JP2002030457A - Aluminum plated steel sheet excellent in galling resistance and white rust resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum plated steel sheet free from the need of imparting lubricity by a resin film and excellent in galling resistance and white rust resistance. SOLUTION: In this aluminum plated steel sheet, a chromate film or a phosphoric acid-chromium-aluminum composite film in which granular materials essentially consisting of aluminum fluoride are dispersed is deposited on the surface of an aluminum plated layer. Preferably, the granular materials contain aluminum phosphate, potassium fluoride or the like as well as aluminum fluoride as the main component, have the average grain size of 0.01 to 10 μm and are dispersedly precipitated in a ratio of >=5% as area ratio occupied in the surface of the aluminum plated layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum-plated steel sheet which is free from defects such as galling in a plated layer during working such as press working and has excellent white rust resistance.

[0002]

2. Description of the Related Art Aluminum-plated steel sheets are susceptible to white rust and deteriorated appearance when plated as they are. Therefore, white rust resistance is improved by chromate treatment. However, the conventional chromate film has insufficient lubricity, so when processing into a product shape by pressing or the like, galling may occur on the chromate film or plating layer depending on the target shape even if press oil is applied. . The place where galling occurs where the base steel is exposed becomes a starting point of rust generation such as white rust. Galling of the chromate film and the plating layer, which causes rust, can be suppressed by providing a resin film containing a polymer resin powder on the chromate film prior to press working (Japanese Patent Application Laid-Open No. H08-08488).
No. 4,1651, JP-A-8-319550).

[0003]

The resin film suppresses the occurrence of galling by enhancing the lubricity of the surface of the material to be pressed and promoting the flow of metal into the mold. However, when assembling a product on a production line that includes a welding process after press working, the resin film is thermally decomposed at the time of welding to generate smoke and odor, thereby deteriorating the working environment. Also, products made of aluminum-based plated steel sheets are often used for heat-resistant applications by utilizing the properties of the aluminum-based plated layer. However, when a product having a resin film is used in a high temperature atmosphere of about 200 to 350 ° C., the appearance is significantly impaired due to partial discoloration of the resin film.

[0004] In order to avoid an adverse effect caused by the resin film, a method has been proposed in which a film-removal type resin film is adopted and the resin film is removed after press working. But,
Since a film removing step for removing the resin film is required after press working, there are problems in workability and manufacturing cost.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been devised to solve such a problem, and disperses and deposits a particulate material having a lubricating effect on a chromate film or a phosphoric acid-chromium-aluminum composite film. An object of the present invention is to provide an aluminum-plated steel sheet which does not require formation of a resin film, does not generate defects such as galling even when processed, and has excellent white rust resistance.

In order to achieve the object, the aluminum-plated steel sheet of the present invention has a chromate film or a phosphoric acid-chromium-aluminum composite film in which particulates containing aluminum fluoride as a main component are dispersed. It is characterized by being formed in. The granular material contains aluminum phosphate, potassium fluoride, and the like in addition to aluminum fluoride as a main component, and has an average particle size of 0.0
It is preferable that the particles are dispersed and precipitated at a ratio of 1 to 10 μm and an area ratio of 5% or more to the surface of the aluminum-based plating layer. Pure A for aluminum plating
1, Al-Si alloy, Al-Zn alloy, Al-Zn-M
g alloy, Al-Mg-Si alloy, Al-Mn alloy, Al
-Mg alloy and the like. Considering workability such as press working, an Al-Si alloy plating layer containing 5 to 13% by mass of Si is preferable.

[0007]

In the aluminum-plated steel sheet of the present invention, a chromate film or a phosphoric acid chromate film (hereinafter referred to as a chemical conversion coating film) in which granules mainly composed of aluminum fluoride are dispersed and formed is formed on the surface of the aluminum-based plating layer. (FIG. 1). It is considered that the particulate matter is dispersed and deposited on the chemical conversion coating by the following mechanism.

When the chemical conversion solution comes into contact with the aluminum-based plated steel sheet, the surface of the aluminum-based plating layer is activated by the etching action of the chemical conversion solution by fluorine, and a part of Al is ionized and eluted into the chemical conversion solution. I do.
The eluted Al ions react with fluorine in the chemical conversion treatment liquid to form aluminum fluoride AlF ₃ and are dispersed and deposited on a large number of deposition sites on the surface of the aluminum-based plating layer. With the precipitated aluminum fluoride AlF ₃ as the core,
The aluminum fluoride AlF ₃ generated in the subsequent reaction is sequentially deposited, and a granular material grows. The generated and grown granular material contains aluminum fluoride AlF ₃ as a main component, aluminum phosphate AlPO ₄ , potassium fluoride KF and the like.

On the other hand, on the surface of the aluminum-based plating layer other than the deposition sites, the elution rate of Al decreases with an increase in the Al ion concentration near the surface, and poorly soluble reaction products such as CrPO ₄ and AlPO ₄ are deposited. To form a film. When the film is formed, Al is no longer eluted from the surface of the aluminum-based plating layer, and there is no Al supply source necessary for the growth of the granular material. Therefore, the particulate matter does not grow excessively.

