JPH0657391A - Production of galvannealed steel sheet excellent in low temp. chipping resisting property - Google Patents

Abstract

PURPOSE:To improve low temp. chipping resisting property by galvanizing a base steel sheet ground by a prescribed thickness by hot dip zinc-coating, then, limiting the Fe content in a plated film by heating for alloying at a prescribed temp. rising rate. CONSTITUTION:A fresh surface is disclosed by previously grinding the surface of the base steel sheet by 0.1 to 5g/m<2> thickness. The ground steel sheet is galvanized by hot dix zinc-coating and then heated for alloying in the region of 420 to 650 deg.C plate temp. at 20 deg.C/sec or higher temp. rising rate. Fe content in the plated film is limited to 7 to 13wt.% and only the boundary of the steel sheet-plating is made to be a high Fe content. In such a manner, both prevention of stripping caused by chipping generating at the boundary of the plating-steel sheet and maintenance of the powdering resistance and sacrifice corrosion inhibition capability are made compatible with each other.

Description

Detailed Description of the Invention

The present invention relates to a method for producing an alloyed hot dip galvanized steel sheet having excellent low temperature chipping resistance, which is particularly suitable for automobile bodies.

[0001]

2. Description of the Related Art In recent years, the demand for improving the rust preventive power of automobile bodies has been increasing year by year, and particularly in North America where snow-melting salt is sprayed. For such requests,
The introduction of surface-treated steel sheets into the body of automobiles has been promoted, and in the past, surface-treated steel sheets have been used as materials for inner plates such as floor panels, member panels, and inner panels as countermeasures against perforated corrosion. With the sophistication of the rust prevention target as described above, the use of surface-treated steel sheets has recently been extended to exterior materials such as doors, hoods, fenders, sills, and even pillar panels.

[0002] As a surface-treated steel sheet used for automobile bodies, alloyed hot-dip Zn plated steel sheets (hereinafter referred to as GA steel sheets) are increasingly used. However, in order to use the GA steel plate as the outer plate, it is necessary to enhance its low temperature chipping resistance.

That is, when an impact is applied to a coated GA steel sheet, the plating film may be peeled from the steel sheet-plating interface to expose the base steel sheet. The lower the peeling temperature, the larger the stress of the coating resin exerted on the plating film, and therefore the peeling diameter tends to increase. For this reason, the plating film peels off relatively easily when it is subjected to the impact of pebbles or the like in winter, and if the snow-melting salt is sprinkled on the plating film, local corrosion occurs and the corrosion resistance is significantly reduced.

Many means for improving the low temperature chipping resistance of the plated steel sheet have been reported for those other than the GA steel sheet, but there are few reports for the GA steel sheet, for example, Japanese Patent Application Laid-Open No. 3-243756, entitled "Chipping A GA steel sheet for automobiles, in which the Fe concentration in the coating on the outer surface to be received is 5 to 11%, the thickness of the Γ phase at the steel plate-plating interface is 1.0 μm or less, and the main peak of the X-ray diffraction of the plating layer is the ξ phase ” Is to some extent.

Regarding the general plating adhesion of GA steel sheets, in order to reduce film peeling (powdering or flaking) during forming, the influence of operating conditions of GA steel sheets such as alloying temperature and heat pattern is considered. Research (The Iron and Steel Institute of Japan Conference "CAMP-ISIJ VO
L. 1 (1988) ". Also, JP-A 64-68
There is an improvement in the plating film structure, formation mode, composition, etc. as proposed by Japanese Patent Publication No. 457 and Japanese Patent Publication No. 2-39585.

[0006]

However, the chipping resistance of GA steel sheets, especially the film adhesion to impact at low temperature, is different from general film adhesion. Therefore, general countermeasures for film adhesion are less effective for low temperature chipping resistance.

Further, the GA steel sheet for automobiles proposed in Japanese Patent Laid-Open No. 3-243756 does not have the low temperature chipping resistance enough to be used as the outer plate of automobiles. This is because even if the ξ phase itself has more ability to absorb the impact energy generated when a high-speed pebbles collides than the other alloy phases, the difference is slight, and as a result, the crack develops the δ1 phase below it. This is because they pass through and easily propagate to the plating-steel plate interface.

