JPH11200000A - Hot dip galvannealed steel sheet excellent in low temperature chipping resistance and workability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the low temp. chipping resistance of a steel sheet and to furthermore improve its resistance to powdering and flaking by composing a plating film substantially of the δ1 , single phase and providing the surface with the specified number of cracks. SOLUTION: The plated film is substantially composed of the σ1 single phase through, partially, the other phase is contained on the base metal, and the amt. is preferably regulated to >=80 wt.%. For forming this plating film, at first, it is rapidly cooled to >=500 deg.C, preferably, at >=20 deg.C/sec. After that, it is rapidly cooled to 420 deg.C, preferably, at >=10 deg.C/sec. In this way, the plated film substantially composed of the δ1 phase is obtd., and into which, cracks are introduced by >=10 pieces per mm. For forming the cracks, the method of mechanically applying strains to the steel sheet is used. Concretely, the application of the strains by mechanical elongation is regulated to 0.4 to 2.0%, and as the applying means, skinpass rolling by, a tension leveller or the like are utilized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an alloyed hot-dip galvanized steel sheet having excellent low-temperature chipping resistance and workability and a method for producing the same.

[0002] An alloyed hot-dip galvanized steel sheet is used on the outer surface of an automobile body. When pebbles collide with a painted surface on the outer surface of the automobile at a low temperature in winter or the like, not only a coating but also a plating layer and a base steel sheet are formed. A phenomenon in which the interface is peeled off (hereinafter, simply referred to as chipping, and durability against this is referred to as low-temperature chipping resistance) may be observed. The present invention is excellent in such low-temperature chipping resistance, and also has a plating powder peeling phenomenon (powdering) and a plating film peeling phenomenon during press working.
Has excellent workability that does not cause (flaking)
The present invention relates to a galvannealed steel sheet and a method for producing the same.

[0003]

2. Description of the Related Art Galvannealed steel sheets are excellent in paintability and corrosion resistance and are used for various purposes including automobiles. However, when the alloyed hot-dip galvanized steel sheet is used as an exterior panel for an automobile, the coating film and the plating film are apt to be peeled off by the impact of chipping due to stepping stones or the like during running. In addition, red rust is likely to be generated at a portion where the plating is peeled off by chipping, which is fatal for use as an automobile exterior plate. Further, in the alloyed hot-dip galvanized steel sheet, during processing, the plating film is powdery, that is, powdering that peels off in a powder shape, and flaking in which the plating film peels off in a scale-like shape occurs with mold galling with a mold. . Such peeling of the plating due to the press working of the steel sheet has problems such as poor forming such as cracks and poor processing appearance due to the adhesion of the peeled pieces to the steel sheet.

For example, in order to improve press formability,
As disclosed in JP-A 1-279738, after forming a hot-dip galvanized film, during the alloying treatment, rapidly heating to a temperature range where the ζ phase does not grow, within the range of the degree of alloying that the powdering resistance does not deteriorate. After holding for a sufficient time so that Fe diffuses from the base material in the plating film, and then rapidly cooling, the formation of a phase that is inferior in flaking resistance is suppressed as much as possible. There is disclosed a technique for providing an alloyed hot-dip galvanized steel sheet which has excellent workability while ensuring workability and powdering resistance.

[0005]

However, the alloyed hot-dip galvanized film obtained by such an alloyed heat pattern can ensure a good workability to some extent, but has a sufficient resistance to be applied to a steel sheet for vehicle exterior. There is a problem that low-temperature chipping property cannot be ensured.

[0006] It is an object of the present invention to provide an alloyed hot-dip galvanized steel sheet having improved low-temperature chipping resistance and excellent resistance to powdering and flaking, and a method for producing the same.

[0007]

Means for Solving the Problems Accordingly, the present inventors have:
As a result of various investigations on the effect of the manufacturing conditions of the alloyed hot-dip galvanized film on low-temperature chipping resistance, in order to secure the flaking resistance, in the rapid cooling process implemented when suppressing the ζ phase formation, It was found that the increase in residual stress (strain) in the galvannealed coating resulted in a decrease in low-temperature chipping resistance.

