JPS61213364A - Improvement of corrosion stripping resistance of hot dip galvanized steel sheet - Google Patents

Abstract

PURPOSE:To improve the corrosion stripping resistance of a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet by irradiating beams having high energy density on the surface of the steel sheet. CONSTITUTION:The surface of a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet having an Fe-Zn alloy layer formed between the Zn layer and the steel base is heated with beams having high energy density such as laser beams, electron beams or infrared rays. At this time, only the Zn layer is rapidly heated with the beams, held at a high temp. for a short time and rapidly cooled, so a hot dip galvanized steel sheet whose Zn layer has superior corrosion stripping resistance, that is, superior adhesion is obtd. without forming an Fe-Zn alloy layer or growing the formed Fe-Zn alloy layer.

Description

[Detailed Description of the Invention] [Industrial Application Field 1] The present invention relates to a method for improving the corrosion peeling resistance of a hot-dip Zn-plated steel sheet, and more specifically, a method for heating so as not to develop an alloy layer to the surface of the plating layer. The present invention relates to a method for improving the corrosion and peeling resistance of hot-dip Zn-plated steel sheets (including hot-dip Zn alloy-plated steel sheets).

[Prior Art 1 Generally, hot-dip Zn-plated steel sheets are shipped after being manufactured.
After that, they are often used while being stored for a long period of time, and when processing is performed during this use, a phenomenon may occur in which the hot-dip Zn plating layer peels off.

The reason why the hot-dip Zn plating layer peels off is that the hot-dip Zn plating layer is gradually corroded by moisture in the atmosphere while the hot-dip Zn-plated steel sheet is stored, and eventually peels off.

The process in which the peeling of the hot-dip Zn plating layer on the steel plate progresses is considered as follows.

(1) Corrosion of Zn grain boundaries (boundaries of Zn crystals during solidification) on the surface of the plating layer.

(2) Corrosion along the Zn grain boundaries inside the plating layer.

(3) Interfacial corrosion between the molten Zn layer and the Zn-Fe-Al alloy layer.

(4) Peeling off the hot-dip Zn plating layer.

Corrosion of Zn grain boundaries in these plating layers and molten Zn
Corrosion at the interface between the plating layer and the Zn-Fe-Al alloy layer is caused by suppressing the reaction and diffusion between the base iron added to the plating bath and the molten Zn plating, thereby suppressing the growth of the Fe-Zn alloy layer. This is thought to be due to the action of local batteries resulting from the segregation of At, which ensures plating adhesion, and Zn crystals during solidification of molten Zn, that is, PB, which is added to adjust the size of spangles. It will be done. That is, (1) Grain boundary corrosion A local battery between the Zn grain boundaries in the hot-dip Zn plating layer where A1, PB, etc. in the hot-dip Zn plating bath are solidified and segregated, and within the Zn grains where they are not segregated. Action.

(2) Interfacial corrosion: PB is solidified and segregated in the hot-dip Zn plating layer in contact with the ZnFe-Al alloy layer, resulting in local battery action between the Pb-concentrated hot-dip Zn plating layer and the Zn-Fe-Al alloy layer.

As a method for preventing such corrosion, Japanese Patent Application Laid-open No. 57-026155 describes reducing the amount of A1, PB, etc. added to the hot-dip Zn plating bath. It is necessary to control the plating bath components in a timely manner, taking into account ensuring plating adhesion and adjusting the spangle size, and such control reduces productivity.

In addition, a method for heating hot-dip Zn-plated steel sheets was published in Japanese Patent Application Laid-open No. 5
Publication No. 5-152166 and Japanese Unexamined Patent Publication No. 7-140867
However, these use oil baths, solder baths, etc. as heating means, and therefore the melted Z
The entire n-plated steel sheet is heated, and the molten Zn
There is a problem that the adhesion of the plating layer decreases and the quality of the material deteriorates.In particular, when the alloy layer develops to the surface of the plating layer, there is a problem that the quality characteristics such as the gloss and color tone of the hot-dip Zn plating surface are reduced. However, it is extremely difficult to obtain a hot-dip Zn-plated steel sheet that suppresses the development of the alloy layer and has good corrosion and peeling resistance.

[Problems to be Solved by the Invention] The present invention has been made in view of the problems of the prior art with respect to grain boundary corrosion, interfacial corrosion, etc. that occur in hot-dip Zn-plated steel sheets as described above, and is based on the inventor's findings. As a result of intensive research, we have suppressed the development of the alloy layer by rapidly heating only the hot-dip Zn plating layer, holding it at high temperature for a short time, and rapidly cooling it, without heating the entire hot-dip Zn-plated steel sheet. We have developed a method for improving the corrosion and peeling resistance of hot-dip Zn-plated steel sheets, which can also prevent corrosion and peeling.

