JPS61272400A - Manufacture of hot dipped steel sheet - Google Patents

Abstract

PURPOSE:To obtain a hot dipped steel sheet having superior suitability to chemical conversion treatment and superior corrosion resistance after painting by subjecting a hot dipped steel sheet to electrolysis including anodizing or by immersing the steel sheet in an acidic soln. CONSTITUTION:A steel sheet coated with Zn or an Zn alloy by hot dipping is subjected to electrolysis including essentially anodizing in an electrolytic soln, to strip and remove metal segregated on the steel sheet. The steel sheet may be immersed in an acidic soln. Thus, the suitability of the hot dipped steel sheet for an automobile to chemical conversion treatment and the corrosion resistance after painting are considerably improved, so a significant industrial effect is produced.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for producing hot-dip galvanized steel sheets, and in particular, from the viewpoint of chemical conversion treatment properties and post-painting corrosion resistance improvement, steel sheets for building materials and steel sheets for automobiles are used. The present invention relates to a method of manufacturing a hot-dip galvanized steel sheet suitable for use as a hot-dip galvanized steel sheet.

[Conventional technology and problems]

In recent years, materials used in the field of building materials, such as roofing and wall materials, are required to have a longer life and higher performance.

Conventionally, galvanized iron sheets have been mainly used in this field. The reason for this is that corrosion products generated in a corrosive environment protect the interior, and Zn has a sacrificial anticorrosion effect and suppresses the elution of iron. This is also because Zn itself is relatively inexpensive and can be easily plated.

However, in response to the increase in Zn corrosion loss due to the recent deterioration of the usage environment, there is no other way than to increase the plating area weight due to the corrosion prevention mechanism, and this method has its own limitations.

Under these circumstances, various alloy hot-dip coated steel sheets have been developed with the aim of having a longer life and higher performance than galvanized iron sheets.

For example, Zn-At-81 hot-dipped steel sheet as seen in Japanese Patent Publication No. 46-17161, or Zn-
ht-Mische metal (Ra, Co) type hot-dip plated steel sheet, or Zn-At-X (X: Mg, Ca, Be, Ti,
There are Cu) type plated steel sheets, Zn-Mg type plated steel sheets, etc.

However, although these new hot-dip galvanized steel sheets have slightly improved bare corrosion resistance compared to Zn-plated steel sheets, they are often inferior in chemical conversion treatment properties and corrosion resistance after painting.

On the other hand, surface-treated steel sheets for automobiles have traditionally been produced in North America,
In Canada, Scandinavia, and other countries, large amounts of rock salt are sprinkled to melt snow on the roads, which exposes automobile bodies to an extremely corrosive environment.

So far, electrolytic Zn plating, hot-dip Zn plating, alloyed hot-dip Zn galvanized steel sheets, etc. have been mainly used as materials that are resistant to such corrosive environments. However, the corrosion resistance after painting is not necessarily sufficient, and various safety problems have arisen, such as red rust forming within a relatively short period of time, damaging the appearance and forming holes. Under these circumstances, efforts have been made to develop surface-treated steel sheets that can withstand this salt damage and have post-coating needle corrosion resistance.

For example, these include Zn-N1-Co system, Zn-Fe system, Zn-Ni system, F
e-Zn-N1 series, Zn-At series, Zn-Mg series, Zn
-Electroalloy plated steel sheets such as Tl type are known.

However, even though the naked corrosion resistance of these materials is slightly improved, the phosphate treatment properties and the corrosion resistance after painting may be inferior. Therefore, various methods are known to improve the chemical conversion properties of both surface-treated steel sheets for building materials and surface-treated steel sheets for automobiles.

For example, "Iron and Steel Manufacturing Law No. 4" published on June 30, 1971,
As described in page 234 of ``Bunshu'', methods to improve phosphate treatment include spraying a suspension of phosphoric acid crystals before phosphate treatment, and adding heavy metal salts to the phosphate treatment solution. There are known methods to do this.

However, these methods have some disadvantages in terms of equipment and cost, and although they improve the phosphate treatment to some extent, they are by no means sufficient.