[0008] Based on this fact, in order to minimize the formation of the ζ phase and to ensure low-temperature chipping resistance as a steel sheet for exterior parts of a vehicle, it is necessary to reduce the distortion generated during rapid heating and rapid cooling. It has been found that it is effective to release the stress of a large alloyed hot-dip galvanized film, and it is effective to form cracks as small as possible as a way of releasing the stress at that time. The reason for this is that when subjected to an impact such as low-temperature chipping or when subjected to shrinkage deformation during press working, cracks occur at the plating-base metal interface and propagate, but dense cracks occur. This is presumed to be because the formation of the crack stops the propagation of the crack at the crack generating portion, and does not lead to large plating delamination.

[0009] The inventors of the present invention have made it clear that finer cracks can be easily formed by releasing the stress of the plating film, and the ζ phase remaining on the outermost surface layer is removed.
In addition to the method of controlling such stress by mechanical elongation control, it has been found that it is effective to perform pickling after forming a plating film. At that time,
It has been found that it is more effective to perform the pickling treatment after the surface is cleaned by performing the alkali treatment.

Here, the present invention provides (1) a hot-dip galvannealed film obtained by heat-treating a hot-dip coated film on the surface of a steel sheet, wherein the plated film substantially consists of a δ ₁ single phase. A galvannealed steel sheet excellent in low-temperature chipping resistance and workability, characterized in that the surface of the plating film has 10 or more cracks per 1 mm, (2) the galvannealed steel sheet according to the above (1), A coated steel sheet with a coating film on a galvanized film, (3) An alloyed hot-dip galvanized steel sheet in which the steel sheet is immersed in a hot-dip galvanizing bath and the resulting hot-dip galvanized film is alloyed by heat treatment In the manufacturing method of
In the alloying treatment, the maximum plate temperature of the heat treatment is set to 500 ° C. or higher, and in the subsequent cooling process, the steel plate is cooled to 420 ° C. at a cooling rate of 10 ° C./sec or more.
Method for producing alloyed hot-dip galvanized steel sheet with excellent low-temperature chipping resistance and workability, characterized by imparting strain at an elongation of 2.0% or less and generating cracks; (4) hot-dip galvanizing steel sheet In the method for producing an alloyed hot-dip galvanized steel sheet in which the hot-dip galvanized film is immersed in a bath and subjected to an alloying treatment by heat treatment, the alloying treatment is performed by setting the maximum temperature of the heat treatment to 500 ° C or more. Then, in the subsequent cooling process, it is cooled to 420 ° C. at a cooling rate of 10 ° C./sec or more, and then subjected to an acid washing treatment, or
A method for producing an alloyed hot-dip galvanized steel sheet excellent in low-temperature chipping resistance and workability, which is characterized in that a crack is generated by performing an alkali treatment.

Thus, according to the present invention, a crack as dense as possible is generated in an alloyed hot-dip galvanized film having a structure substantially consisting of a δ ₁ single phase with substantially no ζ phase remaining. Low temperature chipping resistance,
A good alloyed hot-dip galvanized steel sheet and a method of manufacturing the same can be obtained without complicating press formability and relying on upper layer flash plating as a body exterior material.

The term “substantially δ ₁ single-phase plating” in the present invention refers to a plating in which the ratio of δ ₁ phase in a plating film is 80% or more (% by weight). This can be achieved, for example, electrochemically by measuring the dissolution time at which the δ ₁ dissolution potential is reached from the dissolution time of the entire film.

In order to obtain such a substantially δ ₁ single-phase plating, it is important to minimize residual Δ phase.
Therefore, it is necessary to rapidly heat and cool rapidly to 500 ° C. or higher where the ζ phase does not grow, but as described above, the alloyed hot-dip galvanized film alloyed with such a heat pattern Inevitably, there is a problem that the residual stress of plating increases. At that time, it is effective to release the stress in advance, and at the time of releasing the stress, reduce the crack generation density as much as possible, and suppress the propagation of plating cracks at the time of impact and working.

