JP4249092B2 - Method for producing Al-Mg-Si aluminum alloy sheet with excellent corrosion resistance after painting - Google Patents

Description

The present invention relates to a method for producing an Al—Mg—Si based aluminum alloy plate having excellent corrosion resistance after painting, and in particular, an Al—Mg—Si based aluminum alloy plate having excellent corrosion resistance after painting that can be suitably used for automobile outer plates. It relates to the manufacturing method .

  Aluminum alloy materials for automobile outer panels, which are increasing year by year from the viewpoint of weight reduction of automobiles, include formability, shape freezing properties, dent resistance, corrosion resistance, product surface quality that does not cause rough skin and ridging marks in press molding, etc. In order to satisfy such characteristics, conventionally, as an aluminum alloy for an automobile outer plate, an Al-Mg-based (5000-based) alloy or an Al-Mg-Si-based (6000-based) alloy has been used. .

  Among these, since the 6000 series alloy is a heat treatment type alloy, further weight reduction can be expected by the gauge down using the bake hardenability at the time of electrodeposition coating, but the zinc phosphate treatment compared to the 5000 series alloy. There is a problem of inferiority. Normally, in an automobile body assembly process, an aluminum alloy plate is joined to a steel plate component, and then the chemical conversion treatment, zinc phosphate treatment to electrodeposition coating are performed as a continuous process. Zinc treatability is required.

  Aluminum alloy materials for automobile outer plates are used after solution treatment of cold rolled material. In the case of 5000 series alloys, a strong magnesium oxide film is formed on the surface by solution treatment and phosphoric acid. In order to reduce the zinc treatability, improve the zinc phosphate treatability, and improve the adhesion of the electrodeposition coating and the coating corrosion resistance after that, after the solution treatment, final cleaning with an acidic detergent is indispensable. In addition, dedicated cleaning equipment was necessary in the process.

On the other hand, in the 6000 series alloy, in the solution treatment, a strong oxide film is not formed as in the 5000 series alloy, but in consideration of the decrease in the amount of zinc phosphate film produced due to seizure of the oxide film or rolling oil, The final cleaning with acid or alkali is performed, and further, washing with water for removing acid and alkali and drying are carried out to improve the zinc phosphate treatment property and maintain post-coating corrosion resistance equivalent to 5000 series alloys (for example, Patent Document 1).
JP-A-9-195019

  As described above, the inventors further manufactured an Al-Mg-Si-based (6000-based) alloy for automobile outer plates, which can be expected to be lighter by gauge-down, while maintaining the same zinc phosphate treatability as conventional. In order to achieve cost reduction, the amount of oxide film generated in the solution treatment and factors that inhibit the zinc phosphate treatment were investigated, and as a result, the amount of carbon on the alloy plate surface was reduced before the solution treatment. Thus, it was found that the same zinc phosphate treatment characteristics as those obtained when the final washing was performed after the solution treatment as in the prior art can be obtained without performing the washing with the acid or alkali after the solution treatment.

  The present invention has been made as a result of further studies based on the above findings, and the purpose thereof is to eliminate the final cleaning step, and to perform Al-Mg based (5000) that has been subjected to acid cleaning after solution treatment. It is an object of the present invention to provide an Al—Mg—Si-based aluminum alloy plate excellent in post-coating corrosion resistance and capable of obtaining the same zinc phosphate treatability and post-coating corrosion resistance as those of an alloy and a method for producing the same.

Method for producing Al-Mg-Si based aluminum alloy sheet having excellent corrosion resistance after painting of claim 1 for achieving the above object, Al-Mg-Si system is put into use without cleaning after solution treatment a manufacturing method of an aluminum alloy plate, the aluminum alloy plate which is cold-rolled, cold is used between the cold rolling rolling oil, or conductivity 50 [mu] S / cm or less in the purified water over 60 ° C. After washing with heat-purified water heated to temperature, solution treatment was performed, and the total carbon (C) content on the plate surface determined by the combustion-infrared absorption method was 10 mg / m ² or less , detected by fluorescent X-ray analysis. The C detection amount is 0.9 kcps or less .

