Manufacturing method for electrolytic Zn-Ni alloy plated steel sheet

Abstract

To develop a manufacturing technology to increase the metallic sheen of an electrical Zn-ni alloy plating steel plate to make an appearance of the electrical Zn-ni alloy plating steel plate based on dull original plate (assuming an arithmetic mean roughness Ra of 0.5 μm to 2.5 μm) that is commonly used for forming and processing closer to that of cold-rolled steel plate.SOLUTION: In a plating bath containing not less than 0.25 mol / L and not more than 0.5 mol / L of Zn2+ and not more than 0.5 mol / L and not more than 1.0 mol / L of Ni2+, with a Ni2+ / Zn2+ ratio (molar ratio) of not less than 1.5 and not more than 2.5, and a pH of not less than 1.0 and not more than 2.0, a steel plate having an arithmetic mean roughness of Ra not less that 0.5 μm and not more than 2.5 μm is used as a cathode, and the steel plate s electrolytically treated with plating solution at a relative flow velocity of 4 m / sec or more and with a current density of 10 A / dm2 or more and 30 A / dm2 or less and a bath temperature of 40°C or more and 70°C or less, and a Zn-Ni the Ni alloy plating layer the Ni content of which is 10 mass% or more and less than 16 mass%, the specular gloss (60°) of which is 60 or more, and the coating weight of which is 15 g / m2 or more per one side is formed on the surface of the steel plate.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing an electrolytic Zn-Ni alloy plated steel sheet, in which a Zn-Ni alloy plating layer is formed on the surface of the steel sheet by electrolytic treatment in an acidic plating bath using the steel sheet as a cathode. [Background technology]

[0002] Representative electrolytic galvanized steel sheets are electrolytic galvanized steel sheets and electrolytic Zn-Ni alloy plated steel sheets, and are widely used primarily for home appliances, furniture, building materials, and automobiles.

[0003] Electrogalvanized steel sheets are characterized by their uniform and beautiful appearance because they are produced by an electroplating method, but there is always a demand for improvement in the appearance.

[0004] Electrogalvanized steel sheets are often used after undergoing a thin-film chemical conversion treatment, and the original white appearance of the electrogalvanized steel sheet is the mainstream appearance of the final product. Therefore, there have been many studies on increasing the whiteness of the plating appearance.

[0005] Patent Document 1 discloses a technology for improving the whiteness of zinc plating by adding one or more metal ions of Sn, Mo, and Cr, which are electrochemically more noble than zinc and more noble than iron, to an electrolytic zinc plating bath and then performing electrolytic zinc plating.

[0006] Zn-Ni alloy electroplated steel sheets have superior corrosion resistance compared to electroplated zinc steel sheets, and are widely used mainly in automobiles and home appliances. Zn-Ni alloy electroplated steel sheets have a significantly lower whiteness than electroplated zinc steel sheets, and there is no demand for an appearance similar to that of electroplated zinc steel sheets, so there is no demand for improved whiteness.

[0007] On the other hand, electrolytic Zn-Ni alloy plated steel sheets have a higher metallic gloss than electrolytic galvanized steel sheets and have an appearance closer to that of cold-rolled steel sheets. In automotive and home appliance applications, electrolytic Zn-Ni alloy plated steel sheets and cold-rolled steel sheets are often used in combination, and most of these are painted before use. When painting is applied, the appearance after painting is strongly affected by the surface gloss before painting. Therefore, from the viewpoint of appearance uniformity in the final product, there is a potential demand for further improvement in metallic gloss to make the appearance of electrolytic Zn-Ni alloy plated steel sheets closer to that of cold-rolled steel sheets.

[0008] However, no prior art has been found that aims to improve the appearance, particularly the metallic gloss, of Zn-Ni alloy electroplated steel sheets.

[0009] Regarding appearance, the plating coverage is about 5g / m per side. 2 It is possible to obtain electrolytic Zn-Ni plated steel sheets with high gloss if the coating weight is less than this, but the gloss level decreases as the coating weight increases, so the coating weight actually used is 15 g / m 2 In the above range, it has been difficult to achieve a target gloss level of 60 or more, for example.

[0010] It is known that reducing the surface roughness of the steel sheet used for plating is an effective way to improve metallic luster. Zn-Ni alloy electroplating is mostly applied to cold-rolled steel sheets, but the substrate for cold-rolled steel sheets is often classified into two types: bright and dull. Zn-Ni alloy electroplated steel sheets manufactured by normal manufacturing methods using bright base sheets with small surface roughness exhibit metallic luster equivalent to that of cold-rolled steel sheets. However, when bright base sheets are used, the surface roughness is too small, so the friction coefficient of the steel sheet becomes too small during press forming, making forming difficult, and it is not practical for applications where forming is performed.

