JPS6092483A - Production of galvanized steel sheet - Google Patents

Abstract

PURPOSE:To improve chemical convertibility by chromate or phosphate, etc. by polishing preliminarily at least one plated surface of the steel sheet plated with Zn on both surfaces by means of a polishing material contg. abrasive grains. CONSTITUTION:Zn is plated by electroplating or hot dipping on both surfaces of a steel sheet and thereafter the steel sheet is subjected to a chemical conversion treatment such as chromate treatment or phosphate treatment for the purpose of improving corrosion resistance and paint adhesion. At least one surface of the galvanized steel sheet is polished with a polishing material such as a brush roll, scotch bright roll or the like contg. abrasive grains consisting of about #100-1,000 alumina or silicon carbide to activate uniformly the entire surface prior to said chemical conversion treatment. The chemical conversion film is thereafter formed thereon by the chromate treatment, phosphate treatment, etc. The uniform chemical conversion film free from unevenness is formed on the galvanized surface, by which the uniformity of the succeeding painting and the corrosion resistance of the painting are improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a zinc-based plated steel sheet, and particularly to a method for producing a galvanized steel sheet with excellent chemical conversion treatment properties.

BACKGROUND OF THE INVENTION Due to its excellent corrosion resistance, so-called galvanized steel sheets plated with zinc or zinc alloy are widely used in fields such as home appliances, building materials, and automobiles.

The production of these galvanized steel sheets can be roughly divided into electroplating methods and hot-dip plating methods, and either method is selected in consideration of the quality and economic efficiency required for the product. Recently, in the automobile field, the use of electrical alloy-plated steel sheets such as zinc-nickel and zinc-iron, which have good corrosion resistance even at low coating weights, has been increasing in order to prevent corrosion of car bodies caused by antifreeze agents sprayed on roads. .

In many cases, galvanized steel sheets are subjected to reactive chemical conversion treatment such as chromate or phosphate immediately after being shipped to the user or after being plated at the steel sheet manufacturer for the purpose of improving corrosion resistance and paint adhesion. will be applied. At this time, uneven treatment often occurs due to non-uniformity of the plated surface, dirt, etc., and an insufficient amount of chemical conversion film is generated. If this kind of development occurs, it becomes a big problem not only when the chemical conversion treatment is used as a product but also when it is painted, as it leaves traces of unevenness and deteriorates corrosion resistance and paint adhesion.

Particularly in the field of automobiles, double-sided plating is increasingly being used for exterior panels as a measure against rust, but when phosphate treatment causes unevenness, so-called three-coat coatings (electrodeposition, intermediate coating, and top coating) are being used. Even after it's done, the paint will still be uneven. Therefore, for galvanized steel sheets for exterior panels where a beautiful finished appearance is an essential condition, uneven chemical conversion treatment poses a serious problem.

In the case of electroplating, chemical conversion treatment defects such as insufficient amount of uneven film produced in reactive chemical conversion treatment are caused by stains and non-uniformity on the plating surface due to the following causes.

(a) Dirt in the washing solution after plating and unevenness during washing and drying.

(b) Partial drying of the board surface when passing between the plated electrodes.

(c) Scratching due to energized rolls, cell dam rolls, etc.

2) Unevenness in the electrodeposition state due to non-uniform plating conditions such as variations in liquid flow and distance between electrodes.

(e) Effect of impurities in the plating solution.

In particular, plating is usually carried out in a bath at 50° C. or higher, and in addition to the fact that the plated surface immediately after electrodeposition is very active and highly reactive, the temperature of the plate increases while it is passed through the bath. For this reason, it reacts sensitively to dirt in the washing water after plating and non-uniformity in the plating solution squeezing, water washing and water squeezing drying process, and the plated surface is likely to become dirty or uneven.

On the other hand, in the case of hot-dip plating, defects in chemical conversion treatment occur due to the following reasons.

(a) Zinc oxide on the surface.

(b) Surface defects due to zinc scum, etc.

(c) Surface unevenness due to non-uniform wiping performed immediately after plating.

2) Unevenness in the alloy that is heated and alloyed after plating.

Conventionally, many methods have been proposed to improve the chemical conversion treatability of galvanized steel sheets. That is, JP-A-52-6883
1 is a pretreatment step of chromate treatment at a concentration of 10 to 25 f of one or more of sodium hydroxide, sodium carbonate, sodium peroxide, and sodium hydroxide.
/l of aqueous alkaline solution with hydrogen peroxide solution added to solution temperature 4
This is a method of activating the galvanized surface with a treatment solution at 0 to 60°C, and Japanese Patent Application Laid-Open No. 56-23279 discloses that zinc or galvanized products before chemical conversion treatment are activated at temperatures of 0.3 to 10°2 F.
This is a zinc surface activation treatment method in which the surface is treated with an alkaline solution containing /l of hydrogen peroxide to remove the oxide film on the surface.

However, all of these methods use chemicals to activate the surface before chemical conversion treatment, and are complicated in bath management, replenishment, and waste liquid treatment. Furthermore, since the baths for activating the zinc surface are used in a heated state, the temperature of the zinc surface increases, and the activated surface is heated during the process of chemical squeezing, water washing, water squeezing, and drying. , the risk of uneven staining and unevenness again cannot be ignored.