The granules dispersed and deposited on the chemical conversion coating prevent the chemical conversion coating from directly contacting the mold during molding such as press working, and also act as a solid lubricant to improve the formability of the aluminum-based plated steel sheet. Let it. As a result, even when the formed aluminum-based plated steel sheet is observed, defects such as galling and cracks are not detected in the chemical conversion coating and the aluminum-based plating layer, and the properties of the chemical conversion coating and the aluminum-based plating layer main body are expressed. A product made of an aluminum-plated steel sheet having excellent white rust resistance can be obtained.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Low-carbon steel, medium-carbon steel,
High carbon steel, alloy steel, etc. are used. Particularly, when good press formability is required, a steel sheet for deep drawing such as a low carbon Ti added steel and a low carbon Nb added steel is preferable. The base steel is hot-dip aluminum plated according to the usual method, pure Al, Al
-Si alloy, Al-Zn alloy, Al-Zn-Mg alloy,
An aluminum plating layer of an Al-Mg-Si alloy, an Al-Mn alloy, an Al-Mg alloy or the like is formed. Above all, considering workability such as press working, 5 to 13% by mass
Al-Si alloy plating layer containing Si is preferable. That is, by including 5% by mass or more of Si in the aluminum-based plating layer, formation of a fragile intermediate layer due to an interfacial reaction between Fe / Al is suppressed, and workability is improved.
However, when the Si content exceeds 13% by mass, the primary crystal Si
The aluminum-based plating layer is easily hardened due to crystallization or the like.

The aluminum-plated steel sheet sent from the hot-dip plating line is then subjected to a chromate treatment or a phosphoric acid chromate treatment. In order to form a sufficient chemical conversion treatment film in a short time, a treatment solution having a total Cr content of 0.1 g / l or more is preferable. However, when a treatment solution containing an excessive amount of Cr exceeding 20 g / l is used, the amount of chromate adhering becomes too large and the resistance weldability decreases.

Fluorine ions are usually contained in the treatment liquid to activate the surface of the aluminum-based plated steel sheet by etching. In the present invention, however, the chemical conversion treatment film formed is made of granular material containing aluminum fluoride AlF ₃ as a main component. A chemical conversion treatment solution having a fluorine ion concentration of 0.1 g / l or more is preferred for dispersion precipitation. However, at a high concentration exceeding 30 g / l, the etching action of the chemical conversion treatment solution becomes strong, and the precipitated particulate matter becomes solubilized. As a supply source of the fluorine ions, a fluoride such as KF, NaF, and NH ₄ F, which easily dissociates the fluorine ions, is preferable. Even a compound such as a silicon fluoride having a small dissociation of fluorine ions can serve as a fluorine ion supply source when used in combination with a fluoride which can easily dissociate fluorine ions.

The concentration and ratio of Al ions and F ions in the chemical conversion treatment liquid have a great effect on the formation of particulate matter. Then, the type of fluoride to be applied, the concentration ratio of fluoride ion to the concentration of chromic acid or phosphoric acid,
By adjusting H and the processing temperature, the particle size and area ratio of the granular material can be controlled. The concentration of F ions in the chemical conversion treatment liquid is preferably adjusted so that the plating layer can be sufficiently etched and does not become excessive enough to solubilize the particulate matter. In order to improve the lubricity and galling resistance of the chemical conversion coating by dispersing and precipitating the granules, it is preferable to adjust the average particle size of the granules to 0.01 to 10 μm and the area ratio to 5% or more. If the average particle size is less than 0.01 μm, the lubricity improving effect is insufficient, and if the average particle size is greater than 10 μm or the area ratio is less than 5%, the granular material tends to fall off during press working of the aluminum-based plated steel sheet. On the contrary, the galling resistance decreases.

When phosphate ions are added to the chromate treatment solution, poorly soluble CrPO ₄ and AlPO ₄ are generated and precipitated on the surface of the plating layer, and a phosphate-chromium-aluminum composite film exhibiting more excellent white rust resistance. Is formed. The improvement of the white rust resistance becomes remarkable when the mass ratio of P / total Cr is 0.1 or more. However, if the coating composition contains an excessive amount of AlPO ₄ , unreacted hygroscopic phosphoric acid is likely to remain in the coating and, on the contrary, the white rust resistance is reduced. Preferably, it is set to zero.

[0016]

EXAMPLE A 0.8 mm-thick ordinary steel sheet was used as a plating base sheet, and after reduction annealing, at a line speed of 100 m / min and a bath temperature of 6 mm.
It was introduced into a hot-dip aluminum plating bath at 40 ° C. and 9.5% by mass of Si, and the steel sheet pulled up from the hot-dip aluminum plating bath was gas-wiped to adjust the coating weight to 35 g / m ² per side. The obtained aluminum-plated steel sheet has an average layer thickness of 13 μm containing Si: 9.0% by mass.
m of Al-Si alloy plating layer was formed.