The object of the present invention is to provide an impact-resistant adhesiveness which can be applied as an outer plate without lowering general performances such as blister resistance, flaking resistance, and powdering resistance, especially at low temperature. The object of the present invention is to provide a method for manufacturing a GA steel sheet having excellent low-temperature chipping resistance that can secure the above-mentioned properties.

[0009]

The chipping resistance of GA steel sheet is greatly influenced by the composition of the plating film, and the Fe content in the film such that the η phase remains in the upper layer is low FeG of less than 7%.
In the A steel sheet, the η phase absorbs impact, so the peeling diameter is small, but when this η phase disappears, cracks reach the steel sheet-plating interface, and there is a tendency for peeling propagation along the interface to increase the peeling diameter. Can be seen.

On the other hand, increasing the amount of Fe in the plating film,
If a plating film mainly composed of δ1 with a Γ phase formed at the steel plate-plating interface is used, the chipping resistance is restored again, and especially Fe
When the amount is 15% or more, low temperature chipping resistance applicable as an automobile outer plate is obtained.

However, in terms of general performance, if the Fe concentration in the plating film is less than 7%, the blister resistance and flaking resistance are poor, and if it exceeds 13%, the powdering resistance and sacrificial anticorrosion ability are deteriorated. , Porosity resistance and the like are reduced. In other words, Fe, which is convenient for securing chipping resistance,
The amount sacrifices general performance, which is the main reason why it is difficult to secure the chipping resistance of GA steel.

In order to solve this difficulty, the present inventors set the Fe content of the plating film to 7 to 13% from the viewpoint of ensuring general performance.
It was planned that the high Fe content be applied only to the steel plate-plating interface. That is, since peeling due to chipping occurs at the plating-steel plate interface, if this interface is made high in Fe, peeling due to chipping can be prevented without increasing the amount of Fe in the film, and chipping resistance and general performance can both be achieved. It will be possible.

Therefore, when GA steel sheets having the same amount of Fe in the coating produced by changing the temperature rising rate during alloying were compared, the higher the heating rate, the larger the Fe concentration gradient in the film and the lower the Fe concentration. Steel plate compared to the case of heating at a high speed
An alloy phase with a large amount of Fe was formed at the plating interface. Regarding this, it is reported that when the heating rate increases, it passes through the temperature range where stable growth of ξ can occur in a short time, so that the growth of ξ phase is small and the stable δ1 and Γ phases grow at high temperature immediately. There is.

However, in order to improve the low temperature chipping resistance of the GA steel sheet by increasing the temperature rising rate during alloying, a temperature rising rate of 40 to 45 ° C./sec is required, and such a high temperature rising rate is required. At high temperatures, when an untreated cold-rolled steel sheet was used as the plating base material, there was also an adverse effect such that the plating film was considerably roughened as compared with a general GA steel sheet. Therefore, it is difficult to secure a temperature rising rate that can improve the low temperature chipping resistance in actual operation.

Therefore, as a result of further investigations by the present inventors, in order to find an Fe enrichment means to replace the increase in the temperature rising rate,
If the surface of the steel sheet is ground as a pretreatment of the steel sheet to be subjected to alloying hot-dip Zn plating, Fe at a level at which the low temperature chipping resistance can be sufficiently improved without significantly increasing the temperature rising rate during alloying is present in the steel sheet-plating interface. It was found that it is concentrated in.

The present invention was made based on the above findings, and in the production of alloyed hot dip Zn plated steel sheet, the surface was ground to a thickness of 0.1 to 5 g / m ² as a pretreatment of the base material steel sheet. After performing hot dip Zn plating in a predetermined process, the plate temperature of 420 to 650 ° C. is alloyed and heated at a temperature rising rate of 20 ° C./sec or more, so that the amount of Fe in the plating film is 7
A method for producing an alloyed hot-dip Zn-plated steel sheet excellent in low-temperature chipping resistance is characterized by producing an alloyed hot-dip Zn-plated steel sheet of 13 wt%.

