating layer, which is advantageous for weldability. For the above-mentioned effects and reasons, and as can be seen from Tables 1 and 2 showing the results detailed later, Co-W or Co-W as the upper plating layer.
The amount of the Cr alloy plating layer deposited is preferably in the range of 0.003g/m ² to 3g/m ² . If the plating weight of the Co-W or Co-Cr alloy plating layer is less than 0.003 g/ ^m2 , no effect will be observed, and if it exceeds 3 g/ ^m2 , the uniform coverage will be greatly improved, resulting in improved corrosion resistance. It is also unfavorable from an economical point of view because Co and W are both expensive. Next, although the theoretical basis for the effect of W or Cr contained in the Co-W or Co-Cr alloy plating layer is not necessarily clear, the following is confirmed from the experimental results of the present inventors. There is. That is, when W or Cr is contained in Co in an alloy or eutectoid state, the corrosion resistance especially after processing is significantly improved. This is Co-W or Co-Cr during processing.
This is thought to be due to fewer cracks entering the alloy plating layer. Furthermore, in Co-W or Co-Cr alloy plating, it is thought that the synergistic effect of Co and W or Cr further promotes the formation of uniform, dense, and stable Zn(OH) ₂ . From the above, it has been found that the appropriate W or Cr content in the Co-W or Co-Cr alloy plating layer is in the range of 0.01 to 10 wt%. The reason is,
This is because if the W or Cr content is less than 0.01 wt%, the above-mentioned effects are not observed, whereas if it exceeds 10 wt%, the effect reaches saturation and the plating appearance deteriorates, which is not preferable. The rust-proof steel sheet of the present invention is manufactured by plating a steel sheet with Zn or a Zn-based alloy, and then subsequently plating with a Co-W or Co-Cr alloy using an electroplating method. The composition of the Co-W or Co-Cr alloy plating bath is 10 to 100 g/co of metal Co.
A water-soluble tungstate or chromium compound is added to the aqueous cobaltate solution containing 0.01 to 10 g of metal W or Cr, and the pH thereof is adjusted to an appropriate level. Usable cobalt salts include cobalt sulfate, cobalt chloride, cobalt sulfamate, cobalt pyrophosphate (alkaline PH8.3), cobalt citrate (acidic PH4, alkaline PH10), cobalt naphthenate, etc. Representative examples include ammonium tungstate, sodium tungstate, potassium tungstate, calcium tungstate, and chromium compounds include chromium chloride, chromium sulfate, and chromic anhydride. In addition, Co-W or Co
- In order to stabilize the plating operation in the Cr alloy plating bath, conductive additives such as ammonium chloride and sodium sulfate, boric acid, ammonium citrate, etc.
A PH buffer or the like may be added. Co-W or Co
- Since Cr alloy plating is applied by electroplating, it can be precisely controlled even when the amount of deposit is small. In addition, Co-W or
The W or Cr content in the Co-Cr alloy plating film is
The higher the current density and the lower the plating bath temperature, the higher the plating bath temperature.
Cr content can be controlled. Moreover, by performing insoluble treatment such as chromate and tannic acid on the Co-W or Co-Cr plating layer, corrosion resistance can be further improved. Hereinafter, the present invention will be specifically explained with reference to examples. Example After electrolytically degreasing and pickling a cold-rolled steel plate according to the conventional method,
Various types of first layer Zn or Zn-based alloy plating were performed under the following conditions. (A) Electric Zn plating (1) Bath composition Zinc chloride 210g/ Potassium chloride 360g/ (2) Bath temperature 55℃ (3) PH=5 (4) Current density 30A/dm ² (5) Plating amount 20g /m ² (B) Zn-Ni alloy plating (1) Bath composition Zinc chloride 130g/ Nickel chloride 240g/ Ammonium chloride 60g/ (2) Bath temperature 45℃ (3) PH=4 (4) Current density 30A/dm ² (5) Plating amount 20g/m ² (Ni content 13wt)
%) (C) Zn-Fe alloy plating (1) Bath composition Zinc chloride 130g/ Ferrous chloride 120g/ Ammonium chloride 60g/ Citric acid 5g/ (2) Bath temperature 50℃ (3) PH=2 (4) Current density 30A/dm ² (5) Plating amount 20g/m ² (Fe content 15wt
%) (D) Zn-Al composite plating (1) Bath composition Zinc sulfate 0.5M/ Zinc hydroxide 0.5M/ Aluminum hydroxide 0.2M/ Boric acid 30g/ Aluminum powder (-250mesh)
20～30g/ (2) Bath temperature 40℃ (3) PH=5 (4) Current density 30A/dm ² (5) Plating amount 20g/m ² (Al content 12wt
%) (E) Zn-Cr alloy plating (1) Bath composition Zinc chloride 130g/ Chromium chloride 60g/ Ammonium chloride 60g/ (2) Bath temperature 50℃ (3) PH=4.5 (4) Current density 30A/dm ² (5) Plating amount: 20g/m ² (Cr content: 3wt%) (F) Molten Zn plating Molten Zn plating was performed using the general Sendzimer method. Plating amount: 90 g/m ² After plating the first layer of Zn or Zn-based alloy as described above, wash with water and immediately apply the second layer of Co-W or Co-Cr alloy under the following conditions. I went to meet someone. (G) Co-W alloy plating (1) Bath composition Cobalt sulfate 200g/ Ammonium chloride 60g/ Boric acid 30g/ Sodium tungstate 10
g/ (2) Bath temperature 40℃ (3) PH=6 (4) Current density 1 to 10A/dm ² (5) Plating deposition amount Controlled by the amount of electrolytic electricity, W in the Co-W alloy plating layer was The content was controlled to be approximately 1wt%. (H) Co-Cr alloy plating (1) Bath composition Cobalt chloride 150g/ Ammonium chloride 60g/ Boric acid 30g/ Chromium chloride 20g/ (2) Bath temperature 40℃ (3) PH=3 (4) Current density 1~ 10 A/dm ² (5) Plating deposition amount The amount of plating was controlled by the amount of electrolytic electricity, and the Cr content in the Co-W alloy plating layer was controlled to be approximately 1 wt%. Various performance tests as described below were conducted on the various laminated plated steel sheets thus obtained.
The results are summarized in Tables 1 and 2.
As is clear from the test results in Tables 1 and 2, the coating weight of Co-W or Co-Cr alloy plating is
By setting it as 0.003~3g/ ^m2 , and Co-W
Or by keeping the W or Cr content in the Co-Cr alloy plating layer within the range of 0.01 to 10 wt%,
In particular, a steel plate with excellent bare corrosion resistance, corrosion resistance after coating, weldability, etc. required as a rust-proof steel plate for automobiles can be obtained. (1) Bare corrosion resistance The Co-W alloy was subjected to the cold cycle corrosion test (CCT) shown below. In addition, the Co-Cr alloy was subjected to a composite corrosion test shown below. (1-1) Cold cycle corrosion test (a) Salt spray test (JIS Z 2371)...4 hours (b) Dry (60℃)...2 hours (c) Wet (60℃, relative humidity 95% or more) ...4
Time (d) Freezing (-15°C)...2 hours The above (a) to (d) were treated as one cycle, and 2 cycles/day were carried out for 144 and 240 hours.

