JPS60228662A - Alloyed hot dip galvanized steel sheet - Google Patents

Abstract

PURPOSE:To improve the adhesion of a hot dip galvanized steel sheet to a coated film and the corrosion resistance after the formation of the coated film by heat treating the hot dip galvanized steel sheet to form a layer consisting of an upper layer of practically pure iron and a lower layer contg. GAMMA phase as the principal phase. CONSTITUTION:The steel sheet is hot dip galvanized and heat treated in an atmosphere having <=5,000ppm concn. of oxygen. By this heat treatment the resulting zinc layer is alloyed with the base steel. At the same time, zinc in the surface part of a formed alloy layer is vaporized to form a layer consisting of an upper layer of practically pure iron and the lower layer contg. GAMMA phase as the principal phase.

Description

[Detailed Description of the Invention] (Technical Field) The technical contents described in this specification regarding alloyed hot-dip galvanized steel sheets (including steel strips, hereinafter the same) with excellent paintability are applicable to heat treatment following alloying treatment. The goal is to propose development results derived from the investigation of HHZ n behavior.

<Background Art> So-called alloyed hot-dip galvanized steel sheets, which are made by hot-dip galvanizing a steel sheet and then heating it to alloy the coating layer with the base steel, have excellent post-painting corrosion resistance (particularly pitting corrosion resistance). Because it has resistance weldability, it is widely used as a material for automobiles, home appliances, and construction.

Conventionally, single-sided plated steel sheets, which have no plating on the surface of the steel sheet serving as the outer surface of the vehicle body, have often been used for automobile exterior panels. On the other hand, in recent years, consideration has begun to be given to the use of metal fittings on the outer surface of the vehicle body for the purpose of preventing heavy outer surface steel.

In this regard, as mentioned above, alloyed hot-dip galvanizing itself has excellent pitting corrosion resistance (perforation resistance) after painting, but it is difficult to use it on the outside of a car body because it suffers from blistering or blistering in a corrosive environment. Because of problems such as deterioration of paint film adhesion, it was difficult to say that it had sufficient performance.

In addition, with cationic electrocoating paints that have been increasingly used as primer coatings in recent years, when the coating voltage is increased to improve coating coverage, pinholes may occur due to gas generation from the surface of the material. It also had the disadvantage that it was prone to the so-called crater phenomenon, in which it remained even after baking.

Here, in conventional alloyed hot-dip Zn plating, Fe is 8 to 15
It mainly consists of the δi phase (Zn 7 Fe), and has a low Fe content, so it is inferior to cold-rolled steel sheets in coating film adhesion and cationic electrodeposition.

(Prior art) Fe-Z with a Zn of 40% or less is separately applied to the surface of the anti-rust coated steel plate.
Japanese Patent Publication No. 15554/1983 proposes a plated steel sheet that prevents cratering when coating with a single cation coating by forming an n-based alloy coating, but this technology is based on electroplating. 7n by method or hot-dip plating method
- Multi-layer plated steel sheet with Fe alloy plating as the inner layer and 7040% or less e-rich Zn-Fe plating on the upper layer by electroplating or thermal diffusion using Fe powder, etc. It is characterized by the fact that an Fe-rich /:n-Fe alloy is applied to the upper layer, that is, the double plating operation, and the double plating operation is troublesome and has the disadvantage of increasing the processing cost. be.

In addition, as a similar technique, layered on 7n plating is used.
JP-A-57-1999 also describes the formation of an Fe layer and thermal alloying to obtain an Fe-rich 7n-Fe gold-containing layer as an upper layer.
No. 114692, No. 58-58263, No. 58-61
No. 261 and No. 58-91164, but in both of these, the upper plating layer is entirely 7-
Since it is an n-Fe alloy and the surface layer is not substantially made of pure iron, it is impossible for this surface layer to achieve the expected significant improvement in paintability.

(Motivation for the Invention) In view of the above situation, the inventors have conducted extensive studies to improve the paintability of alloyed hot-dip galvanizing, and as a result, they have developed a method of post-heating treatment in an atmosphere with a low oxygen concentration after alloying treatment. They found that by doing this, Zn, which has a significantly higher vapor pressure than Fe, is preferentially evaporated and removed from the plating surface layer, and as a result, a matted surface with significantly superior paintability can be easily obtained, and the present invention was created. This is what I came to do.

