JPH01172553A - Alloying hot dip galvanized steel sheet and its production - Google Patents

Abstract

PURPOSE:To manufacture an alloying hot dip galvanized steel sheet excellent in corrosion resistance and press formability by subjecting a hot dip galvanized steel sheet to two-stage treatment while changing alloying temp. and also specifying Fe content in a plating layer and alloy structure. CONSTITUTION:A steel sheet after hot dip galvanizing is first heated to 400-520 deg.C and subjected to alloying treatment until eta-phase is dissipated. Subsequently, alloying treatment is continued at 300-500 deg.C, by which alloying treatment is carried out so that average Fe content in a plating layer is regulated to >13-17wt.% and the plating layer is practically composed of delta1-phase and GAMMA1-phase. By this method, the alloying hot dip galvanized steel sheet excellent in powdering resistance and press formability and having high corrosion resistance can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an alloyed hot-dip galvanized steel sheet with excellent corrosion resistance and press formability, and a method for manufacturing the same.

(Conventional technology and its problems) Alloyed hot-dip galvanized steel sheets (hereinafter abbreviated as GA) are superior in weldability and corrosion resistance after painting compared to galvanized steel sheets, and are inexpensive, so they are used as rust-proof steel sheets. It is widely used as Generally, GA has a Fe content of 9 to 13 in the film.
−1%, and from the steel plate base side, r”Fl (Fe3
It has a structure similar to a layered structure of intermetallic compounds such as Zn+o), r' 1 layer (Fe, Zn21), and 61 layers (PeZnt). or those processed at low alloying temperatures,
The uppermost layer may include an intermetallic compound of a ζ layer (FeZn13).

By the way, as mentioned above, GA is excellent in weldability and corrosion resistance after painting, but it has a drawback that a so-called powdering phenomenon occurs in which the plating film peels off during press molding. This phenomenon generally becomes more pronounced as the amount of plating and the Fe content in the film increases.
The amount of powdering increases rapidly. The reason for this is thought to be that the plating film generally has a characteristic of being brittle against compression deformation.

Therefore, according to conventionally known techniques, highly flexible η-Zn remains in the film, or three layers (FeZn +
It is said that powdering resistance can be improved by increasing the composition ratio of , ). That is, the aim of these known techniques is to reduce the Pe content in the film to improve powdering resistance. but,
If the Fe content in the plating layer is reduced, the corrosion resistance will be reduced. In particular, the corrosion resistance after painting is greatly reduced.

By the way, especially in recent years, there has been a strong movement in the automobile industry to strengthen the rust prevention of car bodies as a countermeasure against salt damage in cold regions, and plated steel sheets with even better rust prevention properties are desired. One possible immediate method for improving the rust prevention properties of plated steel sheets is to increase the amount of plating deposited. GA is relatively economical, and even if the amount of plating is increased, the increase in manufacturing costs is not so significant.
As mentioned above, this leads to an increase in powdering.

It would be very desirable for the automobile industry if it were possible to manufacture particularly thick GA with less powdering and excellent paintability and corrosion resistance.

However, using conventionally known manufacturing methods to produce thick GA, for example, the amount of deposited Fe and Zn on one side is 50 g/
Even if a material larger than m" is manufactured, it is extremely difficult to manufacture one that satisfies all of the above-mentioned characteristics.

In order to improve this point, the present inventors used a conventionally known manufacturing method to increase the deposition amount per side of Fe+Zn with different degrees of alloying as shown below (1) to (3) from 55 to 65.
A thick GA with a weight of 1.5 g/m" was manufactured and its characteristics were investigated.

(+1 Fe content in the film: 12 to 15 wt% ■Alloying temperature exceeds 550°C ■ Film layer structure δ, 10r, + (2) Fe content in the film 28.5 to less than 12 wt% ■Alloying temperature 5
Less than 00℃ ■ Film layer structure ζ 161 + "1 (3) Fe content in the film: 1 to B, less than 5 wt% ■ Alloying temperature 450 to 550 °C ■ Film layer structure η + ζ is mainly 61 and ", It may be included.

As a result, the following performance problems were confirmed.

Items in parentheses ( ) were processed at the alloying temperature conventionally used in the production of GA.

Products manufactured using this general method suffer from a large amount of powdering at the shrinkage deformation portion. (2) is 500
Alloying treatment is performed at a low temperature of ℃ to reduce the Fe content in the film to 8.
．． It is suppressed to less than 5 to 1211 t%.

Since the composition ratio of the ζ phase is increased, there is little powdering at the shrinkage deformation part. However, when press molding is actually performed, seizure may occur in the mold or cracks may occur during molding. In addition, flaky peelings are observed. (3
) was subjected to alloying treatment at a temperature of 450 to 550°C to reduce the Fe content in the film to less than 7 to 8.5 wt%. Since the Fe content is low and the η phase remains, powdering hardly occurs at the shrinkage deformation part. However, some seizure may occur during pressing, and the weldability and corrosion resistance after painting are significantly inferior to other materials.

