JPH02267251A - Highly corrosion resistant double layer plated steel sheet capable of preventing occurrence of bubble-like electrodeposition coated film defect and excellent in coating suitability - Google Patents

Abstract

PURPOSE:To obtain a highly corrosion resistant double layer plated steel sheet free from bubble-like electrodeposition coated film defect and excellent in coating suitability by forming a Zn-type plating film and a ferrous plating film of respectively specified coating weights on the surface of a steel sheet and controlling the amount of hydrogen occlusion as a whole. CONSTITUTION:A monolayer or double-layer plating film consisting of Zn alloy plating and/or one or more Zn-type composite platings or the above plating film and further a double layer plating film consisting of Zn plating are formed on the surface of a steel sheet. This Zn-type plating film is formed by means of alloyed hot-dip galvanizing, etc., while regulating the coating weight to >=25g/m<2>, and further, the above Zn-type composite plating is obtained by forming Al2O3, Cr2O3, etc., into eutectoid. Subsequently, the above Zn-type plating film is electroplated with a ferrous plating film consisting of Fe plating or/and one or more kinds among ferrous alloy platings of >=50% Fe content by 1-10g/m<2> coating weight. Then, heat treatment is carried out at >= about 100 deg.C, by which the total amount of hydrogen occlusion in the plating film and the steel is regulated to <=300ml/m<2> per side. By this method, the highly corrosion resistant double layer plated steel sheet capable of preventing the occurrence of bubble-like electrodeposition coated film defect and excellent in coating suitability can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a rust-proof steel plate for automobiles that has excellent paint compatibility.

[Prior art]

Automobile bodies are mainly composed of thin steel plates, so in order to maintain the beauty and mechanical strength of the car body over a long period of time, the steel plates used must have excellent corrosion resistance and paint compatibility. is essential. In order to improve the corrosion resistance of steel sheets, Zn-Fe,
Alloy electroplated steel sheets such as Zn-Ni have been developed.

On the other hand, regarding coating compatibility, since crater-shaped coating defects occur when applying cationic electrodeposition to Zn-based plated steel sheets, a technology is needed to prevent this.
A two-layer plated steel sheet was developed in which an Fe-Zn alloy plating with a Fe content of 50% or more was applied as an upper layer plating on a lower layer Zn-based plating (Japanese Patent Publication No. 58-1555
No. 4, Special Publication No. 57-28754, Plating a
nd 5surface Finishing, 72(1
985) No, 8. P52), therefore, above 2
By using layer-plated steel sheets, it is possible to achieve both high corrosion resistance and excellent paint compatibility.

[Problem to be solved by the invention]

However, as vehicle bodies are used for longer periods of time, it has become necessary to have even better corrosion resistance than before. As a method of imparting corrosion resistance to a steel plate, it is most preferable to apply Zn-based plating, which has sacrificial anti-corrosion properties, in consideration of damage to painted parts.

Methods to obtain high corrosion resistance with Zn-based plating include improving the composition and making it thicker, but improvements in one composition have already been thoroughly studied, and new systems and systems that show better performance than ever have been developed. There is little chance of finding the composition. Therefore, increasing the thickness is the most realistic method for achieving high corrosion resistance. Of course, even thick materials need to be compatible with coating, so to prevent craters that occur during electrodeposition coating, Fe-Zn alloy plating with an Fe content of 50% or more,
Alternatively, it is necessary to apply other Fe-based plating as an upper layer plating. However, according to the studies conducted by the present inventors, a multi-layer thick-grained rust-preventing steel sheet that has been electroplated with Fe-Zr+alloy plating with an Fe content of 50% or more or other Fe-based plating as an upper layer plating. Although it was possible to prevent the generation of craters during electrodeposition coating, it was found that the finished appearance of the electrodeposition coating was significantly deteriorated. These coating defects appear as convex bumps, concave bumps, or marbled unevenness depending on the electrodeposition and baking conditions. It was noticed. When examined using a scanning electron microscope, it was found that this defect was a bubble-like coating defect that had occurred within the electrodeposited coating, as seen in the photograph in FIG. This is a completely different type of coating defect from the craters that have been reported.As will be explained later, absorbed hydrogen is released from the plating layer or inside the steel during the baking process of the electrodeposited coating, creating air bubbles within the coating. This is what became. This bubble-like coating film defect does not occur in a conventional thin-grained multi-layer plated steel sheet that is electro-deposited under the same conditions, so it can be said that it is a defect specific to thick-grained multi-layer plated steel sheets.