Further, in order to increase the crack generation density and to remove a small amount of the remaining Δ phase, it is desirable to perform a treatment with an acid solution, that is, a pickling treatment. By performing the alkali treatment on
Since the cleanliness of the surface before the pickling treatment is ensured, a further improvement effect can be expected.

Various attempts have been made in the prior art for the idea of causing fine cracks in the plating film. However, such a prior art does not suggest anything about the present invention. Will be clear from the following explanation.

For example, Japanese Patent Application Laid-Open No. Hei 4-218653 discloses a method of forming a plating alloy layer by using small blocks in order to improve low-temperature chipping resistance and impact peeling resistance.
However, this method is based on the premise that the alloyed vapor-deposited galvanized steel sheet is used, and it is suggested that the ζ phase is rather positively left. In addition, in order to ensure flaking resistance, it is fundamentally different from the one that attempts to minimize residual phase.

In Japanese Patent Application Laid-Open No. 4-280953, the adhesion to a coating film is improved by forming a plating film having as many concave portions of an appropriate shape as possible on a galvannealed steel sheet. There is disclosed a method of improving impact peeling resistance by an anchor effect. However, such a method is intended to improve the adhesion between the coating film and the plating film,
It is not intended to improve the adhesion between the plating film and the base material by positively releasing the stress of the plating film as in the present invention. In addition, in order to form a plating film having such a shape, it has been proposed to use a temper rolling roll having a special shape. There is no idea to stretch the base steel sheet in an appropriate range in order to have cracks.

Further, Japanese Patent Application Laid-Open No. 5-247620 discloses an alloyed hot-dip galvanized steel sheet for the purpose of improving low-temperature chipping resistance and powdering resistance. Although a cavity is provided in the present invention, its definition is unclear,
It is presumed that the difference between the overgrown portion and the growth retarded portion of the alloyed hot-dip galvanized steel sheet has been studied to minimize the difference. However, the present invention does not discuss the formation of a plating layer having large irregularities, but suggests that it is necessary to form an appropriate crack in the formed plating film for low-temperature chipping resistance. Is what you do. Further, when the void ratio and the crack are synonymous, the present invention recognizes that having more cracks is unavoidable for improving low-temperature chipping resistance, whereas the invention described in the above-mentioned publication discloses a crack ( It has been recognized that it is more effective to suppress the cavities) as much as possible to improve the low-temperature chipping resistance, and the direction of the means for achieving the expected effect is completely contradictory.

An example in which formation of a crack in a plating film has been studied is disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 4-2366752. However, this is based on the premise that an upper layer flash plating is formed on an alloyed hot-dip galvanized steel sheet, and by providing fine cracks, the stress of the upper layer flash plating is dispersed,
The purpose of the present invention is to suppress the occurrence of large cracks in the plating film during processing and to disperse fine cracks, thereby dispersing trap sites for gas generation generated during electrodeposition coating, and to achieve a good coating finish. This is completely different from the alloyed hot-dip galvanized steel sheet without upper plating, which is expected to improve low-temperature chipping resistance.

A method of improving the press formability by forming an oxide film layer after the formation of an alloyed hot-dip galvanized film is disclosed in, for example, JP-A-7-18400. However, this method presupposes the presence of the ζ phase, and compensates for such a decrease in the slidability of the plating film with a thick oxide film. Have been. But,
As in the present invention, in order to achieve both low-temperature chipping resistance and press formability, to promote crack generation in the plating film, and to remove the phase, after forming the alloyed hot-dip galvanized film, the acid solution There is no way to make contact.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Next, an embodiment of an alloyed hot-dip galvanized steel sheet having excellent low-temperature chipping resistance and workability according to the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

First, the alloyed hot-dip galvanized film, which is a premise of the present invention, is partially coated on the base material as shown in FIG.
Includes a phase, has also ζ phase on the surface, is excellent powdering resistance consisting essentially [delta] ₁ single phase, the anti-flaking property. At this time, the amount of δ ₁ is 80% or more by weight, preferably 90% or more. In order to ensure anti-flaking properties, the amount of haze should be 10% or less,
Preferably, 6% or less, in order to ensure powdering resistance,
ΓThe amount of phase is 10% or less, preferably 7% or less.

