JPH0581673B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention relates to a method for producing hot-dip galvanized steel sheets with excellent blackening resistance. (Prior Art) Today, as demand for hot-dip galvanized steel sheets increases and their uses diversify, further performance improvements are required in terms of appearance, workability, chemical conversion treatment properties, and corrosion resistance. In order to improve the appearance, a so-called zero spangle treatment is carried out in which water mist or zinc powder is sprayed onto the surface of the plating layer to finely form the zinc crystals before the zinc layer solidifies after plating. Further, after the zero spangle treatment, a skin pass treatment may be performed to further improve the appearance and surface shape. However, if skin pass treatment is followed by chromate treatment to improve corrosion resistance, the plating surface will turn grayish-black during storage or transportation for a relatively short time after manufacture (so-called blackening over time). ) Problems often occur. This blackening over time is sometimes observed to occur even in non-Skinpass materials when stored for long periods of time, but Skinpass materials treated with cello spangles are unique in that they occur in a very short period of time. As a technology to prevent blackening over time, JP-A-59-
The one described in Publication No. 177381 is known. This technology is characterized by treating zinc-plated steel sheets with zinc or zinc alloy hot-dip plating on the surface with a solution that fuses Ni ions, Co ions, or both. It is in the manufacturing method of galvanized steel sheets. (Problems to be Solved by the Invention) As a result of studies to establish a more effective technology for preventing blackening over time, the present inventors found that plating bath components are most suitable for treatment with solutions containing Ni ions and Co ions. I found out. Results of various studies on elements in the plating bath
Pb was found to be harmful to blackening over time. However, if Pb is not added, it is not possible to produce spangled material, so it is possible to produce spangled material.
Furthermore, as a result of examining additive elements that can provide a sufficient effect of preventing blackening over time by treating with a solution containing Ni ions and Co ions, it was found that Sb was the most suitable. The major differences between the present invention and the prior art, JP-A-59-177381, are as follows. While the prior art attempts to alleviate the effect of the formed local battery, which is the mechanism of blackening over time (described later), by using a metal or oxide of Co or Ni, the present invention This is intended to suppress the concentration of metal in the plating layer, which is a contributing factor. (Means for Solving the Problems) Accordingly, the present invention provides a manufacturing process in which the surface of a steel sheet is hot-dip galvanized and further chromate-treated on the surface, Al: 0.10 to 0.30%, Pb: 0.1%.
Hereinafter, after plating with a hot-dip plating bath consisting of Sb: 0.10 to 0.30% with the balance zinc and unavoidable impurities, a skin pass treatment with an elongation rate of 0.3 to 1.5% is performed as necessary, and then Ni ions or Co ions or their This is a method for producing a hot-dip galvanized steel sheet with excellent blackening resistance, which is characterized by performing chromate treatment by treating with a solution that mixes both. (Operation) Regarding the processing conditions of the present invention, the Pb concentration is
If it is 0.01% or less, a sufficient effect of preventing blackening over time can be obtained, and the lower this value is, the better. If the Sb concentration is less than 0.10%, beautiful spangles will not be formed. Also 0.30
If it exceeds %, not only will spangle formation not change, but a sufficient effect of preventing blackening over time will not be obtained. Further, the Al concentration is determined from the viewpoint of plating adhesion, and if it is less than 0.10%, the plating adhesion will be insufficient, and if it exceeds 0.30%, the improvement effect will not change. Regarding processing liquids containing Ni ions and Co ions,
An acidic bath in which sulfates, chlorides, nitrates, citrates, etc. of Ni and Co are dissolved may be used, or an alkaline bath containing a complexing agent such as ammonia and organic acids may be used. The ion concentration, temperature, and pH in the treatment solution are not particularly limited, and have a wide appropriate range. The metal ion concentration is preferably in the range of 1 to 40 g/. The treatment temperature is preferably in the range of 30 to 60°C. Regarding pH, in case of acidic bath, PH1~
4. For alkaline baths, a pH range of 11 to 13.5 is appropriate; if the pH is outside this range, the plating surface may deteriorate.
A phenomenon occurs in which Ni or Co is deposited too much, or conversely, it is difficult to deposit. In the case of the present invention, Ni or Co
The adhesion amount is preferably in the range of 0.2 to 15 mg/m ² . The immersion or spray treatment time is within a range where the amount of coating is preferred. The concentration mentioned above so that it is 0.2 to 15 mg/ ^m2 ,
All you have to do is select the temperature and pH conditions. If the elongation of the skin pass is less than 0.3%, not only will a beautiful surface appearance not be obtained, but elongation at the yield point will remain and so-called "leveler break" will occur in the subsequent leveler treatment. on the other hand
If the elongation rate exceeds 1.5%, the material will harden and flexible material properties will not be obtained. The mechanism of preventing blackening over time according to the present invention is presumed to be as follows. When a plating bath containing Pb is used, Pb tends to concentrate on the surface layer after plating, forming local batteries, and electrochemical corrosion tends to progress in a corrosive environment with a chromate layer that easily absorbs moisture. When the Zn layer corrodes, fine irregularities occur and a thin film layer of zinc hydroxide and zinc carbonate, which are corrosion products, is present on top of the irregularities, resulting in a blackening phenomenon due to light interference. In this case, even if Ni or Co is precipitated in the form of metal or oxide, it is not sufficient to suppress the progress of corrosion due to local batteries, and it is thought that blackening occurs over time. However, when the amount of Pb added in the plating bath is reduced and Sb is added instead, Sb concentrates less on the surface layer after plating than Pb, so corrosion due to the precipitation of Ni or Co metals or oxides may occur. It is believed that the deterrence of progress is effective. (Example)

[Table] Test material Size 0.5×100×200mm Skin pass elongation 1.0 Pretreatment Ni treatment NiSO ₄・6H ₂ O 50g/PH2 Temperature 40℃ Immersion time 2 seconds Co treatment CoSO ₄・7H ₂ O 50g/PH2 Temperature 40℃ Immersion time: 2 seconds Ni+Co treatment NiSO ₄・6H ₂ O 30g/
CoSO ₄・7H ₂ O 30g/PH2 Temperature
40°C Immersion time 2 seconds Washing Purified water Chromic acid treatment CrO ₃ 30g/50°C Immersion time 3 seconds Blackening acceleration test Sample size 100×100mm Atmosphere 70°C 98%RH Standing time 5 days Judgment Criteria Judgment Blackening condition ◎ Black No change 〇 Very light graying △ Less than 30% blackening × 30% or more blackening

[Table] All specimens plated using the plating bath of the present invention have excellent ability to prevent blackening over time. (Effects of the Invention) As described above, the present invention provides a hot-dip galvanized steel sheet that has a more effective effect of preventing blackening over time than conventional methods.

Claims (1)

[Claims] 1. In the manufacturing process where the surface of the steel plate is hot-dip galvanized and the surface is further chromated,
Al: 0.10~0.30%, Pb: 0.1% or less, Sb: 0.10~
After plating with a hot-dip plating bath consisting of 0.30% zinc and unavoidable impurities, the elongation rate can be adjusted as necessary.
Apply 0.3 to 1.5% skin pass treatment and then Ni
A method for producing a hot-dip galvanized steel sheet with excellent blackening resistance, characterized by chromate treatment by treatment with ions, Co ions, or a solution containing a mixture of both.