[0017]

[Function] A steel sheet to be subjected to alloying hot-dip Zn plating has an easily oxidizable element that is more easily oxidized than iron on the surface before annealing. Since these elements are not reduced even after the annealing reduction step, the steel sheet is immersed in the plating bath while having these oxidizable elements concentrated on the surface, and undergoes alloying treatment. In particular, elements such as Si and P hinder the diffusion of iron and delay the alloying. Therefore, the surface of the steel sheet is ground to remove the surface layer enriched with these elements to expose the new surface.
Thereby, the diffusion of iron into the plating film is promoted in the alloying step, and the temperature rising rate at the time of alloying necessary for improving the low temperature chipping resistance can be reduced.

Here, when the grinding depth is less than 0.1 g / m ² ,
The effect of relaxing the temperature rising rate in the alloying treatment is small, and conversely, when it exceeds 5 g / m ² , the effect of relaxing the temperature rising rate is saturated and the yield decreases. Therefore, the grinding depth is 0.1-5g
/ M ² .

The grinding means is not particularly limited, and any means such as a wire brush, a nylon brush containing abrasive grains, and an elastic whetstone roll may be adopted.

The steel sheet that has undergone grinding and other necessary pretreatments is subjected to a reduction annealing furnace and then subjected to hot dip Zn plating,
Then, it undergoes alloying treatment.

The composition of the Zn plating bath is not particularly limited. For example, a slight amount of Al is added to improve the adhesion, but in addition, Pb, Sb, Si, Fe, Sn, Mg, Mn, N are added.
A small amount of one or more of i, Cu, Ca, Li, Ti, and misch metal may be contained.

The material of the steel sheet is not particularly limited, and may be, for example, C, Si, Mn, P, S, sol. A
The inclusion of 1, Ti, Cr, Nb, Cu, Ni, etc. in a commonly used range does not impair the gist of the present invention.

In the alloying process, the plate temperature is 420 to 65.
The temperature range of 0 ° C. is heated at a heating rate of 20 ° C./sec or more.

In the present invention, the heating rate can be lowered to 20 ° C./sec. If it is less than 20 ° C./sec, it is difficult to form a high Fe alloy phase at the steel plate-plating interface, which is enough to improve chipping resistance. This rapid heating is used for initial heating immediately after leaving the plating bath and passing through the wiping device, and is effective only for a plated material in which the η phase remains in the upper layer. This is because the high-speed heating is for heating the interfacial alloy phase that is initially formed by the high-speed temperature increase to have high Fe,
This is because the plated material that has been alloyed has no effect.

The upper limit of the temperature rising rate is 40 to 45 because of the unevenness of the plating film in the conventional method, that is, as cold rolled.
C./sec was the limit, but in the present invention, the unevenness of the plating film can be suppressed even at 40 to 45.degree. C./sec or more, and the lowering of the lower limit significantly widens the temperature raising rate range. However, it is necessary to increase the power supply capacity as the temperature rise rate increases,
20-100 ° C /
It is desirable to set it within the range of seconds.

As the heating means, any of gas combustion heating, radiant heating, direct heating burner, energizing heating, high frequency induction heating, etc. may be adopted without particular limitation.

The plate temperature range of the high-speed heating is set to 420 to 650 ° C. The reason why the alloy phase is slightly formed below 420 ° C., and above 650 ° C., the alloying of the plating film progresses too much, This is because general performance such as powdering property is deteriorated.

The Fe concentration of the plating film is 7 to 13%. If it is less than 7%, the blister resistance and flaking resistance are inferior, and if it exceeds 13%, the powdering resistance is inferior, and neither general performance is satisfied.

After heating at a high speed, an appropriate heat treatment is performed so as to obtain a predetermined degree of alloying, and the hide pattern is not limited. However, the plate temperature is 42
It is desirable to maintain the temperature within the range of 0 to 650 ° C. for 1 second or more in order to promote the Fe concentration at the interface.

The hot-dip galvannealed steel sheet obtained according to the present invention has excellent general performance and low temperature chipping resistance, and can be used as an inner plate as well as an outer plate of an automobile without any problem.

[0031]

EXAMPLES Examples of the present invention will be described below.

