JPH0336918B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a galvanized steel sheet that prevents the surface of the plating layer from discoloring to grayish black by applying electrolytic chromate treatment to the steel sheet after hot-dip galvanizing. In recent years, as the uses of hot-dip galvanized steel sheets have diversified, there have been demands for further improvements in corrosion resistance, surface shape, and appearance, and these improvements have also been made in manufacturing. For example, regarding corrosion resistance, in order to improve the corrosion resistance of the plating layer, zinc added with corrosion-resistant elements is hot-dipped, and the plating layer contains one or more of Al, Mg, and rare earth elements. Hot-dip galvanized steel sheets have been developed. In addition, in order to improve the surface shape and appearance, a tension leveler and temper rolling mill are installed in the plating line to perform light rolling (the elongation rate is 1-3% for the former and 0.5-3% for the latter).
3%) is being added. By the way, in the case of hot-dip galvanized steel sheets, chromate treatment is performed by dipping as a post-treatment to prevent white rust before or after skin-pass rolling. There was a problem in that the surface layer of the internal plating layer turned grayish-black (hereinafter referred to as black discoloration) for a short period of time, impairing quality.
In particular, the occurrence of this black discoloration was accelerated when the plating layer contained Al or Mg. Until now, the cause of this black discoloration has not been elucidated in detail, but since it occurs in the cases described above, the processing after plating, the elution of components from the chromate film, and the humid environment interact with each other, resulting in black oxidation. It is thought to be used to produce zinc. Therefore, in order to prevent black discoloration, measures such as lowering the rolling rate during skin pass rolling or non-chromate treatment can be taken as post-treatment.
If the rolling reduction in skin pass rolling is reduced, the shape and appearance will be inferior, making it unsuitable for use in fields that require high quality, such as base plates for painting, and post-treatment with non-chromate treatment. In reality, chromate treatment by dipping cannot compete with chromate treatment in terms of corrosion resistance and high-speed processing, and is therefore difficult to adopt. Therefore, the present inventors have studied various ways to obtain a product that does not discolor even when subjected to conventional skin pass rolling when producing hot-dip galvanized steel sheets containing one or both of Al and Mg in the plating layer. As a result, they found that black discoloration resistance could be improved by using electrolytic chromate treatment with a treatment solution containing chromium ions and other metal ions instead of the conventional chromate treatment by dipping. The present invention will be explained in detail below. First, a steel plate is hot-dipped, and zinc containing one or both of Al and Mg is used as the plating metal. Here Al and Mg
are contained in the corrosion-resistant top plating layer in an amount of 0.05% by weight or more and 0.002% by weight or more, respectively. However, when Mg exceeds 0.5% by weight, the surface of the plating layer exhibits a purple interference color, impairing the appearance, so the content is preferably 0.5% by weight or less. Next, after hot-dip plating, temper rolling and electrolytic chromate treatment are performed. Although these steps may be performed in any order, in consideration of the destruction of the electrolytic chromate film due to temper rolling, it is better to perform temper rolling first. In temper rolling, conventional rolling reduction (elongation rate of 0.5 to 3%) may be applied depending on the degree of improvement in surface shape and appearance. On the other hand, in electrolytic chromate treatment, chromium ions are electrolyzed in a treatment solution containing other metal ions using a hot-dip galvanized steel sheet as a cathode and an insoluble electrode as an anode. Although black discoloration resistance can be improved by using only a chromic acid-based aqueous solution in which chromium oxide such as chromic anhydride or dichromate is dissolved as a treatment solution, electrolytic chromate coating is applied to shorten the treatment time. This is because if the amount of the coating is small, the effect of improving the black discoloration resistance is small, and even if the amount of the coating is large, the white rust resistance is inferior to that of the chromate coating formed by dipping. For this reason, in addition to containing chromium ions in the treatment solution, cobalt ions (0.002 to 0.1 mole/metal ion) are added.
, molybdenum ion 0.001-0.1 mol/, titanium ion 0.002-0.1 mol/, manganese ion
0.001-0.05 mol/, Nickel ion 0.002-0.1
mol/, aluminum ion 0.002-0.2 mol/
Add one or more of the above and perform electrolysis, or electrolyze in advance with a non-chromic acid acidic aqueous solution containing the above ions (however, immersion may be used in the case of nickel ions) and then process the chromic acid aqueous solution. It is preferable to improve the black discoloration resistance and the white rust resistance when the amount of the film is small by electrolyzing the resin. When metal ions other than chromium ions are added and electrolytic chromate treatment is performed in one or two stages as described above, the electrolytic deposits of ions are not single elements depending on the type of added ions, but include, for example, titanium ions or aluminum. Some ions are estimated to become hydroxides or oxides, but in order for the added ion precipitate to be effective, the total amount of added ion precipitates, including such electrolytic deposits, must be It must be at least 0.1% by weight of the total amount of chromium in the treated film.
However, if the total amount of added ion precipitates exceeds 50% by weight of the total chromium amount in the post-treatment film, the effect of the precipitates will be saturated, leading to an increase in manufacturing costs, so the upper limit is 50% or more.
It is preferable to make it less than % by weight. Even when the post-treatment according to the present invention is carried out in one or two stages, it is desirable that the total precipitated amount of added ions be 0.1 to 50% by weight based on the total amount of chromium in the post-treated film. The amount of post-treatment film is 3 to 300, including electrolytic deposits of chromium ions and precipitates of added ions.
It is preferable to use mg/m ² (one side). Film amount is 3
If it is less than 300 mg/m ² (one side), the surface of the plating layer will not be sufficiently covered and the effect of improving black discoloration resistance will be insufficient, and even if it exceeds 300 mg/m ² (one side), the effect of improving black discoloration resistance will be insufficient. This will only lead to saturation and increase production costs. Note that the post-treatment according to the present invention is performed when ions other than chromium ions are added to the treatment solution as described above.
Although it may be carried out in stages or in two stages, carrying out in two stages is disadvantageous in terms of equipment and production costs, so it is preferable to carry out in one stage. It is not clear why the electrolytic chromate film according to the present invention is effective for black discoloration resistance, and the chromate film formed by dipping is harmful to black discoloration resistance, but this is because the electrolytic chromate film composition does not contain chromium even under high humidity. It is thought that the composition is such that it does not elute. Example After pre-treating a steel plate using a conventional method, it is hot-dipped with zinc containing one or two of Al and Mg [plating coverage: 120 g/m ² (one side)], and then melted in the following process. A galvanized steel plate was produced. 1 Method of the present invention Hot-dipping → Temper rolling → 1-stage electrolytic chromate treatment → Product 2 Conventional method (1) Hot-dipping → Temper rolling → Chromate treatment by dipping → Product (2) Hot-dip plating → Chromate treatment by dipping →
Product (3) Hot-dip rolling → Temper rolling → Product (4) Hot-dipping → Product Temper rolling is performed so that the elongation rate is 1.0%, and electrolytic chromate treatment and chromate treatment by dipping are shown in Table 1. The conditions shown in were used. All products are subjected to accelerated black discoloration tests (humidity tests, relative humidity tests).
60%, ambient temperature 50°C, test time 800 hours) and white rust acceleration test (salt spray test (according to JISZ2371, test time 100 hours)) and evaluated according to the following criteria.