Under these circumstances, some of the inventors proposed a method for greatly improving the phosphatizing properties and post-painting corrosion resistance of the above-mentioned new alloy electroplated steel sheets for automobiles.
In JP 8-210194, after electroplating, the plated steel sheet is subjected to cathodic electrolysis treatment, anode-cathode electrolysis treatment, in an acidic, neutral, or alkaline solution.
It was proposed to perform one or more of Kando anode electrolytic treatment, anode treatment, and immersion treatment. The purpose of this was to remove the oxide layer generated on the surface of the plating layer during the electroplating process.

On the other hand, in the case of hot-dip galvanized steel sheets, the formation of the plating layer goes through a metallurgical process that is completely different from the electrochemical process that produces a uniform plated metal structure as in the case of electroplating. That is, the plating metal is melted in advance, the molten metal is passed through the steel plate, and the molten metal is then drawn to a predetermined basis weight and then solidified. As a result, the plated metal develops a solidified structure.

During this process, the plated metal is exposed to the atmosphere at a relatively high temperature, so naturally the plated surface oxidizes and oxides are formed, but the inventors have conducted various studies and found that the formation of such oxides. In addition, although the cause is not fully clear, it is probably because the plating metal becomes a solidified structure that various elements added to the bath, such as Pb, At, Mg, Si, and S.
It has been found that n, Ti, etc. may segregate on the surface in a metallic state.

If such metals exist on the surface, they act as if they are inhibiting the reaction between the chemical conversion treatment solution and the plating metal, and therefore, simply removing the oxide film will not improve chemical conversion treatment properties or paint corrosion resistance. However, there was still room for improvement in this regard.

[Means to solve the problem]

Therefore, the present inventors conducted various electrolytic treatments and treatments on various hot-dipped steel sheets using the technology proposed in the above-mentioned Japanese Unexamined Patent Publication No. 58-210194, and newly confirmed the evaluation of chemical conversion treatment properties and paint corrosion resistance. We conducted an experiment. The type of melt-plating, processing conditions, and evaluation results in this case are shown in Table 1. The steel plates used were a Zn-At-Mg alloy plated steel plate, a Zn-Mg alloy plated steel plate, and an alloyed hot-dip Zn plated steel plate. All plate thicknesses are 0.8 mm s
The basis weight is 90, 100, and 40777m, respectively.
It is. The chemical conversion treatment was a normal Zn phosphate dipping treatment. As shown in the same table, although almost no silica oxide film was observed by Auger electron spectroscopy in the products subjected to cathodic electrolytic treatment, chemical conversion treatment properties and paint corrosion resistance did not improve. . On the other hand, only those subjected to anodic electrolytic treatment showed significant improvements in both chemical conversion treatment properties and paint corrosion resistance.

Furthermore, for similar test materials, 501
When we conducted an experiment similar to that shown in Table 1 on samples immersed in a sulfuric acid aqueous solution with a concentration of 7/It for 10 seconds, we found that the chemical conversion treatment properties and paint corrosion resistance were greatly improved in the same way as with anodic electrolytic treatment. did.

As a result of the above, in the production of hot-dip galvanized steel sheets, hot-dip galvanized steel sheets can be plated by either electrolytic treatment that always includes at least 7-node electrolysis in an electrolytic solution or acidic solution immersion treatment. It has been found that not only the heterogeneous layer existing on the surface layer, that is, the dispersion film, but also the segregation layer of added elements can be removed, and chemical conversion treatment properties and paint corrosion resistance are greatly improved.

Table 1 [Structure 1 of the invention] The present invention has been made based on the above-mentioned knowledge, and the gist thereof is that in the production of hot-dip galvanized steel sheets, at least a coated steel plate is heated in an electrolytic solution after hot-dipping. The present invention provides a method for producing a hot-dip galvanized steel sheet, characterized by performing either an electrolytic treatment that necessarily includes an anode electrolytic treatment or an acidic solution immersion treatment.

The present invention will be explained in detail below.

First of all, the hot-dip galvanized steel sheets to which the present invention is directed are the aforementioned Zn-plated steel sheets, Zn-At-8i galvanized steel sheets, and Zn-At-Mitshu metal galvanized steel plates.