C: 0.002%, Si: 0.01%, Mn:
0.25%, P: 0.06%, solAl: 0.025%, T
i: 100 × 250 mm test material was taken from a full-hard cold-rolled steel sheet having a composition of 0.01% and a thickness of 0.8 mm. This was ground as a pretreatment. Nylon brush with abrasive grains is used for grinding, and the rotation speed is 400-800rp
m, the reduction amount is 1 to 3 mm, the number of grinding times is 5 to 20 times,
The grinding depth was changed variously. As other pretreatments, solvent degreasing, electrolytic cleaning in a Na ₂ CO ₃ + NaOH aqueous solution, water cleaning and drying were performed.

The pre-treated sample material was 5% O ₂ + N ₂ −500 ° C. × 30 using a hot dipping simulator.
Second heat treatment, followed by reduction annealing at 850 ° C. × 60 seconds in an atmosphere of 25% H ₂ + N ₂ and then 0.1
Plating was performed in a Zn bath at 460 ° C. to which 0% Al was added. Furthermore, the adhesion amount is 60 g / m ² by wiping treatment.
After adjusting to, alloying treatment was performed.

As the alloying treatment, heat treatment was carried out at a frequency of 100 kHz using a high frequency induction heating device. The heat pattern was initially heated at a constant rate, and when the temperature reached a certain temperature, the heating was stopped, or when the alloying was further advanced, the reached temperature was maintained. The following test was performed on the test material after the alloying treatment.

1) Powdering resistance evaluation test The test material was punched out into a disk shape having a diameter of 60 mm, a punching diameter of 30 mm and a die drawing radius of 3R were subjected to a cylindrical drawing test. Was visually observed and evaluated according to the following criteria. (Good) ◎ ○ △ × (poor)

2) Flaking resistance evaluation test After the test material was cut into 50 × 230 mm and subjected to a beaded hat forming test (abrasive paper # 60, cushion pressure 3 ton, bead height 3 mm, forming height 55 mm) The tape was peeled off from the inner wall side of the bead contact, and the flaking property was evaluated based on the blackening degree according to the following criteria. ○: No problem in use △: Slightly peeled off ×: Not usable

3) Low temperature chipping resistance The test material was cut into a size of 150 × 70 mm, and was subjected to a dipping-type phosphoric acid conversion treatment, a cationic electrocoating, an intermediate coating,
A three-coat coating (total thickness 100 μm) was applied as the top coating.
Then, the coated plate was cooled and kept at -20 ° C, and 10 gravel stones with a diameter of 4 to 6 mm were air-pressed to 2.0 with a gravure tester.
A chipping test was performed in which the particles collided under the conditions of kg / cm ² and an impact velocity of 100 to 150 km / hr, and the peeled diameter was measured.

The test results are shown in Tables 1 to 4. When the surface of the test material is not ground in advance, the general performance (powder resistance) is obtained when the temperature rising rate during alloying is 30 to 40 ° C./sec. Ring resistance and flaking resistance) and low temperature chipping resistance are not compatible. Even if the surface of the test material is ground, if the grinding depth is less than 0.1 g / m ² , general performance and low temperature chipping resistance are not compatible. If the grinding depth is 0.1 g / m ² or more and the temperature rising rate is 20 ° C./sec or more, general performance and low temperature chipping resistance are compatible at a high level. Even if the grinding depth exceeds 5 g / m ² , there is no problem in performance.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

[0042]

[Table 4]

[0043]

As is clear from the above description, the method for producing an alloyed hot-dip galvanized steel sheet excellent in low temperature chipping resistance of the present invention ensures general performance by limiting alloying of the plating film. . Despite limiting the alloying of the plating film, the surface of the steel sheet is ground in advance so that Fe is concentrated at the steel sheet-plating interface and the low temperature chipping resistance is also secured. Surface grinding can limit the rate of temperature rise during alloying and prevent the plating film from becoming uneven.

Claims (1)

[Claims] 1. In the production of an alloyed hot-dip galvanized steel sheet, as a pretreatment of a base steel sheet, the surface is ground to a thickness of 0.1 to 5 g / m ² , and hot-dip Zn plating is performed on the surface by a predetermined process. After that, the area of the plate temperature of 420 to 650 ° C. is alloyed and heated at a temperature rising rate of 20 ° C./sec or more, and Fe in the plating film is heated.
A method for producing an alloyed hot-dip Zn-plated steel sheet having excellent low-temperature chipping resistance, which comprises producing an alloyed hot-dip Zn-plated steel sheet in an amount of 7 to 13 wt%.