  According to the Al-Mg-Si-based aluminum alloy plate of the present invention, the final cleaning step can be omitted, and phosphoric acid similar to the Al-Mg-based (5000-based) alloy that has been subjected to acid cleaning after the solution treatment. Zinc processability and post-coating corrosion resistance can be obtained, and a reduction in manufacturing cost can be achieved. On the other hand, according to the method for producing an Al—Mg—Si-based aluminum alloy plate of the present invention, the zinc phosphate treatment property and after coating are the same as those of an Al—Mg-based (5000-based) alloy subjected to acid cleaning after solution treatment. Corrosion resistance can be obtained. The Al—Mg—Si based aluminum alloy plate having excellent post-painting corrosion resistance according to the present invention can be suitably used particularly as a member for automobile hoods, fenders, trunk lids, roofs, doors and the like.

  The present invention contains an Al—Mg—Si based alloy, for example, Si: 0.2 to 2.0% (mass%, the same applies hereinafter), Mg: 0.2 to 1.3% as essential components, As a selective component, one or two of Cu and Zn are contained in a total of 2.0% or less, and if necessary, one of Ti, B, V, Mn, Cr, Zr, and Fe Alternatively, an aluminum alloy containing a total of 1.5% or less of two or more types and the balance Al and impurities is targeted.

The Al—Mg—Si-based aluminum alloy plate according to the present invention is used without washing after the solution treatment, and the total carbon (C) amount on the surface of the aluminum alloy plate determined by the combustion-infrared absorption method is used. It is characterized by being 10 mg / m ² or less. In this case, carbon (C) to be quantified refers to the following.

  That is, the Al-Mg-Si-based aluminum alloy plate according to the present invention is ingot formed by ordinary DC casting, and the obtained ingot is subjected to homogenization treatment, hot rolling, cold rolling according to a conventional method, and then solutionized. It is manufactured through the process of processing, but the rolling oil remains on the surface of the cold-rolled aluminum alloy plate, and the aluminum alloy plate in which this rolling oil remains is used for solution treatment. When it is charged in an annealing furnace, for example, a secondary annealing furnace, and heat-treated, a part of the rolling oil does not volatilize and remains burned on the plate surface. In the present invention, it is C that remains in the state of being baked on the surface of the plate that is to be quantified.

  C remaining in the state of being baked on the surface of the plate continues to remain even when degreasing with a solvent such as degreasing with hexane or degreasing treatment with a degreasing liquid in the previous step of zinc phosphate treatment. C on the surface of the plate is increased by application of rust preventive oil or press oil after solution treatment, but the amount of C to be quantified in the present invention is hardly affected by the oil after solution treatment. .

  C remaining in the state of being baked on the plate surface delays dissolution of aluminum during the zinc phosphate treatment, lengthens the critical time until the reaction starts during the zinc phosphate treatment, and C remains unevenly on the plate surface. As a result, partial reactions occur from time to time, thus making it electrically unstable and also easier to promote growth than nucleation of zinc phosphate, inhibiting uniform growth of the zinc phosphate coating. As a result, the corrosion resistance after coating (yarn rust resistance) decreases.

In the aluminum plate according to the present invention, the above C is quantified by a combustion-infrared absorption method, and the total carbon (C) amount on the plate surface is 10 mg / m ² or less, more preferably 7 mg / m ² or less, and further preferably 6 mg / m ^2. m ² or less, and by controlling the total carbon content within this range, a sufficient amount of the zinc phosphate coating can be uniformly formed without inhibiting the formation of the zinc phosphate coating. Formed and excellent corrosion resistance is obtained after painting.

The total amount of carbon on the plate surface determined by the combustion-infrared absorption method has a very good correlation with the amount of C detected on the plate surface detected by fluorescent X-ray analysis, and is therefore detected by fluorescent X-ray analysis. The C detection amount on the plate surface can be used as a simple index for managing the total carbon amount on the plate surface within a predetermined range, and the C detection amount on the plate surface detected by fluorescent X-ray analysis is 0. If it is 9 kcps or less, more preferably 0.75 kcps or less, the total amount of carbon (C) on the plate surface determined by the combustion-infrared absorption method is restricted to 10 mg / m ² or less.