As shown in FIG. 2, one or more cracks per 0.1 mm, that is, ten or more cracks per mm are formed on the surface of the alloyed hot-dip galvanized film having such an alloy phase structure. In addition, it is necessary to ensure low-temperature chipping resistance. That is, when the number of cracks per 1 mm is less than 10, the propagation distance of plating cracks increases, and not only the low-temperature chipping resistance but also the powdering resistance deteriorates. The number of plating cracks generated is preferably 50 or more per 1 mm.

The crack penetrates the plating layer as shown in FIG. 2, and penetrates the ζ phase and the Γ phase to reach the substrate. As a measuring method at that time, an average value of 10 points obtained by observing an arbitrary portion at a measuring width of 1 mm at × 500 (magnification) in the cross section of the plating film was adopted. Also,
The upper limit is not particularly defined, but if cracks are too dense, there is a possibility that a large number of fine plating exfoliations may occur. If desired, the upper limit is set to 500 per mm.

Next, a method for producing the galvannealed steel sheet according to the present invention will be described. First, a steel sheet is dipped in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet.
In the present invention, the type of the steel sheet, the composition of the plating bath and the like are not particularly limited, and for example, a commonly used steel sheet may be used. After the hot-dip plating, an alloying process is performed.

Here, the influence of the heat pattern at the time of the alloying treatment is specified. In order to form a δ ₁ single-phase alloyed hot-dip galvanized film as described above, first, it is important to suppress the ζ phase. Therefore, the temperature range where the ζ phase does not grow is 500
C., preferably above 520.degree. C., preferably as fast as possible, in practice heating at a rapid rate above 20.degree. C./sec. If the temperature is lower than 500 ° C and lower than 20 ° C / sec, the ζ phase remains,
In some cases, a problem that the flaking resistance is deteriorated may occur. Also, if secured present invention generates the [delta] _1-phase, but not the temperature prescribed upper limit of the heating should preferably be in the 600 ° C. or less. This is because, since the Γ phase exceeds 600 ° C. is a large amount generated, because substantially [delta] ₁ Single-phase plating film becomes difficult to manufacture.

Further, after the heating, when cooling rapidly to 420 ° C. where substantially no alloying reaction can occur, the cooling temperature is increased.
A temperature of 10 ° C./sec or more, preferably 40 ° C./sec or more is effective in suppressing the residual Δ phase. If the cooling rate is slower than this, the ζ phase remains on the surface layer and the problem that the anti-flaking property cannot be secured arises, and the residual stress of the plating film decreases. This is because it is not possible to secure a sufficient plating crack generation density when performing. The rapid cooling method at the time of cooling is not particularly limited to a cooling method such as air-water cooling or air cooling as long as the cooling rate of the present invention can be secured.

Next, the substantial δ ₁ thus generated
Preferably, 10 or more cracks per mm are introduced into the plating film composed of the phase. Here, mechanical strain imparting as one means for introducing such a crack will be described.

In the present invention, in order to ensure the low-temperature chipping resistance, which is important, cracks are formed in the alloyed plating film provided on the steel sheet in advance. As a method therefor, there is a method of mechanically imparting strain to a steel sheet. The elongation at that time must be 0.4% or more. If it is less than this, a sufficient crack density cannot be secured, and a problem occurs in low-temperature chipping resistance. On the other hand, if the elongation is greater than 2.0%, the crack generation density increases, but the deformation of the steel plate does not accompany the deformation of the plating, and cracks occur at the plating-base metal interface. There is a problem that the performance is deteriorated. Therefore,
The strain imparted by mechanical elongation of the steel sheet is 0.4 to 2.0
%, Preferably 0.8 to 1.6%. In this case, the distortion imparting means is not particularly limited,
After alloyed hot-dip galvanizing, a method using temper rolling with a skin pass or using a tension leveler is conceivable.