3- (Objective of the invention) The object of the invention is to provide an alloyed hot-dip galvanized steel sheet that has excellent coating film adhesion and cationic electrodeposition.

(Structure of the Invention) The present invention is an alloyed hot-dip galvanized steel sheet characterized in that the upper layer consists of substantially pure iron and the lower layer mainly consists of P phase.

The alloyed hot-dip galvanized steel sheet of the present invention is obtained by hot-dip galvanizing a steel sheet and then continuing the heat treatment in an atmosphere with an oxygen concentration of 5000 ppm or less when alloying the plated layer with the base steel by heating. This can be advantageously obtained by vaporizing Zn in the surface layer to form a plating layer in which the upper layer is made of substantially pure iron and the lower layer is made of F phase.

The surface of the alloyed hot-dip galvanized steel sheet according to this invention has Zn
Since it consists essentially of an iron layer obtained by evaporation of iron, the adhesion of the coating film is extremely good, and the activity of the plating surface is particularly high, making it easy to form a dense chemical conversion film in phosphorus 4-acid chemical conversion treatment. As a result, the propagation of scratches after painting, especially the progression of paint film, S, etc., is significantly reduced.

Furthermore, the phenomenon of cratering during cationic electrodeposition coating is also reduced to the same level as that of cold-rolled steel sheets.

In particular, it is important that the lower layer is in the F phase with respect to the above-mentioned upper layer. The lower layer is the δ1 phase (Zn 7 Fe), which is the main component of conventional alloyed hot-dip galvanizing, and has poor powdering properties, whereas the F phase (Zn□.Fe 3 ) has poor powdering properties.
This is because it is particularly good in the case of .

Now, Figure 1 schematically shows the change behavior of the surface state due to the Zn-removal heat treatment that follows the alloying melting treatment. is vaporized, and finally a pure iron layer remains on the surface, and the lower layer becomes F phase (Zn□.Fe5).
By advantageously utilizing this mechanism, the alloyed hot-dip galvanized steel sheet of the present invention can be easily obtained.

Here, the following range of Zn removal heat treatment conditions is particularly suitable.

Heat treatment atmosphere 02: 0-11000pp plate temperature
650°C to 750°C Processing time 1 second to 20 seconds Next, examples of the present invention will be shown.

(Example) Ultra-low carbon Nb stabilized steel for deep drawing (0,002%C-0,
23%Mn-0.04%A, f2-0.02%Nb)
The plate was hot-rolled, pickled, and cold-rolled to a thickness of 0.7 mm, and then galvanized at 60 mm (1/m2) per side under normal operating conditions in a continuous hot-dip galvanizing line using a non-oxidizing furnace. Following alloying treatment according to conventional methods, Zn removal was performed under various conditions.
Heat treatment is applied to improve surface roughness (PPI) and coating adhesion (1
The effects on post-coating adhesion and water-resistant adhesion), post-coating corrosion resistance (external rust resistance), and cratering characteristics during electrodeposition coating were investigated.

For painting here, a commercially available cationic electric paint was used as an undercoat, and the coating temperature was 27°C and the coating voltage was 220v1.
A coating with a film thickness of 18 μm was applied under the condition that the current was applied for 8 minutes, and for primary and water resistant adhesion and external rust resistance, a total coating of 80 μm was applied using commercially available intermediate and top coats. did.

The cratering property test was conducted at an electrodeposition coating voltage of 80V.
The test was carried out under the condition that the current was applied for 80 seconds.

The results are shown in Table 1.

(Effects of the Invention) According to the present invention, excellent paint film adhesion equivalent to or better than that on the surface of a cold rolled steel sheet is achieved with virtually no craters occurring during cationic electrodeposition coating, and excellent post-coating properties are achieved. Corrosion resistance is ensured.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing Zn removal behavior during alloying heat treatment. Patent applicant: Kawasaki Steel Corporation 10- Figure 1

Claims (1)

[Claims] 1. An alloyed hot-dip galvanized steel sheet characterized in that the upper layer consists of substantially pure iron and the lower layer mainly consists of P phase.