In addition, JP-A-49-134530 discloses that in chemical conversion treatment of galvanized steel sheets, degreasing is performed in an alkaline degreasing tank incorporating a brush roll, or alkaline degreasing is performed after washing in a water washing tank incorporating a brush roll. Discloses a method of performing pretreatment, washing with water, and chemical conversion treatment. This case also involves the above-mentioned problems since an alkaline degreasing solution at a temperature of 50 to 60° C. is used. Furthermore, all of the above methods focus on removing oxides from the very surface layer of zinc, and remove dirt on the zinc surface embedded between crystals, uneven electrodeposition, or molten metal. K
It is not effective against stains or irregularities that have a certain depth in the plating layer, such as gas leaks in the plating layer.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for manufacturing a zinc-based plated steel sheet that can strongly improve the chemical conversion treatment properties of the plated surface at room temperature and without using chemicals.

The gist of the present invention is as follows.

That is, a method for manufacturing a zinc-based plated steel sheet in which the plated surface is subjected to chemical conversion treatment after the plating treatment is characterized by comprising the step of polishing at least one plated surface with an abrasive containing abrasives after the plating treatment. This is a method for manufacturing zinc-based plated steel sheets.

In the present invention, zinc-based plated steel sheets include electrogalvanized steel sheets and their heat-alloyed steel sheets, or zinc-nickel, zinc-iron, and other electrozinc alloy plated steel sheets.
Further, it includes hot-dip galvanized steel sheets, heat-alloyed steel sheets thereof, hot-dip zinc alloy-plated steel sheets alloyed with aluminum, magnesium, etc. The present invention also covers electroplated steel sheets in which zinc-iron is plated on zinc-nickel or zinc-nickel is plated on nickel. Furthermore, these plated steel sheets include single-sided plated materials and double-sided plated materials, both of which are included. When double-sided plating is used for the exterior of automobiles, the uniformity of the chemical conversion treatment is particularly important on the surfaces corresponding to the exterior, as described above, but this is not so strict for the interior. The invention applies to at least one plating surface.

In the present invention, after the above-mentioned zinc plating treatment, the plated surface is polished with an abrasive containing abrasives. As the abrasives containing abrasives, a nylon brush containing abrasives, Scotchbride, a grinding belt, etc. are applicable. The abrasive grains used are alumina, silicon carbide, or the like. For example, in the case of a brush roll, the ability of the abrasive varies depending on the particle size of the abrasive grains, wire diameter, reduction blade, rotation speed, number of brushes, etc. If the polishing power is increased too much, the applied area weight will decrease and the corrosion resistance will deteriorate. Therefore, the polishing reduction should be about 10% or less of the applied area weight. It must also be able to be removed. Furthermore, if the abrasive grain size is too coarse, polishing scratches may become noticeable. Although pure zinc plating has a soft hardness, zinc alloy plating such as zinc-nickel and zinc-iron is much harder. Therefore, the type of abrasive and the polishing conditions must be set in consideration of the type of plating to be applied and the line speed ring passing conditions. Note that it is essential to polish while applying water to prevent damage to the polishing material due to frictional heat. According to the experience of the present inventors, brush rolls containing abrasive grains of about 100 to 1000 or Scotch Bride rolls containing abrasive grains of about 150 to 1000 have uniform activity for both zinc plating and zinc alloy plating. This was suitable for obtaining a surface.

Upon completion of the polishing process using the abrasive-containing abrasive material that characterizes the present invention as described above, the polished surface is immediately washed with water, and if chemical conversion treatment is performed in-line, it is left as it is or once dried. Enter the chemical treatment tank. In addition, when chemical conversion treatment is performed on the user's side, after washing and drying with water, oil is applied to prevent oxidation of the activated mating surface, and after being pressed on the user side, chemical conversion treatment is performed.

The effect of polishing with the abrasive material containing abrasive grains of the present invention is not only to remove dirt and oxides on the very surface layer, but also to remove dirt embedded between plating crystals and unevenness of coated crystals, etc. According to K, it is possible to remove even defects with oxidation, resulting in a uniform and highly active surface.

In addition, the method of the present invention makes it possible to stably and easily obtain a zinc-based plated surface with excellent chemical conversion treatability by properly managing the abrasive material. Furthermore, since this is a processing method that does not use any chemicals and can be carried out at room temperature, the surface that has been activated during normal washing with water or washing and drying after polishing will not become dirty or oxidized again.

From the above, when the polished zinc-based plating surface of the present invention is subjected to chemical conversion treatment with chromate or sinate, etc., a uniform and dense chemical conversion film is formed, and treatment unevenness and film formation are prevented. It is possible to avoid chemical conversion defects such as insufficient quantity or coarse phosphate crystals called walleye.

EXAMPLES The effectiveness of the present invention will be explained below using four types of examples and four types of comparative examples corresponding to the respective examples.