[Table] (1-2) Combined corrosion test (a) Salt spray test (JIS Z 2371)...4 hours (b) Hot air drying (60℃)...1 hour (c) Humidity test (50℃, relative temperature over 70)…
...3 hours The above (a) to (c) were carried out in 4 cycles/day for 168 and 240 hours.

[Table] (2) Phosphate treatment Bonderite #3128 (Nippon Parkerizing
Co., Ltd.) was sprayed at 50°C for 140 seconds, and evaluation was made based on the amount of adhered particles that produced uniform, dense, and fine crystals. In addition, in the case of x evaluation, the crystals were non-uniform, porous, and coarse.

[Table] (3) Corrosion resistance after electrodeposition coating After phosphate treatment with Bonderite #3128, Power Top U-30 Black (manufactured by Nippon Paint Co., Ltd.)
A cationic electrodeposition coating of 20 μm was performed using a cationic electrodeposition coating, and the coating was evaluated based on the width of the coating film swelling from the cross-cut area in accordance with the above-mentioned cold/heat cycle corrosion test and composite corrosion test.

[Table] (4) Weldability Using single-sided plated steel plates with a wall thickness of 0.7 mm, spot welding was performed by overlapping the plates with the plated side facing outward, and evaluation was performed based on the number of continuous welding points.

[Table] The present invention is not limited to the above-mentioned fields, but can be applied to various fields as described below. (1) The laminated plating of the present invention is also suitable as a base for a zinc-rich primer (a conductive organic coating containing metal powder such as Zn powder), and exhibits extremely excellent corrosion resistance. (2) The laminated plating of the present invention can be applied to one or both sides of a steel plate, and in the case of double-sided plating, differential thickness plating or different types of plating can be performed depending on the application. (3) In the Co-W or Co-Cr alloy plating of the present invention, similar effects can be obtained even if metals such as vanadium, titanium, molybdenum, and zirconium are used instead of W or Cr.

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Claims (1)

[Claims] 1. On a steel plate, the amount of plating deposited as the first layer is
Zn or Zn-based alloy plating layer of 3 g/m ² or more and a plating amount of 0.003 to 3 g/m ² as the second layer,
W or Cr content of plating film is 0.01~10wt%
A highly corrosion-resistant surface-treated steel sheet, characterized by forming a Co-W or Co-Cr alloy plating layer. 2. Apply Zn or Zn-based alloy plating to a steel plate, then add 10 to 100 g of Co and 0.01 to 0.01 g of W or Cr.
A method for producing a highly corrosion-resistant surface-treated steel sheet, which comprises electroplating Co-W or Co-Cr alloy in a cobalt plating bath containing 10 g/cobalt.