[0011] Therefore, in applications where forming is performed, which is the main application of Zn-Ni alloy electroplated steel sheets, it is a prerequisite that a dull base sheet is used in the case of cold-rolled steel sheets.

[0012] On the other hand, a technique of adding organic brighteners to the plating bath is widely used as a method of increasing the gloss of the plating surface. However, the addition of a small amount of organic brighteners often changes the gloss significantly, and controlling the brightener concentration is extremely complicated in order to obtain a stable appearance. In addition, brighteners often increase the internal stress of the plating film and reduce the plating adhesion during forming, making them unsuitable for the production of electrolytic Zn-Ni alloy plated steel sheets. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] JP 2006-316329 A Summary of the Invention [Problem to be solved by the invention]

[0014] The objective of this project is to enhance the metallic luster of electrolytic Zn-Ni alloy plated steel sheets, which are made from dull base sheets (with an arithmetic mean roughness of the surface of 0.5 to 2.5 μm) that are commonly used for forming, in order to make their appearance closer to that of cold-rolled steel sheets. The manufacturing technology for this purpose will be developed. [Means for solving the problem]

[0015] As a result of intensive research aimed at solving the above problems, the present inventors have found that in a method of electroplating using a Zn-Ni alloy plating bath, the metallic gloss of the surface of a plated steel sheet can be significantly improved even when a dull base sheet is used, by bringing the plating solution into contact with the steel sheet at a specific flow rate and plating at a specific current density. The surface roughness of a typical dull base sheet is often in the range of 0.5 μm or more and 2.5 μm or less in terms of arithmetic mean roughness Ra.

[0016] The Zn-Ni alloy plating bath used can be a commonly used plating bath. Zn-Ni alloy plating is considered to have the best corrosion resistance when the Ni content of the alloy is in the range of 10% by mass to 16% by mass. Therefore, in order to obtain an alloy with such a composition, the Zn in the plating bath is 2+ The concentration is 0.25 mol / L or more and 0.5 mol / L or less. Ni in the plating bath 2+ The concentration is 0.5 mol / L or more and 1.0 mol / L or less. At the same time, the Ni 2+ Concentration / Zn in plating bath 2+ The concentration ratio (molar ratio) must be 1.5 or more and 2.5 or less.

[0017] The plating bath pH and plating bath temperature are selected within the commonly used ranges, and conditions are selected such that the Ni content of the alloy is 10% by mass or more and 16% by mass or less. The plating bath pH is in the range of 1.0 to 2.0. The plating bath temperature is in the range of 40°C to 70°C.

[0018] The plating bath contains Zn 2+ , Ni 2+ In addition, additives such as a conductivity enhancer may be added.

[0019] As described above, a normal plating bath can be used as the plating bath, but a Zn-Ni alloy plating having excellent gloss cannot be obtained under normal plating conditions.

[0020] The main plating conditions that affect the plating appearance, such as the gloss of plated steel sheet, are thought to be the flow state of the plating solution on the steel sheet surface in the plating bath and the plating current density.

[0021] Typical Zn-Ni alloy plating conditions are: the flow state of the plating solution on the steel sheet surface in the plating bath is evaluated as the relative flow velocity of the steel sheet and the plating solution of 0.5 to 2 m / s, and the plating current density is 50 to 100 A / dm 2 Under these conditions, it is not possible to obtain a Zn-Ni alloy plating with high gloss.

[0022] However, the relative flow velocity between the steel sheet and the plating solution is set to 4 m / s or more, which is not usually selected, and the plating current density is set to 10 A / dm, which is lower than the usual lower limit. 2 More than 30A / dm 2 It has been found that a Zn-Ni alloy plating having excellent gloss can be obtained by the following.

[0023] The arithmetic mean roughness Ra (μm) of a steel sheet can be measured in accordance with JIS B 0633:2001 (ISO 4288:1996).

[0024] The specular gloss can be measured using a commercially available gloss meter, for example, a handy gloss meter PG-1M manufactured by Nippon Denshoku. The measurement is preferably performed at a standard angle of 60° (Gs(60°)). The Zn-Ni alloy plating coating weight is 15 g / m per side from the viewpoint of corrosion resistance. 2 The above is necessary.