As described above, even if conventional known techniques are simply applied to thick GA, it is extremely difficult to produce a GA that satisfies all properties such as powdering resistance, paintability, and corrosion resistance.

The present invention was developed in view of the above-mentioned circumstances, and its purpose is to improve the antinomy situation of corrosion resistance, weldability, and powdering resistance after painting, and to improve all of these properties. The purpose is to provide an excellent alloyed hot-dip galvanized steel sheet and a new method for producing the same.

Another object of the present invention is to provide a thick alloyed hot-dip galvanized steel sheet that suppresses film build-up and seizure on a mold during press forming and does not cause flaking, and a method for manufacturing the same.

(Means for solving the problem) In order to achieve the above object, the present inventors have developed a thick GA
The plating peeling behavior during press forming was investigated using the cylindrical deep drawing method used in the evaluation of powdering of GA in -i, and the following findings were obtained by analyzing the results.

For example, in the case of GA (plate thickness 0.8 m+n) with an adhesion amount of 60 g/m'' per side, if the alloying temperature is taken as a parameter, the relationship between the degree of alloying and powdering will be as in the first case. In addition, in the figure, ○ marks are those processed at an alloying temperature of 450°C, [stock] marks are processed at an alloying temperature of 500°C, and 0 marks are processed at an alloying temperature of 550°C. and ■ marks are those processed at an alloying temperature of 600°C.

What can be seen from Figure 1 is that powdering can be suppressed by suppressing the degree of alloying, that is, by reducing the Fe content in the plating film, or by lowering the alloying temperature. This means that the amount will decrease. This is also a conventionally known knowledge. For this reason, it is considered desirable to produce GA with a suppressed degree of alloying to such an extent that no η layer remains.

However, when such a GA is produced and actually press-molded, a large amount of build-up and seizure occurs on the mold, as described above. The reason for this is that in cylindrical drawing, the influence of surface sliding is small and no significant seizure occurs, whereas in actual press forming, the influence of surface sliding is quite strong, causing seizure. It turns out that there is something. Therefore, the present inventors developed a method using GA with an adhesion amount of 60 g/m" per side.
(plate thickness 0.8 mm) was used to investigate the influence of surface sliding during press forming, and the following became clear.

The investigation was conducted using a ■-shaped bead drawing method using a bead tip diameter of 0.5 mmR. Figure 2 summarizes the results in terms of the relationship between alloying degree and powdering using alloying temperature as a parameter. In the figure, O marks are those processed at an alloying temperature of 450°C, ■ marks are processed at an alloying temperature of 500°C, and 0 marks are processed at an alloying temperature of 550°C.

It can be seen from Figure 2 that even when the degree of alloying is suppressed, the Fe content in the film is between 9.5 and 13.
There is a region where the amount of peeling is extremely large within the wt% range, and seizure due to surface sliding is likely to occur. The reason why the amount of powdering specifically increases in this range is not clear, but it is thought to be related to the presence of the ζ layer formed on the surface layer.

In addition, the present inventors have found that the amount of Zn deposited on one side with different degrees of alloying after alloying at a low temperature of 480°C is 55"6
The relationship between the degree of alloying and forming load was investigated using a cylindrical deep drawing method using a GA of 0g/m'' (plate thickness 0.8mm).Part 3
The results are shown here. F in the film
It can be said that in the range where the e content is 9 to 13 bit%, the molding load increases abnormally and the risk of cracking is high.

From the above results, it is clear that the Fe content is higher than that of conventional general materials (GA with Fe content of 9 to 13 wt%). The present invention was completed by discovering that GA can be obtained and that such GA can be manufactured by changing the alloying temperature and performing a two-step process.

The gist of the present invention lies in the following items (1) and (11): the alloyed hot-dip galvanized steel sheet and its manufacturing method.

(i) The average Fe content of the plating layer exceeds 13 wt%, 1
An alloyed hot-dip galvanized steel sheet, characterized in that the plating layer substantially consists of a 61 phase and an RL phase at a content of 7 wt% or less.

(ii) The steel plate after hot-dip galvanizing is 400 to 520'
The alloying process is carried out in the temperature range of C until the η phase disappears, and then the alloying process is continued in the temperature range of 300 to 500'C, so that the average Fe content of the plating layer exceeds l:%t%,
When the amount is 17 wt% or less, the plating layer substantially has 61 phases and “1
1. A method for producing an alloyed hot-dip galvanized steel sheet, the method comprising alloying the steel sheet so that it consists of a phase.

(Function) Hereinafter, the alloyed hot-dip galvanized steel sheet and the manufacturing method thereof according to the present invention will be explained in detail.