On the other hand, as another means for introducing such cracks, there is an acid solution treatment, that is, an acid washing method. Here, the acid solution treatment method will be described.

In the present invention, processability,
The reason why the low-temperature chipping resistance is improved is presumed to be the following mechanism. That is, the pickling treatment causes a dezincing phenomenon of the plating film, and at this time, the stress of the plating film is released. At this time, the cracks become finer than the cracks generated in the plating film by the stress release method by the above-mentioned mechanical strain. Therefore, at the time of processing, which is a feature of the present invention,
In order to realize the technical idea of suppressing the propagation distance of the plating-base metal interface crack propagation at the time of impact, a chemical acid treatment for forming fine cracks is effective. In addition, the pickling treatment dissolves a phase that adversely affects the flaking resistance remaining on the surface layer, and thus has an advantage that it is advantageous for the flaking resistance.

Further, by performing the alkali treatment before the pickling treatment, it is possible to promote the effect of the pickling treatment. In an alloyed hot-dip galvanized steel sheet, an oxide layer such as alumina may be formed on the surface layer. The oxide layer is removed once by alkali treatment, and the surface is cleaned. Can be expected to promote the formation of cracks and the dissolution reaction of the ζ phase due to dezincing at room temperature.

Although it is a pickling treatment method having such an effect, in the present invention, although not particularly specified, in order to accelerate the reaction to some extent, the pH is 3.0 or less, preferably 1.0 to 1.0. It is preferable to contact with a pickling solution of 2.0 or less. If the pH is greater than 3.0, no clear improvement effect by the pickling treatment is observed, and if the pH is less than 1.0, the reaction promoting effect is large in the pickling treatment, and particularly, the dissolution of the plating film is too fast, This is because crack control becomes difficult.

Immediately after the pickling treatment, washing with water is carried out. At this time, it is preferable that no residue is left on the surface, and the acid solution used is extremely oxidizing. However, those which form an oxide film are not preferred from the viewpoint of surface appearance, and include acids such as sulfuric acid and hydrochloric acid. Also, based on such acids,
It is also possible to use an electroplating solution containing metal ions. As a treatment method, it is only necessary to bring the acid solution into contact with the galvannealed film, and an immersion method, a spray method, or the like can be considered.

Further, the alkali treatment is carried out before the pickling treatment. The pH is preferably 9.0 or more, though not particularly specified. If the pH is less than 9.0, the effect of the present invention due to the acid treatment is more stable, and the treatment time cannot be shortened. In this case, the alkali treatment liquid is also not preferable because the silicate-based alkali degreasing agent is not preferably left with a residue on the surface by such washing.
Alkali metal hydroxides such as sodium hydroxide and potassium hydroxide are preferred. As a treatment method at that time, an immersion method, a spray method, and the like can be adopted as in the pickling treatment.

Here, as the above-described crack introducing means,
The temper rolling with the skin pass roll and the pickling treatment as described above may be combined. The alloyed hot-dip galvanized steel sheet according to the present invention thus obtained is further used, for example, as an exterior material for automobiles by appropriately providing a coating film. Good.

[0037]

Next, the low-temperature chipping resistance according to the present invention,
The effects of the alloyed molten steel sheet having excellent workability will be described with reference to examples. (Example 1) Cold rolled steel sheets having the chemical composition values shown in Table 1
8 mm) for the test.

[0038]

[Table 1]

G1 material based on this cold-rolled steel sheet and having a coating weight of 45 g / m ² adjusted in a hot-dip galvanizing bath
(No alloying treatment, Al content in bath = 0.13%). An alloyed hot-dip galvanized steel sheet obtained by subjecting this GI material to alloying treatment in a heating furnace according to various heat patterns was subjected to a test. At that time, the heating pattern
° C / sec, the alloying holding time was fixed at 10 seconds, the alloying holding temperature was changed from 480 to 650 ° C, and the cooling rate to 420 ° C or lower was changed from 7 to 50 ° C / sec.