[0025] The present invention has been made based on these findings, and has the following gist. [1] By electrolytically treating a steel sheet with an arithmetic mean roughness Ra of 0.5 μm or more and 2.5 μm or less as the cathode in a plating bath, the Ni content on the steel sheet surface is 10 mass% or more and 16 mass% or less, the specular gloss (60°) is 60 or more, and the plating coating weight is 15 g / m per side. 2 A method for producing an electrolytic Zn-Ni alloy-plated steel sheet for forming the above Zn-Ni alloy plating layer, Zinc 2+ Contains 0.25 mol / L or more and 0.5 mol / L or less, and Ni 2+ Contains 0.5 mol / L or more and 1.0 mol / L or less, and Ni 2+ Concentration / Zn 2+ A plating bath having a concentration ratio (molar ratio) of 1.5 to 2.5 and a pH of 1.0 to 2.0 is used, The relative flow velocity between the steel sheet and the plating solution is 4 m / s or more, and the current density is 10 A / dm 2 More than 30A / dm 2 The method for producing an electric Zn-Ni alloy plated steel sheet is characterized in that electrolysis is carried out at a bath temperature of 40°C or higher and 70°C or lower. Effect of the Invention

[0026] According to the present invention, even when a dull base sheet is used as the steel sheet, it is possible to obtain an electrolytic Zn-Ni alloy plated steel sheet with a high glossiness and a specular glossiness (60°) of 60 or more, which is close to the appearance of a cold-rolled steel sheet. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Hereinafter, an embodiment of the present invention will be described.

[0028] A steel sheet having an arithmetic mean roughness Ra of 0.5 μm or more and 2.5 μm or less is pretreated as necessary by degreasing, pickling, and water washing, and then electrolysis is performed in a Zn-Ni alloy electric plating bath using the steel sheet as the cathode to form a Zn-Ni alloy electric plating layer, followed by rinsing and drying.

[0029] As for the electrolytic Zn-Ni alloy plating bath, Zn 2+ Contains 0.25 mol / L or more and 0.5 mol / L or less, and Ni 2+ Contains 0.5 mol / L or more and 1.0 mol / L or less, and Ni 2+ Concentration / Zn 2+ A plating bath with a concentration ratio (molar ratio) of 1.5 to 2.5 and a pH of 1.0 to 2.0 is used, the relative flow velocity of the steel sheet and plating solution is 4 m / s or more, and the current density is 10 A / dm 2 More than 30A / dm 2 Electrolysis is carried out as follows.

[0030] It is known that Zn-Ni alloy plating has the best corrosion resistance when the Ni content is in the range of 10 mass % or more and 16 mass % or less, and therefore it is necessary to manufacture the plating with a Ni content of 10 mass % or more and 16 mass % or less.

[0031] Using the above plating bath, the relative flow rate between the steel sheet and the plating solution is 4 m / s or more, and the current density is 10 A / dm 2 More than 30A / dm 2 By electrolysis at or below this temperature, the plating deposition amount per side is 15g / m 2Even with the above amount of adhesion, a high-gloss Zn-Ni electroplated steel sheet with a gloss level of 60 or more can be obtained. The mechanism behind this is unclear, but the following possibilities are considered. Here, the relative flow velocity between the steel sheet and the plating solution is the flow velocity of the plating solution as seen from a certain point on the moving steel sheet. The flow velocity of the plating solution can be obtained from calculations including the discharge flow velocity of the jet nozzle that supplies the plating solution between the steel sheet and the anode electrode, the position of the jet nozzle relative to the steel sheet, etc. In general, in zinc-based electroplating, a jet nozzle that sprays the plating solution in the opposite direction to the traveling direction of the steel sheet is often installed. In such cases, the relative flow velocity can be calculated simply by adding the jet nozzle outlet jet velocity (plating solution discharge flow rate / nozzle outlet cross-sectional area) and the steel sheet speed.

[0032] It is known that in Zn-Ni alloy electroplating, the Zn-Ni alloy is deposited through a process in which the pH of the plating solution at the interface between the steel sheet and the plating solution increases. Since the relative flow rate is clearly higher than that in normal Zn-Ni alloy electroplating, the convex parts of the steel sheet surface unevenness are constantly exposed to new plating solution with a lower pH, making it difficult for the pH of the plating solution to increase, while the pH of the plating solution in the concave parts is more likely to increase. Therefore, it is thought that the Zn-Ni alloy is preferentially deposited in the concave parts, improving the smoothness of the surface and increasing the gloss.

[0033] For this reason, the relative flow velocity between the steel sheet and the plating solution must be 4 m / s or more. If it is less than 4 m / s, a high gloss cannot be obtained. From the viewpoint of equipment costs such as pumps for supplying the plating solution, the upper limit of the relative flow velocity between the steel sheet and the plating solution is preferably 10 m / s or less.

