JPS6365088A - Production of hot-dipped steel sheet having excellent blackening resistance - Google Patents

Abstract

PURPOSE:To produce the title steel sheet having excellent blackening resistance by applying zinc hot dipping to a steel sheet, then adjusting the plating deposition quantity, and spraying an aq. soln. of Co hydroxide on the plated layer surface at a specified temp. to form the film of Co oxide. CONSTITUTION:Zinc or zinc-aluminum alloy hot dipping is applied to a steel sheet, and then the plating deposition quantity is appropriately adjusted by a gas throttling method. While the plated layer is kept at >=100 deg.C or after the plated layer is heated to >=100 deg.C, an aq. soln. or a suspension (at 5-8pH) contg. one or both of Co hydroxide and Fe hydroxide is atomized and sprayed on the plated layer surface to form the film of Co oxide and/or Fe oxide on the plated layer surface. Since the oxide film thus obtained firmly adheres to the plated layer, the film is not easily removed even when the steel sheet is worked, and a hot-dipped steel sheet having excellent blackening resistance is obtained.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention uses zinc-based or zinc-based materials that do not easily change color to gray-black even when spangles are minimized after hot-dip plating or when chromate treatment is performed after mechanical processing. The present invention relates to a method for producing hot-dip galvanized steel sheets of zinc-aluminum alloy.

(Prior art) Hot-dip galvanized steel sheets in which the coating layer is zinc-based include those in which the coating layer contains alloy/ffi growth inhibiting growth suppressing element ^1, as well as corrosion resistance improving elements, such as 810. .1-0.30%, Mg0-0.5%, Pb0
．． 0.002 to 0.3%, with the remainder consisting of Zn and unavoidable impurities. *As a hot-dip galvanized steel sheet of zinc-aluminum alloy, it is 14 to 53%,
Mg0-0.5%, Si 0-10%, Pb0.30%
, the remainder consisting of Zn and unavoidable impurities, etc. are known. These hot-dip plated steel sheets are normally manufactured continuously by the steps of heating and reducing the steel strip, immersing it in a hot-dipping bath, and adjusting the amount of coating.

The hot-dip galvanized steel sheets mentioned above have traditionally been used primarily for roofs and walls, but in recent years their use has expanded to areas that require a high degree of appearance, shape, and workability, such as interior decorations and utensils. For this reason, when manufacturing hot-dip galvanized steel sheets, spangles can be minimized by spraying n-shaped water droplets on the unsolidified plating layer immediately after adjusting the adhesion 1 after plating and rapidly cooling it. Mechanical treatments such as puff polishing, leveler or skin pass are applied to improve the appearance, shape and processability.

(Problems to be Solved by the Invention) However, even if the spangles are minimized in this way and the spangles are regular spangles, they are plated with a plating bath containing elements such as M or a zinc-aluminum alloy plating bath. When chromate treatment is applied mechanically to improve corrosion resistance and paint film adhesion, the surface turns grayish-black (hereinafter referred to as black discoloration) within a relatively short period of several months even under normal storage conditions. death,
There was a problem that the silvery white appearance was lost.

This black discoloration is caused by the minimization of spangles and mechanical processing, which changes the elemental state and crystal orientation on the surface of the plating layer, resulting in a film that easily absorbs light (^1, Cr, Zn).
This is presumed to be due to the fact that this film appears grayish-black, but the blackening was particularly severe when chromate treatment was applied. Therefore, in order to make the blackening less noticeable or less noticeable, it is sufficient to use a non-chromic acid-based post-treatment, but a non-chromic acid-based post-treatment has the disadvantage of high processing costs.

- ffi? Acidic or p 1111-13 of put~4 containing Ni ions and/or Co ions after chromate treatment and before chromate treatment.
There is a method of treatment with an alkaline aqueous solution as described in No.
9-177381), this method can be carried out at a lower cost than non-chromic acid post-treatment, but it requires a new treatment tank to be installed before the chromate treatment tank, and if the treatment liquid is a strong acid or a strong alkali. Therefore, a cleaning tank is also required for cleaning.

This increases equipment costs and complicates the process.