[Means for Solving the Problems J The feature of the method for improving the corrosion resistance of hot-dip Zn-coated steel sheets according to the present invention is that high energy density is applied to the surface of hot-dip Zn-coated steel sheets and hot-dip Zn alloy-coated steel sheets. The aim is to irradiate a beam of light.

A method for improving the corrosion and peeling resistance of a hot-dip Zn-plated steel sheet according to the present invention (hereinafter sometimes referred to as the method according to the present invention).

) will be explained in detail below.

Laser beams, electron beams, infrared beams, maser beams, etc. can be used as beams with high energy density in the method according to the invention.

In the method according to the present invention, the hot-dip Zn-plated steel sheet is irradiated with a beam of high energy density.
The surface layer of the hot-dip Zn plating layer of the n-plated steel sheet is locally heated, and at this time undergoes rapid heating and rapid cooling.

The reason why the surface of the hot-dip Zn-plated steel sheet is prevented from peeling due to corrosion when the hot-dip Zn-plated layer is irradiated with a high-energy-density beam is not clear at present, but it is possible that the surface of the hot-dip Zn-plated layer itself It is presumed that the Zn grain boundaries and interfaces in the hot-dip Zn plating layer are improved in the presence of AI, PB, etc. in the vicinity of the Zn grain boundaries and interfaces.

[Example J] Next, an example of a method for improving the corrosion peeling resistance of a hot-dip Zn-coated steel sheet according to the present invention will be described.

Example hot-dip Z1 plated steel plate (single side plating coating weight = 150g/L
112, approximately 21 μm) was subjected to the following treatment.

(1) Laser irradiation treatment (method according to the present invention) Output per irradiation beam (beam diameter 10 mm) = 1.3.5 KW Hot-dip Zn-plated steel plate moving speed = 10 m/+nin (2
) Solder bath heat treatment (comparative example) Bath composition = Pb-8n eutectic bath temperature = 400°C Treatment time = 5 minutes, 10 minutes, 1 hour, 3 hours.

Cooling method after 5-hour treatment = cooling (3) As-plated without laser irradiation or hang bath heat treatment) In order to investigate the corrosion-resistant peeling characteristics of hot-dip Zn-plated steel sheets subjected to such treatment, a wet test was conducted. was carried out, and then the adhesion of the hot-dip Zn plating layer was tested.

Furthermore, the plating adhesion of the hot-dip Zn-plated steel sheet that had been subjected to the above treatment and the thickness of the alloy layer in the hot-dip Zn-plated layer were measured using an optical microscope (400x magnification).

Humidity test = temperature 70℃, relative humidity 98%, time 10 days Adhesion test conditions = Adhesion bending test implementation The test results are shown in Table 1.

Table 1: By using the method according to the present invention, the coating adhesion of the hot-dip Zn-plated steel sheet as it was irradiated with laser was as good as that without any treatment, and no development of an alloy layer was observed. It can also be seen that the adhesiveness after a wet test after laser irradiation, so-called corrosion resistance and peeling properties, is also excellent.

On the other hand, when heating a solder bath for up to 10 minutes, the adhesion of the coating on the heated hot-dip Zn-plated steel sheet is good and no alloy layer development is observed, but the corrosion resistance and peeling properties are insufficient. Furthermore, when heated for more than 1 hour, the alloy layer develops and the adhesion of the plating while heated decreases.In this way, in solder bath heating, it is necessary to balance the inhibition of the development of the alloy layer and the corrosion-resistant peeling property. It is difficult.

[Effect of the Invention 1] As explained above, the method for improving the corrosion peeling resistance of hot-dip Zn-coated steel sheets according to the present invention has the above-mentioned structure. The remarkable increase in corrosion and peeling properties is due to rapid heating by high energy density beam irradiation, high temperature holding for a short time, rapid cooling, and local heating of only the plating layer on the surface of the hot-dip Zn-plated steel sheet, and high energy density beam irradiation. In-line processing is possible in a short time, and the high-energy density beam irradiation output can be easily changed to the minimum necessary in response to changes in plating bath component concentration, plating amount, line speed, etc. It is also excellent in terms of productivity, economy, etc., and has great industrial effects.

Claims (1)

[Claims] A method for improving the corrosion and peeling resistance of a hot-dip Zn-plated steel sheet, the method comprising irradiating the surface of the hot-dip Zn-plated steel sheet and hot-dip Zn alloy-plated steel sheet with a beam of high energy density.