[0034] The amount of Zn-Ni alloy plating is 15g / m per side from the viewpoint of corrosion resistance. 2 However, an increase in the amount of adhesion leads to an increase in manufacturing costs, so the recommended amount is 80 g / m 2 The following is preferred:

[0035] Zn in solution 2+ is 0.25 mol / L or more and 0.5 mol / L or less, Ni 2+The content must be between 0.5 mol / L and 1.0 mol / L. Below the specified concentration, Zn 2 and Ni 2+ The supply is insufficient, and it is not possible to form electrolytic Zn-Ni plating with a gloss of 60 or more. If the concentration exceeds a certain level, it becomes costly to maintain the plating bath. 2+ Concentration / Zn 2+ The concentration ratio (molar ratio) must be between 1.5 and 2.5. If it is lower than 1.5 or exceeds 2.5, plating with a Ni content of 10% by mass or more and 16% by mass or less cannot be obtained. The pH of the acidic solution must be between 1.0 and 2.0. If the pH is less than 1.0, the amount of hydrogen gas generated increases, reducing the efficiency of electrolysis. If the pH exceeds 2.0, the electrical conductivity of the bath increases, resulting in increased power costs.

[0036] The current density is 10A / dm 2 More than 30A / dm 2 The current density must be less than 10A / dm 2 If the current density is less than 30 A / dm, Ni is preferentially deposited, and it is not possible to obtain plating with a Ni content of 10% by mass or more and 16% by mass or less. 2 If it exceeds this value, the gloss level decreases.

[0037] The plating bath temperature must be between 40°C and 70°C. Although the mechanism is not clear, if the plating bath temperature is less than 40°C or more than 70°C, the specular gloss (60°) will be less than 60.

[0038] A conductivity aid such as sodium sulfate or potassium sulfate may be added to the plating bath. After the plating treatment, the plate is washed with water and dried. The method of washing with water and drying is not particularly limited, and a general method can be used. Depending on the application, an inorganic or organic film may be formed on the plated layer. EXAMPLES

[0039] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

[0040] Using a cold-rolled steel sheet with a thickness of 0.6 mm as a cathode, electrolytic treatment was carried out in a plating bath shown in Table 1 under plating conditions shown in Table 1. Next, the sheet was washed with water and then dried.

[0041] The thus obtained Zn-Ni alloy electroplated steel sheets were subjected to the following evaluations. (1) Evaluation of plating weight and Ni content of plating film For test pieces (size: 50mm x 50mm) taken from electrolytic Zn-Ni alloy plated steel sheets, the plating layer was dissolved in hydrochloric acid, and the Zn and Ni adhesion weights were measured using an ICP emission spectrometer. The plating adhesion weight was the total value of the Zn and Ni adhesion weights, and was shown as the adhesion weight per side. The Ni content in the film was calculated by Ni adhesion weight / (Ni adhesion weight + Zn adhesion weight). Based on this Ni content, evaluation was performed according to the following criteria. ○:Ni content is 10% by mass or more and 16% by mass or less ×: Ni content is less than 10% by mass or more than 16% by mass (2) Evaluation of gloss level Test pieces (size: 150 mm x 50 mm) taken from the Zn-Ni alloy electroplated steel sheets were measured at an angle of 60° using a Nippon Denshoku Corporation handheld gloss meter PG-1M, and were evaluated according to the following criteria. ○: 60 or more ×: Less than 60 The results are shown in Table 1.

[0042] [Table 1]

[0043] As shown in Table 1, in all of the invention examples that satisfied the requirements of the present invention, the Ni content was 10% by mass or more and 16% by mass or less, and the gloss level was 60 or more.

[0044] In contrast, all of the comparative examples that do not satisfy the requirements of the present invention either had a Ni content of 10% by mass or more and 16% by mass or less, or had a gloss level of less than 60, or both, and did not exhibit the desired performance. [Industrial Applicability]

[0045] The Zn-Ni alloy electroplated steel sheet of the present invention has an excellent corrosion resistance, a Ni content of 10 mass% or more and 16 mass% or less, and a coating weight of 15 g / m 2 Despite the above, the plated steel sheet has a metallic luster with a specular gloss of 60 or more, making it useful in applications where corrosion resistance and high gloss are required.

Claims

【Request 1】 By electrolytically treating a steel sheet having an arithmetic mean roughness Ra of 0.5 μm or more and 2.5 μm or less as a cathode in a plating bath, the Ni content on the surface of the steel sheet is 10 mass% or more and 16 mass% or less, the specular gloss (60°) is 60 or more, and the plating coating weight is 15 g / m per side. ２ A method for producing an electric Zn-Ni alloy plated steel sheet having the above Zn-Ni alloy plating layer formed thereon, Zinc ２＋ Contains 0.25 mol / L or more and 0.5 mol / L or less, ２＋ Contains 0.5 mol / L or more and 1.0 mol / L or less, and Ni ２＋ / Zn ２＋ A plating bath containing no brightener and having a molar ratio of 1.5 to 2.5 and a pH of 1.0 to 2.0 is used, The relative flow rate between the steel sheet and the plating solution is 4 m / s or more, and the current density is 10 A / dm ２ 30A / dm or more ２ The method for producing an electrically Zn-Ni alloy plated steel sheet is characterized in that electrolysis is carried out at a bath temperature of 40°C or higher and 70°C or lower.

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