In addition, this method prevents blackening by precipitating Ni and Co in the form of metals or oxides on the surface of the active plating layer, but since the precipitation here is performed by displacement plating, the precipitates The adhesion force was weak. Therefore, if slippage occurs between the plates during coil winding, stacking or handling of cut plates, or transportation, or if they are subjected to processing such as roll ohming, pressing, or bending, precipitates can be easily removed. This causes black discoloration to occur in the form of dots, streaks, or stripes.

Furthermore, as the aqueous solution is acidic or alkaline as mentioned above, if cleaning after treatment is insufficient, the remaining acid or alkali will become a corrosion factor, instead inducing blackening and corrosion, and causing white rust. In other cases, the precipitated Ni and Co form local cells with Zn, causing microcorrosion.

Therefore, the present inventors proposed a method for preventing blackening without the above-mentioned drawbacks by first spraying an aqueous solution of cobalt salt, iron salt, or both onto the plating layer at a temperature higher than the decomposition temperature of the salt to thermally decompose the salt. We developed a method for forming an oxide film of cobalt or iron, and a method for spraying the aqueous solution and then applying chromate treatment to form a chromate film on the oxide film.

This method does not require cleaning because the aqueous solution is neutral, and since the oxide film is welded or firmly attached to the plating layer, it is not easy to process hot-dip galvanized steel sheets. Since the oxide film is not removed and is stable, it does not form local cells with Zn and cause microcorrosion, and spraying of an aqueous solution is usually installed in continuous hot-dip plating equipment. If a spraying device is used to minimize spangles, there is no need for a special spraying device and there are no problems with liquid management or waste liquid.

However, in this method, if cobalt nitrate or iron nitrate, which is inexpensive and has a low thermal decomposition temperature, is used, the thermal decomposition will cause T
) Since toxic gases such as NO and NO2 are generated, there is a problem of contaminating the working environment.

For this reason, it was necessary to spray the aqueous solution inside a box, and before releasing it into the atmosphere, it had to be treated with a water mist and absorbed by the mist.

For this reason, it has been necessary to use a treatment agent that dissolves in an aqueous solution that does not generate toxic gases through thermal decomposition.

(Means for Solving the Problems) The present inventors have searched and studied a treatment agent that has the same anticorrosion performance as cobalt nitrate or iron nitrate, and does not generate toxic gas through thermal decomposition. Found oxides and hydroxides of iron.

(Thus, the present inventors hot-dip-plated a steel sheet with zinc-based or zinc-aluminum-based alloy, then adjusted the amount of plating deposited, and then, while the plating layer was at 100°C or higher,
Alternatively, the salt can be thermally decomposed by heating the plating layer to 100°C or higher and atomizing and spraying an aqueous solution or suspension containing one or both of cobalt hydroxide and iron hydroxide onto the surface of the plating layer. , a method for producing a hot-dip galvanized steel sheet with excellent blackening resistance, characterized by forming an oxide film of cobalt, iron, or both on the surface of the plating layer, and a method for producing a hot-dip galvanized steel sheet with excellent blackening resistance, which is characterized by forming an oxide film of cobalt, iron, or both on the surface of the plating layer, and forming an oxide film of cobalt, iron, or both as described above. We have developed a method for producing hot-dip galvanized steel sheets with excellent blackening resistance, which is characterized by forming a chromate film on top of the oxide film by treating it with a chromate treatment solution after forming the oxide film.

In the present invention, the concentration and temperature of cobalt and iron hydroxides (hereinafter rain hydroxides are collectively referred to as treatment agents) in the aqueous solution or suspension (hereinafter both solutions are collectively referred to as treatment liquids) are not particularly limited. Undissolved particles with a particle size of 10 μm or less (
Even if some oxides (including some oxides) are mixed in, the performance and appearance will be affected completely (
no problem.

TreatmentyI! The spraying of step (a) onto the plated steel sheet is carried out by atomizing them and when the temperature of the coating layer is 100°C or higher.