[Table] Table 1 shows the test results by manufacturing method.

Table 1 shows that the method of the present invention improves the black discoloration resistance. Furthermore, if the chromate treatment solution of the electrolytic solution contains added ions, white rust resistance is also improved. As described above, according to the present invention, even if zinc containing one or two of Al or Mg is hot-dipped and then temper rolled, the black discoloration resistance and white rust resistance are improved. . The electrolytic chromate treatment in the present invention can also be applied to post-treatment for preventing white rust during the production of other galvanized steel sheets such as electrolytic zinc or zinc alloy coated steel sheets and vapor deposited galvanized steel sheets.

Claims (1)

[Claims] 1. After hot-dipping zinc containing one or both of Al and Mg to a steel sheet, in addition to chromium ions, cobalt, molybdenum, titanium, manganese,
Electrolytic chromate treatment is performed using a treatment solution containing one or more of nickel and aluminum ions using a steel plate as a cathode, and the content of electrolytic deposits other than chromium in the electrolytic chromate film is determined by A method for producing a galvanized steel sheet with excellent black discoloration resistance, characterized by containing 0.1 to 50% by weight of the total chromium content. 2 The amount of electrolytic chromate film is 3 to 300mg/
^2. The method for producing a galvanized steel sheet with excellent black discoloration resistance according to claim 1, characterized in that the galvanized steel sheet is coated on one side.