First, the reason why the alloyed hot-dip galvanized steel sheet according to the present invention has a plating layer having the above composition and phase structure will be explained.

The composition of the plating layer exceeds 13 wt% in average Fe content,
The reason for setting the content to 17 wt% or less is that if it deviates from this range, the following problems tend to occur, which is not preferable. That is, as is clear from the above-mentioned Figures 1 to 3, when the average Fe content is less than 13 wt%, for example, when the average Fe content is in the range of about 9 to 13 wt%, powdering is significant. In most cases, seizure occurs due to surface sliding during pressing, and the forming load is abnormally increased, which tends to cause cracks.

Ungrooved steels with an average Fe content of 9 wL% less than this have fewer problems as described above, but are significantly inferior in weldability and corrosion resistance.

On the other hand, when the average Fe content in the film exceeds 17 wt%, the powdering resistance is poor.

In addition, the reason why the plating layer is made to have a phase composition consisting essentially of 61 phases and 1 phase is that when the plating layer contains any or all of the η phase, ζ phase, and 1 phase, the following This is because the problems described above tend to occur, which is not desirable.

For example, if the η phase remains, there will be less powdering, but the corrosion resistance and weldability after painting will deteriorate. If the ζ phase is included, seizure is likely to occur. Also, ``If phase is included, powdering will be significantly increased.

However, a plating layer that does not contain these phases is substantially δ
, phase and one phase, the friction coefficient of the surface can be lowered and the sliding resistance can be reduced.

And, ``The presence of one phase can prevent the weakening of the δ phase.

For this reason, powdering, seizure, and cracking do not occur.

Generally, when the degree of alloying exceeds 13 wt%, a phase is formed, but in the case of the GA of the present invention, the alloying temperature is suppressed to prevent the formation of the η phase as much as possible. Never.

The presence of a phase can be detected by physical detection means such as EPMA (electron probe microanalyzer), but there is also the problem of measurement error. Refers to those whose thickness (existing thickness) is 0.5 μm or less.

Such an alloyed hot-dip galvanized steel sheet according to the present invention can be manufactured by performing alloying treatment on a hot-dip galvanized steel sheet obtained by ordinary means in two steps. . That is, in the first stage, the steel plate after galvanizing is heated to a temperature range of 400 to 520 °C to alloy it until the η phase disappears, and in the second stage, the steel plate is alloyed with 3
By heating to a temperature range of 00 to 500°C for alloying treatment, the average Fe content of the plating layer exceeds 13 wt%, and 1
When the content is 7iit% or less, an alloyed hot-dip galvanized steel sheet can be obtained in which the plating layer substantially consists of a δ1 phase and a Γ1 phase.

The reason why the alloying temperature in the first stage is set to 400 to 520°C is that at a temperature lower than 400°C, it takes a long time for the η phase to disappear, which is economically unfavorable. Upper limit is 5
The reason for setting the temperature at 20°C is that at alloying temperatures exceeding this temperature,
This is because phases are more likely to be formed.

Further, the reason why this first stage of alloying is performed until the η phase disappears is that if the η phase is present at this stage, unevenness is likely to occur in the subsequent alloying process.

In the present invention, "until the η phase disappears" does not necessarily mean immediately after the η phase disappears, but as long as alloying does not progress to an extreme degree, that is, unless the Fe content reaches about 124% or more, The alloying reaction of
Fe content is 12wt at or above the level where it is less than “g/m”
It refers to the level of alloying degree of less than %. However, if the alloying reaction is 1
When the Fe content is allowed to increase to more than 211 t%, the η phase is likely to be formed.

The disappearance of the η phase can be determined by visually observing the color tone of the surface of the steel sheet, optically measuring the reflectance, or measuring by a fluorescent X-ray method.

The reason why the alloying temperature in the second stage is set to 300 to 500°C is that at temperatures lower than 300″C, the Fe enrichment reaction in the plating layer hardly progresses, making it impossible to achieve an average Fe content of 13 wt% or more. The reason why the upper limit alloying temperature is set at 500°C is because at an alloying temperature exceeding this, Fe enrichment is promoted and a layer is easily formed. By adjusting the alloying temperature to a range of 300 to 500°C, the formation of the phase is suppressed, the Γ1 phase grows, and an average Fe content of 13 to 17 wt% can be achieved.

The preferred second stage alloying temperature is 350-470°C.

Furthermore, in the present invention, the reason why the alloying treatment method is performed in two steps is that the average Fe content exceeds 13-1% and 17w
At t% or less, η phase, ζ phase, and substantially δ phase-free,
This is to economically and efficiently obtain the above-mentioned GA consisting of one phase and one phase.

For example, regardless of the present invention, the steel plate after hot-dip galvanization is
Although it is possible to increase the average Fe content to more than 13 wt% with only one treatment at an alloying temperature exceeding 0'C, the plating layer will contain "phases". Even if treated only once at the second stage alloying temperature in the invention, an average Pe content of more than 13 wt% can be achieved, and δ, phase and ".