This spraying may be carried out while the temperature of the hot-dip plated steel sheet is above 100°C by adjusting the coating amount after plating, or it may be carried out by heating the hot-dip plating wI@ which has been cooled to room temperature to above 100°C. By this spraying, moisture in the treatment agent evaporates at the temperature of the plated steel sheet and is thermally decomposed, forming an oxide film on the surface of the plating layer. Note that if spraying is performed especially while the plating layer is in an unsolidified state immediately after plating, cobalt and iron oxides generated by thermal decomposition will be diffused to the surface layer of the plating layer and become integrated with the plating layer, causing the plating layer to deteriorate. A strong oxide film can be formed on the surface layer.

The reason why the temperature of the plating layer when spraying the treatment liquid is set to 100° C. or higher is because the moisture in the treatment agent does not evaporate efficiently in a short time below this temperature, and it takes time for the treatment agent to thermally decompose.

The form of oxide produced by thermal decomposition of the processing agent is C00S.
It is a single film of CO304 or spinel type CO^1204, or a composite film of two or more thereof.

An oxide is also formed in the method of JP-A-59-177381, but since the oxide in this case is formed by direct conversion plating, the oxide is not a compound with oxygen; Like hydroxide, the valence of metal is increased, and it is different from the oxide formed by the present invention.

Since the oxide formed in the case of the present invention is a compound with oxygen as described above, it is chemically stable and acts as a barrier to protect the plating layer for a long period of time.
5. Suppresses the formation and growth of a blackened film containing Cr2bs, Mg formula 1□01, etc. In addition, this oxide film is welded or firmly adhered to the surface of the plating layer, so even if mechanical processing such as a leveler or skin pass or chromic acid treatment is performed, it cannot be easily removed or dissolved. This point is also clearly different from the conventional method disclosed in Japanese Unexamined Patent Publication No. 177381/1983.

In addition, the treatment liquid dissolves or suspends hydroxide in water, and the pH
Use one with a score of 6 to 8. Since the aqueous solution is neutral, washing with water is not necessary.

The amount of treatment liquid sprayed should be such that the adhesion air is 1 to 100 mg/m'' in terms of Co or Fe metal. 1 m3
If the number of wheels is less than 7, the blackening prevention effect is insufficient, and 1
This is because if it exceeds 00g/l112, it will be colored.

In the present invention, after a treatment liquid is sprayed as described above to form a cobalt or iron oxide film, a chromate treatment is further performed to form a chromate film. The reason why chromate treatment is applied is that the mere formation of a metal oxide film provides long-term corrosion resistance, including prevention of blackening, but the purpose is to further improve corrosion resistance and provide paint film adhesion.

The chromate treatment solution to be used is one that mainly dissolves chromic acid anhydride, and is most preferably adjusted to Cr"/(Cr"÷10Cr"ゝ)=O~0.6.Cr"
A chromate treatment solution having such a composition of `` and Cr needle is known as a known chromate treatment solution (reactive chromate treatment solution, coating type chromate treatment solution, electrolytic chromate treatment solution, etc.). In the case of the present invention, there may be problems in its use.This is because most of the known chromate treatment solutions contain reaction accelerators and etching components that moderately etch the zinc surface. This is not preferable because the zinc is dissolved and the metal oxide film is also removed, and the intoxication effect is lost.

Therefore, the chromate treatment solution used in the present invention is F
-, Cl-, SO42-, NO3-, Po, 3-1cu
It is preferable to use a material that does not contain an etching component consisting of an anion such as , coo-1 oxalate ion.

Note that any inorganic or organic material can be added to the aqueous solution as long as it does not dissolve the zinc surface or metal oxide film (for example, ^1.0..SiO□).

The present invention will be explained below with reference to Examples.

(Example) Example 1 In weight%, ^I 0.17%, r'b O, 30%, F
After hot-dipping a steel plate in a plating bath consisting of e O, 0.3%, balance Zn and unavoidable impurities, plating is performed using a gas coating method! 200-2509/mJ:a
9 L, Pt51 2 to 3 Kg/aqueous solution of the salt shown in the table
A hot-dip galvanized steel sheet was produced by atomizing and spraying with am' compressed air.