The above-mentioned coating defects are not only problematic in terms of appearance, but also harmful in terms of corrosion resistance, and are an essential problem to be solved in order to put thick multilayer plated steel sheets into practical use.

[Means and effects for solving the problem] As a result of intensive research on the above-mentioned coating defects, the present inventors have determined that the cause of the occurrence is hydrogen occluded in the multi-layer thick plated steel plate during the manufacturing process, Furthermore, the hydrogen storage capacity has been increased to 300ml/m
``We have found that the above coating defects can be prevented by following the steps below.

The present invention was made based on such findings, and is characterized by the fact that a single layer or multiple layer consisting of one or more types of Zn-based alloy plating and/or Zn-based composite plating is formed on the steel sheet surface. One or two of layer plating skin 1 or Zn alloy plating and/or Zn composite plating
A multi-layer plating film consisting of Zn plating and Zn plating.

It has a Zn-based plating film with a coating weight of 25 g/ha2 or more, and on the Zn-based plating film, one or more of Fe plating and/or Fe-based alloy plating with an Fe content of 50% or more is applied. and the plating amount is 1 to 10.
A multi-layer plated steel sheet having an Fe-based plating film of g/m2, the total amount of hydrogen storage in the plating film and the steel being 30
0m1t/m" (per one side) or less.

Such a multilayer plated steel sheet has excellent coating compatibility that can simultaneously prevent craters and bubble-like coating defects caused by electrodeposition coating, and also has a high degree of corrosion resistance.

The present invention will be explained in detail below.

The multilayer plated steel sheet of the present invention has an inner layer plating made of a Zn-based plating film having at least a Zn-based alloy plating or a Zn-based composite plating, and an outer layer plating made of a Fe-based plating film. The inner layer plating is a plating layer that imparts corrosion resistance to this multilayer plated steel sheet. The coating weight of this plating layer needs to be 25 g/m'' or more, especially in order to ensure sufficient corrosion resistance for automobile bodies and the like.

As the Zn-based plating film for inner layer plating, for example, Zn
There is a single-phase or multi-layer structure consisting of Zn-based alloy plating or Zn-based composite plating, or a multi-layer structure consisting of Zn-based alloy plating and Zn-based composite plating. Specifically, 1) Z
N-based alloy plating single layer structure, ii) different types of Zn (including cases where the content of alloy components is different; the same applies hereinafter)
iii) Single-layer structure of Zn-based composite plating; iv) Multi-layer structure of different types of Zn-based composite plating; v) Same as one or more Zn-based alloy platings. Usually, a multilayer structure consisting of Zn-based composite plating or two or more types of Zn-based composite plating is considered.

Further, the inner layer plating can be a Zn-based alloy plating, a Zn-based composite plating, or a combination of both and Zn plating. Also in this case, the Zn-based alloy plating and the Zn-based composite plating can be used alone or in combination of two or more different types of plating.

Zn-Fe alloy plating is used as Zn-based alloy plating.

Zn-Ni alloy plating, Zn-Mn alloy plating, Zn-
Co gold alloy plating, etc., and each of these alloys is coated with Ti, Cr,
M. .

This is an alloy plating containing one or more elements such as W.

Methods for obtaining these Zn-based alloy platings include a method of thermally alloying hot-dip galvanizing or electrogalvanizing, an electroplating method, a vacuum evaporation method, an ion blating method, and the like.

Further, the Zn-based composite plating is one in which one or more types of Al1203, Cr2O3°Sin, B a Cr 04, etc. are eutectoided to Zn plating or the above-mentioned Zn-based alloy plating.

The outer layer plating is Fe plating or Fe plating.
It consists of Fe-based alloy plating with a content of 50% or more, or a multilayer plating thereof, and has a coating weight of 1 to 10 g/m2. In order to prevent the occurrence of craters during electrodeposition coating, it is necessary to form such Fe-based plating on the outermost layer. In the case of Fe-based alloy plating, the Fe content is 50%
If it is less than that, the effect of preventing the formation of craters is not sufficient.