Example 1 Plate thickness 0.8 mm, plating area weight 20 f/r111
The surface of the electrogalvanized steel sheet was polished with a nylon brush roll containing alumina abrasive grains under the conditions shown in Table 1, washed with water, and dried, followed by chromate treatment under the conditions shown in Table 1.

Comparative Example 1 The plated surface of the same steel plate as in Example 1 was polished with a nylon brush roll containing no abrasive grains, and the same chromate treatment as in Example 1 was performed. Comparative Example 1 was processed in exactly the same manner as Example 1, except that the polishing brush did not contain abrasive grains.

Example 2 Plate thickness 1゜2 nun, plating area weight 45 f'/m
Alloying Treatment The surface of the hot-dip galvanized steel sheet was polished with a nylon brush roll containing silicon carbide abrasive grains under the conditions shown in Table 1, washed with water, and dried, followed by chromate treatment under the conditions shown in Table 1. Ta.

Comparative Example 2 The same steel plate as in Example 2 was immersed in a sodium orthosilicate-based alkaline degreasing solution (degreaser concentration: 5%) at 70°C for 10 seconds, washed with water, dried, and then subjected to the same chromate treatment as in Example 2.

Example 3 The surface of an electrolytic zinc-nickel alloy plated steel sheet with a plate thickness of 0.7 nwu, a plating weight of 30 f/♂, and a nickel content of the plating layer of 12 t was polished under the conditions shown in Table 1, and after washing and drying. Phosphate treatment was carried out under the conditions shown in Table 1.

Comparative Example 3 The same plated steel plate as in Example 3 was subjected to sic acid salt treatment under the conditions shown in Table 1 without being treated.

Example 4 The surface of a hot-dip galvanized steel plate with a thickness of 1.01 mm and a coating weight of 120 r/m was polished with a Scotch Bride roll containing alumina abrasive grains under the conditions shown in Table 1, washed with water, and dried. Phosphoric acid treatment was performed under the conditions shown in Table 1.

Comparative Example 4 The plated surface of the same steel plate as in Example 4 was polished with a nylon brush roll containing no abrasive grains under the same conditions as in Comparative Example 1, washed with water and dried, and then subjected to the same phosphoric acid treatment as in Example 4.

After observing the external appearance of the eight types of steel plates subjected to the above treatment, the amount of chromium deposited in Examples 1 and 2 and Comparative Examples 1 and 2 was measured, and the corrosion resistance was investigated by a salt spray test.

Regarding Example 3.4 and Comparative Example 3.4, the amount of chemical conversion film was measured, and the melamine alkyd paint was coated with a bar coater so that the film thickness after baking was 15μ, and the coating was applied at 150°C for 10 After baking for a few minutes, a corrosion resistance test using a salt spray test and a paint adhesion test using Erichsen substrate were conducted. These results are shown in Table 2. In addition, bare corrosion resistance is determined by JIS Z2371 Salt Water Spray Test 1 Salt Water Spray Test White Pig Area Corrosion resistance is expressed by the width of the blister bulge in the scratched area after 240 hours of the salt spray test. The adhesion of the paint film was determined by carving 100 base marks at intervals of IITfn on the paint surface, then extruding it at 7mi+ using an Ecxen tester, adhering sellotape to the extruded surface and peeling it off, and measuring the area where the paint film peeled off. is shown in chi.

From Table 2, in the case of chromate treatment, Example 1 of the present invention, in which the plated surface was polished with an abrasive containing abrasive grains before chromate treatment,
2 is polished with an abrasive containing abrasives, and compared to Comparative Examples 1 and 2, the bechromate is evenly finished, and the surface is active, so the amount of chromium deposited (chemical conversion coating) is large. It can be seen that the corrosion resistance is also clearly excellent.

Further, Example 3 also applies when phosphate treatment is performed.

4 is the same as in the case of chromate treatment, each comparative example 3
, 4, it can be seen that the chemical conversion treatment is finished more uniformly. Further, the reason why the amount of the chemical conversion coating in Examples 3 and 4 is lower than that in Comparative Examples 3 and 4 is that the amount of the chemical conversion coating is relatively low because the sulfate crystals with close edges are formed in a dense manner. Note that Examples 3 and 4 are clearly better in corrosion resistance and paint adhesion after painting than Comparative Examples 3 and 4.

As mentioned above, zinc-based coated steel sheets are subject to causes of uneven surface conditions and inertness during the manufacturing process, and to varying degrees, they all carry the risk of poor chemical conversion treatment. However, as is clear from the above examples, the present invention strongly uniformizes and activates the plated surface by polishing the plated surface with an abrasive containing abrasive grains after plating, resulting in good chemical conversion treatment properties. It was possible to manufacture a zinc-based plated steel sheet having the following properties.

Agent: Patent Attorney Takeo Nakaji

Claims (1)

[Claims] (1) A method for manufacturing a zinc-based plated steel sheet in which the plated surface is subjected to chemical conversion treatment after the plating treatment, characterized by comprising the step of polishing at least one plated surface with an abrasive containing abrasives after the plating treatment. A method for manufacturing galvanized steel sheet.