Specimens were cut out from aluminum-plated steel sheets, sprayed with reactive chromate treatment liquids of various compositions shown in Table 1, and the excess treatment liquid was removed with a rubber roll, followed by drying. When the surface of the chromate-treated test piece was observed, as shown in FIG. 2 showing the test piece surface of Test No. 4, many particles were dispersed and deposited on the formed chromate film. The average particle size of the granular material and the area ratio of the granular material to the surface of the plating layer differed depending on the composition of the treatment liquid.

[0018]

The test pieces on which the various chemical conversion coatings were formed were subjected to the following galling resistance test, bare corrosion test, and resistance welding test. Galling resistance test: Using a mold heated to 80 ° C. assuming a temperature rise during continuous press molding, and using a press oil of 1 g / m 2
After application to the test piece at a ratio of ² , the test piece was formed into a cylindrical shape at a draw ratio of 2.0. Under these conditions, 20 test pieces were continuously formed. The fluorescent X after the molding test was performed on the side wall portion of the last twentieth molded product that had undergone the most severe processing.
The degree of damage to the chemical conversion coating was measured by the difference in linear strength, and the degree of damage to the plating layer was measured by observing the cross section. When the degree of damage to the chemical conversion coating or the aluminum-based plating layer is less than 20%, ◎, 20 to 40%, ４０, 40 to 60%, and 60%
The above was evaluated as x, and the galling resistance was evaluated.

Bare corrosion test: After continuous spraying of 5% salt water for 100 hours in accordance with JIS Z2371, the surface of the test piece was observed to examine the occurrence of white rust. When the area ratio of white rust in the whole test piece is less than 3%, ◎, 3 to 10%, ○, 10 to 3
The bare corrosion resistance was evaluated by setting 0% to Δ and 30% or more to ×. Resistance welding test: Two superposed test pieces were spot-welded using a Cr-Cu alloy electrode. As the welding conditions, a method was used in which an appropriate current and an appropriate load were determined in advance for each test piece, and the welding current was increased at a fixed rate at each fixed point. With the increase in the number of welding spots, Al in the plating layer adheres to the Cr-Cu alloy electrode, making it impossible to perform welding due to a decrease in current density due to damage to the tip of the electrode. Sex can be determined.溶 接: 500 to 1000 welding points, ×: 500 or less
The resistance weldability was evaluated.

As can be seen from the results of the investigation in Table 2, the galling resistance was insufficient in Comparative Examples 1 to 3 in which no particulate matter was dispersed and precipitated, whereas in Examples 1 to 10 of the present invention, the galling resistance was excellent. Anti-galling property was shown. Above all, the average particle size is 0.4 μm
In the case where the above granular material is dispersed in an area ratio of 25% or more,
Little galling was detected. Further, Comparative Example 1 having a relatively large amount of phosphate ions was inferior in bare corrosion resistance, and Comparative Example 3 having a high Cr adhesion amount was insufficient in resistance weldability.

[0022]

[0023]

As described above, the aluminum-plated steel sheet of the present invention has a chemical conversion coating formed by dispersing and depositing particles having a solid lubricating action on the surface of the aluminum-based plating layer. Even when processing under harsh conditions, the smooth inflow of the material into the mold is ensured, and it is processed into the product shape without generating defects such as galling and cracks in the chemical conversion coating and the aluminum-based plating layer. You. Therefore, the aluminum-based plating layer effective for heat resistance and corrosion resistance and the chemical conversion coating effective for white rust resistance are maintained in a healthy state, and a product excellent in durability can be obtained. In processing, a resin film for imparting lubricity is not required unlike the prior art, so that film removal after processing into a product shape is unnecessary, and the production process is simplified.

[Brief description of the drawings]

FIG. 1 is a view for explaining that particulate matter is dispersed and deposited on a chemical conversion treatment film.

FIG. 2 is a photograph showing the surface condition of a chemical conversion coating formed in an example.

Continuing from the front page (72) Inventor Yukihiro Morita 5th Ishizu Nishimachi, Sakai City, Osaka Nisshin Steel Co., Ltd. (72) Inventor Hirofumi Takezu 5th Ishizu Nishimachi, Sakai City, Osaka Nissin Steel Technical Research In-house F-term (reference) 4K026 AA02 AA09 AA22 BA06 BA07 BA11 BB08 BB09 BB10 CA16 CA20 CA23 CA28

Claims (3)

[Claims] 1. A chromate film or a phosphoric acid-chromium phosphate on which particulate matter mainly composed of aluminum fluoride is dispersed and deposited.
An aluminum-plated steel sheet having excellent galling resistance and white rust resistance, characterized in that an aluminum composite film is formed on the surface of an aluminum-based plating layer. 2. The aluminum-plated steel sheet according to claim 1, wherein the particulate matter having an average particle size of 0.01 to 10 μm is dispersed and precipitated at a rate of 5% or more as an area ratio occupying the surface of the aluminum-based plating layer. 3. An aluminum-based plating layer comprising Si: 5-1.
2. An Al-Si alloy plating layer containing 3% by mass.
The aluminum-based plated steel sheet according to the above.