After that, a part of this steel plate was treated with CrCh209/JB at a temperature of 4
Chromate treatment was performed by immersing the sample in a chromic anhydride solution at 0° C. for 5 seconds.

Next, test pieces were taken from these hot-dip galvanized light steel sheets, untreated and treated with chromate, and placed in an atmosphere with a temperature of 50 ± 1°C and a relative humidity of 98% or more for 1 day for the untreated and treated steel sheets. A blackening acceleration test was conducted on the chromate-treated samples by leaving them for 30 days. Table 1 shows the results of this accelerated test.

The judgment of black discoloration is ``A'' based on the following criteria.

◎ O pole without blackening (lightly grayed Δ Blackened area is less than 30% of the whole × Blackened area is 30% or more of the whole □ Example 2 In a plating bath with the composition shown in Table 2 After hot-dipping the steel plate and adjusting the amount of KI with plating, Co(OH)2
Treatment solution containing 59/e or Fe(Oll)s 109
A treatment solution containing /e was sprayed on the steel sheet, and then chromate treatment was performed in the same manner as in Example 1 to produce a hot-dip plated steel sheet. Thereafter, a test piece was taken from this steel plate and left in an atmosphere with a temperature of 50±1° C. and a relative humidity of 98% or more for 30 days to perform a blackening acceleration test. Table 2 shows the types of aqueous solutions and the judgment results for blackening. Note that the criteria for determining black discoloration are the same as in Example 1.

(Effects of the Invention) As described above, the method of the present invention can be applied even when spangles are minimized during the production of hot-dipped steel sheets, and when plating is performed using a zinc-based plating bath or a zinc-aluminum alloy plating bath containing elements such as MH. It is possible to suppress the occurrence of black discoloration.

Furthermore, since the treatment agent is hydroxide, the thermal decomposition produces only water, and no toxic gas is generated. There is no need to treat with mist.

Claims (6)

[Claims] (1) After applying hot-dip plating with zinc or zinc-aluminum alloy to a steel sheet, adjust the coating amount, and then heat the plating layer to 100°C or higher while the plating layer is still at 100°C or higher. By atomizing and spraying an aqueous solution or suspension containing one or both of cobalt hydroxide and iron hydroxide onto the surface of the plating layer, a film of cobalt, iron, or both oxides is formed on the surface of the plating layer. A method for producing a hot-dip galvanized steel sheet with excellent blackening resistance. (2) The method for producing a hot-dip galvanized steel sheet with excellent blackening resistance according to claim 1, characterized in that an aqueous solution or suspension having a pH of 6 to 8 is sprayed. (3) As zinc-based plating, Al0.1-0.3%, M
The blackening resistance according to claim 1, characterized in that hot-dip plating is performed in a plating bath consisting of 0 to 0.5% g, 0.002 to 0.30% Pb, and the balance being Zn and unavoidable impurities. A method for producing hot-dip galvanized steel sheets with excellent properties. (4) As zinc-aluminum alloy plating, Al4
~53%, Mg0~0.5%, Si0~10%, Pb0
．． 0.002 to 0.30%, the balance being Zn and unavoidable impurities. Method. (5) After applying hot-dip plating with zinc or zinc-aluminum alloy to a steel sheet, adjust the coating weight and then heat the plating layer to 100°C or higher while the plating layer is still at 100°C or higher. By atomizing and spraying an aqueous solution or suspension containing one or both of cobalt hydroxide and iron hydroxide onto the surface of the plating layer, a film of cobalt, iron, or both oxides is formed on the surface of the plating layer. 1. A method for producing a hot-dip galvanized steel sheet with excellent blackening resistance, characterized by forming a chromate film on the oxide film by forming a chromate film on the oxide film and then treating it with a chromate treatment solution. (6) The chromate treatment solution is a solution mainly containing chromic anhydride, Cr^3^+/(Cr^3^+Cr^
6^+) = 0 to 0.6, and F^-, Cl^-, SO
_4^2^-, NO_3^-, PO_4^3^-, CH
3. The method for producing a hot-dip galvanized steel sheet with excellent blackening resistance according to claim 1, which does not contain _3COO^- and oxalate ions.