It is possible to have a phase configuration consisting of phases. However, such a GA requires a long time and is economically unfavorable.

However, as in the manufacturing method of the present invention, the steel sheet after hot-dip galvanization is first subjected to a first stage treatment at a relatively high temperature.
% or less, and then perform a second stage treatment at low temperature without r phase to exceed 13 wt% and 17 wt%.
If the average Fe content is adjusted to the range of
It is possible to economically and efficiently produce a GA in which the plating layer is substantially composed of the δ1 phase and the r1 phase.

In addition, in the present invention, the holding time in the first and second stage alloying treatments is not particularly limited.
The optimal holding time depends on the amount of Zn deposited, the steel type of the base steel plate, and the Zn
It is set according to the AN concentration in n. For example, if the coating weight is 55 g/m or more, the holding time for the first stage alloying treatment is 10 to 100 seconds, and the holding time for the second stage is 2.
It is preferable to set it as 0-180000 seconds from a point of alloying degree.

A more preferable holding time for the second stage alloying treatment is 60~
It is 36000 seconds.

Furthermore, in the present invention, the first and second stage alloying treatments do not need to be performed entirely in a heating furnace in the hot-dip galvanizing line; for example, the second stage alloying process can be performed offline depending on the temperature conditions. Needless to say, the annealing may be carried out in a box-type annealing furnace.

Next, the present invention will be further explained by examples.

(Example) l is 0.1. Zinc coating amount per side treated in a zinc plating bath with addition of ht% at a bath temperature of 464 to 470°C is 60 to 7.
A 2g/m2 hot-dip galvanized steel sheet was made, and then 0.
A test piece with a thickness of 8 mm, a width of 100 mm, and a length of 200 mm was prepared, and it was alloyed under the heating conditions shown in Table 1 in a hot-dip plating simulator equipped with a vertical infrared heating furnace.

The test pieces thus obtained were subjected to a V-shaped bead drawing test and a cylindrical drawing test, and the thickness of the phase and the presence or absence of the ζ layer were also investigated.

In the (2) type bead pull-out test, the amount of plating peeled off was measured and evaluated using a (2) type bead of 0.5 mm radius. The sliding distance in the pullout test was 125 mm and the width was 25 mm.
A test piece with a length of 215 mm was used. In the cylindrical drawing test, five test pieces were drawn into a 90 mmφ blank cylinder, and evaluation was made based on the number of pieces in which molding cracks occurred during drawing.

“The thickness of the phase was measured by EPMA, and the presence or absence of the ζ layer was measured by X-ray diffraction.

These results are also shown in Table 1.

As is clear from Table 1, the inventive examples (A, B) have less peeling of the plating and less cracking than the comparative examples (C, D) and the conventional examples (E, F). has not occurred at all.

In addition, although the above description has mainly focused on thick GA, the present invention is not limited to this in any way. A thinner coating amount, e.g. 50g/lo”
It goes without saying that similar effects can be obtained with the following. Furthermore, the GA according to the present invention includes both single-sided and double-sided plated steel plates.

(Effects of the Invention) As explained above, the alloyed hot-dip galvanized steel sheet according to the present invention has excellent powdering resistance and press formability, and therefore also has high corrosion resistance. Moreover, since the manufacturing method requires only two steps of processing by changing the alloying temperature, the increase in manufacturing cost is not so large.

Furthermore, if alloy plating such as Fe-Zn or Fe-P is applied on the alloyed hot-dip galvanized steel sheet according to the present invention, there is also the effect that the phosphate chemical conversion treatment properties are greatly improved.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the degree of Fed in the plating film and the amount of powdering when the alloying temperature is used as a parameter in a cylindrical drawing test of an alloyed hot-dip galvanized steel sheet with a coating weight of 60 g/n+", Figure 2 is a diagram showing the relationship between the Fe concentration in the plating film and the powdering amount when the alloying temperature is taken as a parameter in the ■-type bead pull-out test of an alloyed hot-dip galvanized steel sheet with an adhesion ff of 160 g/m''. , FIG. 3 is a diagram showing the relationship between the Fe concentration in the plating film and the forming load in the cylindrical drawing test.

Claims (2)

[Claims] (1) The average Fe content of the plating layer exceeds 13 wt%, 1
7wt% or less, the plating layer is substantially composed of δ_1 phase and Γ_
An alloyed hot-dip galvanized steel sheet characterized by comprising one phase. (2) Alloying the hot-dip galvanized steel sheet at a temperature range of 400 to 520°C until the η phase disappears, and then continuing the alloying process at a temperature range of 300 to 500°C,
The average Fe content of the plating layer exceeds 13 wt% and is 17 wt%
.