JP2006257491A - Method for producing surface-treated steel sheet having excellent corrosion resistance and visual color tone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To produce a surface-treated steel sheet having an excellent visual color tone without impairing corrosion resistance when an acidic aqueous solution comprising either or both of phosphoric acid and primary phosphate is applied to the surface of an electrogalvanized steel sheet, and drying is performed, so as to form an amorphous film. <P>SOLUTION: When an acidic aqueous solution containing either or both of phosphoric acid and primary phosphate is applied to the surface of an electrogalvanized steel sheet, and drying is performed, so as to form an amorphous film, before the acidic aqueous solution comprising either or both of phosphoric acid and primary phosphate is applied, surface control is performed with an aqueous solution comprising condensed phosphate and also having a solid grain concentration of <1.0 g/L. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a surface-treated steel sheet that is most suitable for use in automobiles, home appliances, building materials, etc., product processing, human influence on assembly workers and end users, or elution of harmful substances from products during production and disposal, etc. The present invention relates to a method for producing an environment-adaptive surface-treated steel sheet that does not contain any hexavalent chromium harmful to the human body and the environment, and is adapted to solve the environmental pollution problem.

  Conventionally, for the purpose of improving corrosion resistance (white rust resistance, red rust resistance) on the surface of galvanized steel sheets or aluminum-based galvanized steel sheets, chromic acid, dichromic acid or its Steel plates that have been chromated with a treatment solution containing salts as main components are widely used. This chromate treatment is an economical treatment method that has excellent corrosion resistance and can be performed relatively easily.

  The chromate treatment uses hexavalent chromium which is a pollution control substance. This hexavalent chromium is treated with a closed system in the treatment process, and the chromate film is formed by the sealing action by the organic film formed on the upper layer. Since elution of chromium from the inside can be made almost zero, the human body and the environment are not substantially polluted by hexavalent chromium. However, along with the recent increase in global environmental problems, not only conventional regulations that emphasize work environment and wastewater treatment, but also regulations that emphasize environmental load and environmental harmony are being enacted. In addition, it is an era when manufacturers are evaluated by their environmental contributions, and there is an increasing trend to reduce the use of heavy metals including hexavalent chromium.

Based on this background, many chromate-free technologies have been proposed as white rust suppression technologies for zinc-based plated steel sheets that do not use hexavalent chromium. Among these, many techniques for forming a film by applying and drying a treatment liquid containing phosphoric acid or phosphate as an inorganic component have been proposed. For example, Patent Document 1 and Patent Document 2 propose a surface treatment agent containing Al, a phosphoric acid compound, silica, and a water-based organic resin emulsion, and a metal material coated therewith. Patent Document 3 proposes a galvanized steel sheet in which an organic coating layer is formed after forming an amorphous film by applying and drying a mixed aqueous solution of a polyvalent metal primary phosphate and a metal oxide sol. Yes. Further, in Patent Documents 4 and 5, a composite oxide film layer containing oxide fine particles, phosphoric acid and / or a phosphoric acid compound, and one or more metals selected from Mg, Mn, and Al is used as a lower layer, and an upper portion thereof. A steel sheet having an organic film is proposed.
Japanese Patent Laid-Open No. 11-350157 JP 2000-26980 A JP 2000-129460 A JP 2002-53979 A JP 2002-53980 A

  The method of forming a film by applying and drying a treatment liquid containing one or both of phosphoric acid and primary phosphate as the above-mentioned inorganic component is difficult to dissolve because the plating metal on the substrate reacts with the treatment liquid. By forming phosphate, corrosion resistance, paint adhesion, and the like are exhibited.

  However, when the above film is formed on a zinc-based plating production line, that is, when the above film is continuously formed after the plating film is formed, the appearance color tone, particularly whiteness, is unstable due to changes in production conditions such as line speed changes. Had a serious quality problem.

  In the present invention, when forming an amorphous film by applying and drying an acidic aqueous solution containing one or both of phosphoric acid and primary phosphate on the surface of the electrogalvanized steel sheet, without damaging the corrosion resistance, It aims at enabling it to manufacture the surface treatment steel plate excellent in appearance color tone.