This Fe-based plating film usually has a single layer structure made of Fe alloy plating or Fe plating, but in some cases,
For example, Fe plating and Fe-based alloy plating.

Alternatively, it may have a multi-layer structure made of two or more types of Fe-based alloy plating with different Fe contents.

Here, if the amount of the outer layer plating is less than 14/m2, the surface coverage will be low and the generation of craters cannot be prevented.

On the other hand, if the amount of plating exceeds 10 g/m'', the adhesion of the plating layer will decrease.

This outer layer plating is formed by electroplating. the current,
Electroplating is the only method that can industrially form thinned layers densely and uniformly on wide strips.

As Fe-based alloy plating, for example, Fe, Zn, Ni
There is a plating containing one or more of the following: , Co1, Cr, Mo, W, P, and B.

The present invention aims to reduce the total amount of hydrogen storage in the plating film and the steel to 3 per side of the steel plate in the multi-layer plating structure as shown below.
00d/m" or less (however, the storage amount under standard conditions of latm and 25°C).

When Fe-based alloy plating or Fe plating is formed as the outer layer by electroplating, a large amount of hydrogen is generated on the surface of the inner layer plating, and some of it is occluded in the plating layer and steel. Figures 1 and 2 show 80% Fe on alloyed hot-dip galvanizing.
Qualitative analysis of hydrogen concentration was performed using ion micromass spectroscopy (ion micromass spectroscopy) for multilayer plated steel sheets with Fe-Zn alloy plating containing IMA
) showed the results. Comparing the strength of hydrogen with and without heat removal treatment, it can be seen that hydrogen is significantly occluded in the as-plated state.

Figure 4 shows Ar
While releasing hydrogen by heating in the atmosphere,
The amount of hydrogen absorbed was determined by gas chromatography, and the relationship between this amount of absorbed hydrogen and the degree of bubble defect formation during electrodeposition coating was determined. Here, the test material is a multi-layer plated steel sheet in which the inner layer plating is alloyed hot-dip zinc with a plating amount of 38 g/m2, and the outer layer plating is an 80% Fe-Zn alloy electroplating with a plating amount of 5 g/n+2, and the amount of absorbed hydrogen is It was adjusted before electrodeposition coating by preheating treatment. According to this, in order to prevent the occurrence of bubble defects, the amount of absorbed hydrogen must be 300 d/m.
It can be seen that the following is a necessary condition.

From the above results, it is clear that hydrogen occluded in the plating layer or in the steel is deeply related to bubble-like coating defects. From this, it is presumed that bubble-like paint film defects occur through the following mechanism.

In other words, when electrodeposition is applied to a plated steel sheet in which more than a certain amount of hydrogen is occluded in the plating layer or steel, the surface of the electrodeposition film hardens before sufficient hydrogen is released during the baking process. A thin film forms on the outside. Hydrogen remaining without being released at the beginning of the baking process is trapped inside the coating film, so the hydrogen collects and expands upon heating to form bubbles, which cause defects in the electrodeposition coating film. The baking temperature for electrodeposition coating is generally 170 to 180°C, but as is clear from Figure 5, this temperature range is within the temperature range where occluded hydrogen is released at a high permeation rate; This is a condition that makes it easy for bubbles to form.

Methods for controlling the amount of hydrogen storage include: 1) heating the plated steel sheet after plating the outer layer, 2) selecting conditions for the outer layer plating where hydrogen storage is unlikely to occur, and 3) hydrogen storage that is low or released even if it is stored. methods such as using steel types that are difficult to
Or there is a way to combine these. A necessary condition for releasing occluded hydrogen by heating is that the heating temperature is 100° C. or higher. This is because hydrogen is hardly released in a temperature range below 100°C.

Heating methods include continuous heating in a line and batch heating using an open coil. The heating atmosphere may be any atmosphere that does not contain hydrogen or contains hydrogen to such an extent that hydrogen absorption hardly occurs. If heated in the atmosphere for a long time, the outer layer plating will be oxidized and the chemical conversion treatment property will be inhibited, so there will be restrictions on the heating time.

In addition, when the present invention is a BH rusted steel sheet, it is preferable that the heat treatment for releasing occluded hydrogen be performed at as low a temperature as possible.