In order to regulate the total carbon content of the plate surface within the above range, in the present invention, the cold-rolled aluminum alloy plate is washed with the cold rolling oil used in the cold rolling before solution treatment. Then, it is charged into a solution treatment furnace.

  Prior to the solution treatment, purified water having a conductivity of 50 μS / cm or less, more preferably 30 μS / cm or less is heated to a temperature of 60 ° C. or more, more preferably 70 to 75 ° C., and cold rolling is performed with the heated purified water. The plate may be washed and subjected to a solution treatment.

As described above, after the cold rolling oil used in cold rolling or purified water having a conductivity of 50 μS / cm or less is heated with heated purified water heated to a temperature of 60 ° C. or higher , solution treatment is performed. Thus, as in the past, after the solution treatment, the total carbon content on the surface of the plate can be reduced to 10 mg / m ² or less without performing the final cleaning, and the final cleaning is performed with respect to zinc phosphate processing property and corrosion resistance after coating. It is possible to obtain the same characteristics as those of the conventional ones.

  Examples of the present invention will be described below in comparison with comparative examples. These examples show one embodiment of the present invention, and the present invention is not limited to these examples.

Example 1
An aluminum alloy having the composition shown in Table 1 was ingoted by DC casting, and the resulting ingot was chamfered, and then homogenized at 540 ° C. for 5 hours. Subsequently, hot rolling was performed at a starting temperature of 400 ° C., and a plate material having a thickness of 1 mm was obtained by cold rolling.

In order to change the total amount of carbon on the surface of the aluminum alloy plate after solution treatment, the cold rolled plate is heated to cold rolled oil (mineral oil) and purified water having a conductivity of 50 μS / cm or lower to a temperature of 60 ° C. or higher. After changing the conditions with the heated and purified water, it was charged into the furnace and subjected to a solution treatment at a temperature of 550 ° C., cooled to 30 ° C. at an average cooling rate of 10 ° C./s, and then within 30 minutes Was heat-treated at 100 ° C. for 24 hours.

  For the obtained aluminum alloy plate, the total carbon content on the plate surface was quantified by the combustion-infrared absorption method, and the C detection amount on the plate surface was detected by fluorescent X-ray analysis. The correlation is shown in Table 2. As can be seen in FIG. 1 which shows the correlation between Table 2 and Table 2 in a graph, there is a primary relationship between the C detection amount detected by the fluorescent X-ray analysis method and the total carbon amount quantified by the combustion-infrared absorption method. It was confirmed that there was a very good correlation indicated by the function, and that the total carbon content on the plate surface could be easily detected by detecting the C detection amount on the plate surface by fluorescent X-ray analysis.

The aluminum alloy plate heat-treated at 100 ° C. for 24 hours is immersed in a commercially available zinc phosphate treatment agent, and then subjected to cationic electrodeposition coating using a commercially available automotive exterior coating, and baked at 170 ° C. for 20 minutes. Then, a coating film having a thickness of about 20 μm was formed as a test material, and zinc phosphate treatment property and post-coating corrosion resistance were evaluated by the following methods. Details of the test materials and evaluation results are shown in Table 3 and Table 4 , respectively.

Evaluation of zinc phosphate processability: During the preparation of the test material, the potential is measured during the zinc phosphate treatment, and the surface of the treated plate is observed with an electron microscope (SEM), and the coating weight is measured by dissolving the treated coating. Thus, the uniformity of the film (covering area ratio of the film) and the film weight were evaluated. Regarding the film uniformity, the coating area ratio of the film is 90% or more (◎), 85% or more and less than 90% (◯), 80% or more and less than 85% (△), 80% The film weight is less than (x), and the film weight is that of a film weight of 1.3 g / m ² or more (以上), and the film weight of 1.0 g / m ² or more and less than 1.3 g / m ² (○ ), 0.8 g / m ² or more and less than 1.0 g / m ² is (Δ), and less than 0.8 g / m ² is (x).