  The gist of the present invention for solving the above problems is as follows.

  In the first invention, when forming an amorphous film by applying and drying an acidic aqueous solution containing one or both of phosphoric acid and primary phosphate on the surface of the electrogalvanized steel sheet, the phosphoric acid, first Before applying an acidic aqueous solution containing one or both of the phosphates, the surface is adjusted with an aqueous solution containing a condensed phosphate and having a solid particle concentration of less than 1.0 g / L. This is a method for producing a surface-treated steel sheet excellent in corrosion resistance and appearance color tone.

According to a second invention, in the first invention, the first phosphate is one or a mixture of two or more of an Mg salt, an Mn salt, and an Al salt, and a metal / Surface treatment excellent in corrosion resistance and appearance color tone, characterized in that P ₂ O ₅ molar ratio is 0.60 to 0.98, and Al salt is metal / P ₃ O ₇ molar ratio is 1.02 to 1.20 It is a manufacturing method of a steel plate.

  According to the present invention, after an electrogalvanized coating film is formed, an acidic aqueous solution containing one or both of phosphoric acid and primary phosphate is subsequently applied to the surface of the electrogalvanized steel sheet and dried. In forming the film, surface adjustment under specific conditions is performed as a pretreatment from the viewpoint of controlling the reactivity between the electrogalvanized film and the treatment liquid, or the phosphoric acid concentration in the acidic aqueous solution is restricted to a specific range. As a result, it has become possible to achieve a high degree of compatibility between corrosion resistance and appearance color tone, which was previously impossible. According to the present invention, the surface appearance is not deteriorated even if the line speed changes, and it is possible to manufacture at a wider range of line speeds than in the conventional manufacturing conditions, and it is possible to greatly improve the production efficiency. It was.

As a post-treatment of the zinc-based plating production line, the present inventors applied and dried a treatment solution containing one or both of phosphoric acid and primary phosphate as an inorganic component to form an amorphous film (slightly soluble The appearance color tone generated when the (phosphate) was formed, particularly the cause of the instability of the whiteness, was studied earnestly. As a result, after the formation of the electrogalvanized plating film, when the treatment liquid containing one or both of phosphoric acid and primary phosphate was subsequently applied on the surface of the electrogalvanized plating film, the following phenomenon was observed: Has occurred, and the film has a large surface roughness, and the surface has been blackened.
(1) The plating solution component remaining on the plating film surface is redissolved to activate the galvanized film, and the reaction between the plating film and the treatment solution is significantly accelerated.
(2) Contaminants attached to the surface of the plating film serve as crystal nuclei, and hardly soluble phosphate crystals grow locally.

  In view of the above-mentioned causes, a method of reducing the reactivity of the treatment liquid can be considered as a measure for improving the appearance color tone, but in that case, since a sufficiently poorly soluble phosphate cannot be formed, the corrosion resistance is remarkably lowered, and the treatment liquid composition It was impossible to achieve both corrosion resistance and appearance color by optimization alone.

  As a result of further investigation, in forming an amorphous film by applying and drying an acidic aqueous solution containing one or both of phosphoric acid and primary phosphate on the surface of electrogalvanized steel sheet, electrogalvanizing plating From the viewpoint of controlling the reactivity between the film and the treatment liquid, it has become possible to achieve a high degree of compatibility between corrosion resistance and appearance color tone, which has been impossible in the past, by performing specific surface adjustment as a pretreatment. The present invention is based on this finding.

  The present invention will be described in detail below.

  The surface-treated steel sheet of the present invention is a steel sheet in which an amorphous aqueous film is formed by applying and drying an acidic aqueous solution containing one or both of phosphoric acid and primary phosphate on the surface of the electrogalvanized film. Here, as the electrozinc plating film, besides pure zinc plating, alloy plating such as Zn-Ni alloy plating, Zn-Fe alloy plating, Zn-Cr alloy plating, Zn-Mn alloy plating, Zn-Co alloy plating, etc. Is mentioned. The production method is not particularly limited as long as it is an electroplating method, but as a plating bath, in addition to a sulfuric acid bath mainly composed of zinc sulfate and a chloride bath mainly composed of zinc chloride, a mixture of zinc sulfate and zinc chloride. As a main component, there is a mixed bath.