Further, for automobile outer panels where stretcher strain is a problem, it is preferable to use a material that has been heated and then pressure regulated.

〔Example〕

Example 1 A cold-rolled coil having the components shown in Table 1 was used as a raw coil and subjected to hot-dip galvanizing 1-alloying treatment, temper rolling, and Fe-Zn electroplating in a non-oxidation furnace continuous plating line. A layer-plated steel sheet was manufactured. Hydrogen storage capacity is Fe-Z
Adjustments were made by changing the amount of n plating, or by continuously heat treating the multilayer plated steel sheet at a temperature of 100° C. or higher using an induction heating method, or by performing open coil heat treatment. Note that the Fe-Zn electroplating of the outer layer was carried out under the following conditions.

(1) Plating bath/component Na25o4 30 g/Q/pH1
, 8~2.0 (2) Plating condition bath Temperature: 50°C Flow rate: 2 m/s Current density: 30~120 A/dm2 Also, Fe-Z
n The Fe content in the electroplated film is mainly determined by the Fe salt ratio (
”FeSO4・7)1.0/ (FeS04・7H□0
+ZnSO4.7H20)) by changing the current density, and the amount of adhesion was adjusted by changing the plating time and current density. The evaluation results of the coating film appearance, corrosion resistance, and workability after electrodeposition coating are shown in Table 2.
It can be seen that the Zn electroalloyed plated steel sheet is superior in all performances.

Example ■.

By using the cold-rolled coil shown in Table 1 that has been annealed and temper-rolled, it is subjected to alkaline degreasing, sulfuric acid pickling, Zn-Fe alloy electroplating, and Fe-Zn alloy electroplating on an electrogalvanizing line. , produced multilayer plated steel sheets.

The amount of hydrogen storage was adjusted by the amount of outer layer plating and continuous or batch heat treatment at 100° C. or higher after plating.

Note that both the inner layer plating and the outer layer plating in this example were carried out under the same conditions as the outer layer Fe-Zn electroplating in Example (1). In addition, Fe-Zn electroplated film and Z
The Fe content and adhesion amount of the n-Fe electroplated film were adjusted in the same manner as in Example (1). The performance evaluation results are shown in Table 3, and it can be seen that the multilayer Fe-2n/2n-Fe alloy electroplated steel sheet with a released amount of stored hydrogen of 300 mQ/m2 or less is superior in all performances.

Table 1 Table 4 Test methods and evaluation criteria [Effects of the invention] According to the present invention described above, it has high corrosion resistance and does not cause craters and bubble-like coating defects in cationic electrodeposition coating. We are now able to supply multi-layer plated steel sheets with excellent coating compatibility.

It is possible to achieve both high corrosion resistance and beautiful appearance of automobile bodies.

[Brief explanation of drawings]

FIG. 1 is an enlarged microscopic photograph showing bubble-like defects that occurred in the cationic electrodeposited coating. Figures 2 and 3 show the results of quantitative analysis of hydrogen concentration using ion micro-pawnshop analysis spectroscopy for as-plated multi-layer plated steel sheets and multi-layer plated steel sheets that have been treated to remove stored hydrogen by heating after plating. This shows that. FIG. 4 shows the relationship between the amount of hydrogen storage and the degree of bubble-like coating defects that occur during electrodeposition coating. FIG. 5 shows the relationship between the release of stored hydrogen and temperature. Diagram: Hydrogen storage capacity (rIIl/butt)

Claims (1)

[Claims] On the steel plate surface, a single layer or multilayer plating film consisting of one or more types of Zn-based alloy plating and/or Zn-based composite plating, or one or more of Zn-based alloy plating and/or Zn-based composite plating. A multi-layer plating film consisting of two or more types and Zn plating, with a coating weight of 2.
It has a Zn-based plating film of 5 g/m^2 or more, and the Z
On the n-type plating film, one or more types of Fe plating and/or Fe-based alloy plating with an Fe content of 50% or more are applied, and the plating weight is 1 to 10 g/m^2.
A multi-layer plated steel sheet having a plating film with a total hydrogen storage capacity of 300ml/m^2 in the plating film and the steel.
(per one side) Highly corrosion-resistant multi-layer plated steel sheet with excellent coating compatibility that prevents the occurrence of bubble-like electrodeposited coating defects.