It includes all plated steel plates that undergo a hot-dip galvanizing process, such as Zn-At-Mg-based plated steel sheets, Zn-Mg-based plated steel plates, and 1-alloy hot-dip Zn-plated steel plates. In the present invention, anode electrolytic treatment refers to treatment in which the object to be treated is electrolyzed in an electrolytic solution at a predetermined current density using it as an anode, and acidic solution immersion treatment refers to treatment in which the object to be treated is immersed in an acid solution with a predetermined concentration. Refers to the process of soaking in water.

Next, the reason why the electrolytic treatment must include at least an anodic electrolytic treatment will be described.

That is, in the above-mentioned through-coating and hot-dip galvanizing steel sheets, the plating layer becomes a solidified structure, and various additive elements may segregate on the surface as metals.

In this case, the oxide layer formed on the surface is subjected to anodic electrolytic treatment, 7-node cathode electrolytic treatment,
It is possible to remove metals by cathode-anode electrolysis treatment, cathode electrolysis treatment, or immersion treatment, but as is clear from the above experiment, when removing metals segregated on the surface by electrolysis, it is necessary to use these methods. This method performs anodic electrolysis, which is effective in removing metals.

The reason why only anodic electrolysis is effective in this case is not fully clear, but it is probably because anode electrolysis is sufficiently active to dissolve the plated metal, which causes the plated metal around the segregated metal to dissolve. It is thought that this causes the segregated metal to fall off and be removed.

Since only the anodic electrolytic treatment is effective as described above, it is essential that the electrolytic treatment includes anodic electrolysis. However, as long as anodic electrolysis is included, the treatment may be performed in combination with cathodic electrolysis. By the above means,
It is thought that by removing these segregated metals, the surface of the plating layer becomes chemically active and improves chemical conversion treatment properties, which in turn improves the corrosion resistance after painting.

The solution used for anode electrolysis is not particularly limited, but if an alkaline solution is used, the eluted metal ions will react with the hydroxide ions generated by the electrolysis of water during electrolysis, resulting in the formation of precipitates. It is preferable to avoid it because it may adhere to the plated surface and deteriorate chemical conversion treatment properties and post-painting puncture resistance.

Further, the electrolytic conditions are not particularly limited, but if the current density is too high, uniform dissolution will not occur and the appearance of the plating will be significantly impaired, so a current density of about 1 to IOA/dm2 is appropriate.

Furthermore, metals segregated on the surface can be removed by immersion in an acid solution, probably by a mechanism similar to that of anode electrolysis, and the type of acid is not particularly limited, but for example, Ti, Pb, etc. Since it may take a long time to dissolve, it is effective to appropriately select the type and concentration of the acid depending on the element to be segregated.

The effects of the present invention will be explained in more detail below using Examples.

〔Example〕

The chemical conversion treatment property and post-painting needle corrosion properties were evaluated for coated steel sheet specimens having the coating layer composition shown in Table 2, which were subjected to electrolytic treatment under the treatment conditions such as electrolyte and current density shown in the table. investigated.

The results are also listed in the same table. The * mark in the same table is a comparative material.

The evaluation method is as follows.

a) Chemical conversion treatment properties (zinc phosphate type/immersion tie f) Rod value ○ Crystals are small and uniform, dense Δ crystals are large and some scratches can be seen b) Salt spray test Zinc phosphate chemical conversion treatment - Cross after electrodeposition coating Cuts were made and a salt water spray test was conducted for 4 weeks to check for blisters at the cross-cuts.

Evaluation: ◎ Excellent ○ Excellent Δ Average As is clear from Table 2, the chemical conversion treatment property and post-painting needle corrosion resistance of the hot-dip galvanized steel sheets treated by the method of the present invention are greatly improved.

〔Effect of the invention〕

As described above, it is clear that by carrying out the manufacturing method of the present invention, the chemical conversion treatment properties and post-coating needle corrosion properties of hot-dipped steel sheets are greatly improved, and the industrial effects are extremely significant. be.

Claims (1)

[Claims] A method for producing a hot-dip-coated steel sheet, which comprises subjecting the plated steel sheet to either electrolytic treatment, which necessarily includes at least an anodic electrolytic treatment, in an electrolytic solution, or immersion treatment in an acidic solution after hot-dip plating.