  Before applying an acidic aqueous solution containing one or both of phosphoric acid and primary phosphate, the surface is adjusted with an aqueous solution containing a condensed phosphate and having a solid particle concentration of less than 1.0 g / L. Thus, an electrogalvanized steel sheet excellent in corrosion resistance and appearance color tone can be obtained. Here, the condensed phosphate is limited to an alkali metal salt that becomes water-soluble. Since the alkali metal condensed phosphate aqueous solution is alkaline, the surface of the plating solution neutralizes the plating film surface where the plating solution components remain and forms an inert zinc hydroxide layer on the outermost layer of the plating film. To do. As a result, even if the plating solution component remains on the surface of the zinc-based plating film, the surface of the plating film whose reactivity is uniformly controlled can be obtained by this surface adjustment. The alkali metal condensed phosphate is not particularly limited, but condensed sodium phosphate, condensed phosphoric acid K and the like are preferable from the viewpoint of solubility and the like, and the concentration ranges from 1.0 to 50.0 g / L. preferable. If it is less than 1.0 g / L, the surface conditioning effect is poor, and if it exceeds 50.0 g / L, there is a concern that the surface conditioning solution has a high pH and the galvanized film is dissolved.

  When solid particles are present in this surface conditioning solution, they become crystal nuclei, causing insoluble phosphate crystals to grow locally and the surface to become black. Therefore, it is essential that the solid particle concentration in the surface conditioning liquid is less than 1.0 g / L. Here, the solid particles are not particularly limited as long as they are hardly soluble substances, and examples thereof include organic polymers, metal salts, and oxide colloids. The particle size is not particularly limited, but if solid particles of 0.1 μm or more adhere to the plating film, the appearance color is greatly affected. Therefore, practically, the concentration of solid particles of 0.1 μm or more is set to 1 By making it less than 0.0 g / L, the effect of preventing blackening of the surface can be expressed. The particle size of the solid particles present in the aqueous solution can be measured with a laser diffraction / scattering particle size distribution measuring device or the like.

  In addition, when surface-adjusting with said aqueous solution (surface adjustment liquid), it is only necessary to make aqueous solution contact the surface of a plated steel plate, and specifically, it is preferable to immerse in aqueous solution for 1 second or more.

There is no limitation on phosphoric acid or primary phosphoric acid to be applied to the surface of the electrogalvanized film, and phosphoric acid includes orthophosphoric acid, polyphosphoric acid, etc., and primary phosphate includes primary phosphoric acid Mg, primary phosphoric acid. Examples thereof include Ca, primary phosphate Ni, primary phosphate Mn, primary phosphate Al, and the like. As said first phosphate, metal / P ₂ O ₅ molar ratio is 0.60 to 0.98, primary phosphoric acid Mg, primary phosphoric acid Mn, or metal / P ₃ O ₇ molar ratio is 1. A high degree of corrosion resistance and appearance color tone can be realized by using one or two or more mixed liquids of primary phosphoric acid Al of 0.02 to 1.20. Here, when the metal / P ₂ O ₅ or metal / P ₃ O ₇ molar ratio is less than the lower limit, due to its high reactivity, the plating film is excessively etched to form precipitates in the treatment liquid, and the treatment liquid. Not only will the stability be poor, but the appearance color of the resulting film will be poor. On the other hand, if the metal / P ₂ O ₅ molar ratio exceeds the upper limit, a sufficiently poorly soluble salt film cannot be formed due to its low reactivity, and the corrosion resistance and paint adhesion are poor.

  A water-soluble or water-dispersible organic resin can be added to the treatment liquid from the viewpoint of improving performance such as corrosion resistance. There is no limitation in particular as an organic resin, An acrylic resin, a urethane resin, an epoxy resin, and a phenol resin are mentioned. Moreover, an antirust additive, oxide sol, and wax can also be added as needed. The film thickness of the amorphous film formed by applying and drying an acidic aqueous solution containing one or both of phosphoric acid and primary phosphate is preferably 0.05 μm or more and 1 μm or less, preferably 0.1 μm or more and 0.5 μm. The following is more preferable.