  Evaluation of corrosion resistance after painting: The electrodeposited surface of the test material was subjected to cross-cut reaching the substrate, and sprayed with salt water for 24 hours in accordance with JIS Z2371, followed by constant temperature (50 ° C), constant humidity (90%) Left for 1 month in an environment of, measure the maximum thread rust length from the cross-cut, and the maximum thread rust length is less than 1 mm (◎), 1 mm or more and less than 2 mm (○), 2 mm Those with a thickness of less than 3 mm were designated as (Δ) and those with a diameter of 3 mm or more as (x).

As seen in Table 4, the test material No. 1, no. 3-4, no. Both 6 is the zinc phosphate treatment, more than 85% area coverage of the film, the film weight showed an excellent zinc phosphate treatment of 1.0 g / m ² or more, less than 1mm maximum yarn rust length It has excellent post-painting corrosion resistance. In particular, the test material No. 1, no. 3-4 , the total carbon content of the plate surface of the test material is 7 mg / m ² or less, the covering area ratio of the zinc phosphate treatment coating is 90% or more, the coating weight is 1.3 g / m ² or more, the maximum yarn rust length The thickness is less than 1 mm, and it has extremely excellent zinc phosphate processability and post-coating corrosion resistance.

Comparative Example 1
After chamfering the aluminum alloy ingot having the composition shown in Table 1 that was ingoted in Example 1, homogenization treatment was performed at 540 ° C. for 5 hours. Subsequently, hot rolling was performed at a starting temperature of 400 ° C., and a plate material having a thickness of 1 mm was obtained by cold rolling.

The cold-rolled aluminum alloy plate is placed in a furnace without washing, subjected to solution treatment at a temperature of 550 ° C., cooled to 30 ° C. at an average cooling rate of 10 ° C./s, and then 30 minutes Within 24 ° C. for 24 hours.

  The aluminum alloy plate heat-treated at 100 ° C. for 24 hours was immersed in a commercially available zinc phosphate treating agent in the same manner as in Example 1, and then subjected to cationic electrodeposition coating using a commercially available automotive exterior plate paint. Baking treatment was performed at 20 ° C. for 20 minutes, and a coating film having a thickness of about 20 μm was formed as a test material. By the same method as in Example 1, zinc phosphate treatment property and corrosion resistance after coating were evaluated. Details of the test materials and evaluation results are shown in Table 5 and Table 6, respectively.

As shown in Table 6, the test material No. In No. 7, the coating area ratio of the zinc phosphate-treated film was less than 85%, and the film formation was slightly non-uniform, and the film weight was reduced to less than 1.0 g / m ^2. The post-corrosion resistance was slightly inferior. Test material No. In Nos. 8 to 9, since the total amount of carbon on the plate surface of the test material is large, the zinc phosphate-treated film is not uniform and the film weight is not sufficient, so that the corrosion resistance after coating is remarkably deteriorated.

It is a graph which plots the value shown in Table 2 by correlating the total carbon amount of the plate surface quantified by the combustion-infrared absorption method and the C detection amount of the plate surface detected by the fluorescent X-ray analysis method.

Claims (1)

A method for producing an Al—Mg—Si-based aluminum alloy plate that is used without washing after solution treatment, wherein the cold-rolled aluminum alloy plate is used in the cold rolling. After cleaning with rolling oil or heated purified water heated to a temperature of 60 ° C. or higher with purified water having a conductivity of 50 μS / cm or less , solution treatment was performed, and total carbon on the plate surface determined by combustion-infrared absorption ( C) An Al—Mg—Si aluminum alloy plate having excellent post-coating corrosion resistance, characterized in that the amount is 10 mg / m ² or less and the C detection amount detected by X-ray fluorescence analysis is 0.9 kcps or less . Production method.

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