  Furthermore, an organic resin film layer can be formed on the upper layer of the film. The film thickness at that time is preferably 0.1 μm or more and 2 μm or less.

  According to the present invention, since the surface appearance does not deteriorate even if the line speed changes, it is possible to operate in a wider range of line speeds than in the conventional manufacturing conditions, and it is possible to greatly improve the production efficiency. became.

Examples of the present invention will be described below.
[Production of evaluation materials]
(1) After electrolytic degreasing (cathodic electrolysis, 10 A × 10 seconds) of a zinc-plated steel sheet cold-rolled steel sheet in a sodium orthosilicate aqueous solution (40 g / l, 80 ° C.), zinc sulfate (440 g / l) and Ni sulfate Electroplating (current density = 44 A / dm ² ) of an aqueous solution containing (1.6 g / l) (adjusted to pH = 1.5 with sulfuric acid, 50 ° C.) using a flow cell (flow rate = 2 m / s) A plating film having an adhesion amount of 20 g / m ² was formed.
(2) Immersion of plating solution The electrogalvanized steel sheet was immersed in a dilution solution (concentration = 100 ppm) of the plating solution for 1 second to contaminate the plating film.
(3) Surface adjustment After the electrogalvanized steel sheet immersed in the plating solution was taken out from the diluted solution of the plating solution, it was immersed in various surface adjustment solutions for 3 seconds within several seconds. After immersion, water was removed with a blower without washing with water.
(4) Film Formation An acidic aqueous solution containing various phosphoric acids or primary phosphates was applied to the treatment material and dried (drying temperature = 140 ° C.) to form an amorphous film. For the partially treated material, the organic resin-containing solution was subsequently applied and dried (drying temperature = 140 ° C.) to form an upper organic resin film.

  The evaluation material obtained above was evaluated for corrosion resistance and appearance color tone as follows.

[Evaluation methods]
(1) The area ratio of white rust generated after a JIS Z 2371 salt spray test for 72 hours was performed on the evaluation material having the corrosion-resistant end portion and back surface tape-sealed. The evaluation criteria are as follows. ◎, ○ +, ○ were evaluated as having good corrosion resistance.
◎: No white rust occurrence ○ +: White rust occurrence area rate less than 5% ○: White rust occurrence area rate 5% or more and less than 10% ○-: White rust occurrence area rate 10% or more and less than 25% △: White rust occurrence area Rate 25% or more and less than 50% x: White rust generation area ratio 50% or more (2) The L value of the appearance color tone evaluation material was measured with a color difference meter. The evaluation criteria are as follows. ○ and Δ were evaluated as good appearance color tone.
○: L value of 66 or more Δ: L value of 63 or more and less than 66 ×: L value of less than 63 Table 1 shows the composition of the surface conditioning liquid, the treatment liquid for forming an amorphous film, the organic resin film composition, the thickness of each film It is described in Table 2.

  As shown in Table 1 and Table 2, the evaluation materials of the examples have good corrosion resistance and appearance color tone. Among the evaluation materials of the examples, those satisfying the second invention range are more excellent in corrosion resistance. On the other hand, the evaluation material of the comparative example is inferior in appearance color tone.

Claims (2)

In forming an amorphous film by applying and drying an acidic aqueous solution containing one or both of phosphoric acid and primary phosphate on the surface of the electrogalvanized steel sheet, among the phosphoric acid and primary phosphate, Before applying an acidic aqueous solution containing one or both, the surface is adjusted with an aqueous solution containing a condensed phosphate and having a solid particle concentration of less than 1.0 g / L. A method for producing an excellent surface-treated steel sheet. The first phosphate is one or a mixture of two or more of Mg salt, Mn salt, and Al salt, and the Mg / Mn salt has a metal / P ₂ O ₅ molar ratio of 0.60. The method for producing a surface-treated steel sheet having excellent corrosion resistance and appearance color tone according to claim 1, wherein the metal / P ₃ O ₇ molar ratio is 1.02 to 1.20 for an Al salt of ˜0.98.