Further, in order to examine the effect of mechanical straining, the elongation of the steel sheet was set to 0.0 to 3.0% in a small temper rolling mill.
And temper rolling was performed. As a survey item of the test material thus obtained, the alloy layer structure of the plating film is as follows.
As described above, it was determined by an electrochemical method using a galvanostatic electrolytic stripping method. The electrolysis conditions at that time are as shown in Table 2. The alloy phase structure ratio was measured by measuring the melting time at the potential of each alloy phase.

[0041]

[Table 2]

The method for measuring the crack density of the plating film is as follows.
As described above, surface SEM observation was performed at a magnification of × 500. At this time, 10 mm after observing an arbitrary part with a measurement width of 1 mm
The average value of the points was adopted. Workability was evaluated by powdering resistance and flaking resistance. The test conditions for those properties were as follows.

Under the conditions shown in Table 3, the powdering resistance was measured by reducing the amount of powdering peeled off from the side wall when the cylinder was drawn. As a comparison standard at that time, 20 mg / unit was adopted. Investigation of the relationship with the amount of plating peeling during actual car body processing reveals that if the amount of powdering is less than 20 mg / piece (preferably, 10 mg / piece or less), there is no problem for body exterior use. It is. If the amount of peeling is larger than this, it is not preferable because the plating powder peeled off during plating of the vehicle body is pushed into the steel sheet and appears as a surface flaw.

[0044]

[Table 3]

The flaking resistance was evaluated by a flat sliding test method. As shown in Fig. 3 and Table 4, the test method used was a flat-bead flat plate in which a test steel sheet pressed from both sides by a distance of 50mm was pulled out at a speed of 200mm / min for a distance of 100mm. A pull-out test was performed, and the evaluation was performed by measuring the friction coefficient of the steel sheet surface at that time. As a criterion at that time, 0.14 was adopted as the friction coefficient. As a result of investigating the correlation between actual press cracking and friction coefficient,
If the coefficient of friction was 0.14 or less, it was proved that the steel sheet for exterior use was at a level having substantially no problem.

[0046]

[Table 4]

The low-temperature chipping resistance was evaluated by a coating system (3 coats) shown in Table 5 and a gravelo test.
As an evaluation criterion at that time, the peeling area ratio from the plating-base metal by the gravelo test was measured, and the peeling area ratio was 5%.
If it is below, there is no practical problem.

[0048]

[Table 5]

Table 6 summarizes the effects of the present invention based on these evaluation criteria.

[0050]

[Table 6]

From Nos. 1 and 2 in Table 6, the holding temperature was
If the temperature cannot be maintained at 500 ° C. or higher, the amount of ζ phase is large, and a substantially δ ₁ single-phase alloyed hot-dip galvanized film cannot be obtained, indicating that the flaking resistance cannot be satisfied.

According to Nos. 3 and 4, if the mechanical elongation cannot be maintained at 0.4% or more, it is impossible to obtain a plating crack generation density within the scope of the present invention, and the low temperature chipping resistance is ensured. It turns out that it is not possible.

According to No. 12, if the cooling rate is not 10 ° C./sec or more, even if a mechanical elongation rate is given, the crack generation density, which is the expected effect of the present invention, cannot be secured. It turns out that chipping property cannot be ensured.

(Embodiment 2) Next, in this embodiment, a pickling treatment was performed in place of the temper rolling in Embodiment 1.

The test materials had the same chemical component values as in Example 1,
Using a cold-rolled steel sheet having a thickness, an acid pickling treatment was performed in a laboratory on a production line using a steel sheet subjected to an alloying hot-dip galvanizing treatment.

The plating film composition had a basis weight of 60 g / m ² , an Al content of 0.13% in the bath, an alloying holding temperature of 520 ° C., a cooling rate after the alloying treatment of 10 ° C./sec, and a mechanical strain. Was based on a 0.4% addition in a temper rolling mill in an actual production line. Alloy phase structure in that case, zeta Airyo is 10 wt%, [delta] ₁ Airyo 84 wt%, gamma Airyo is 6 wt%, cracking density was 11 / mm.

The above galvannealed steel sheet was subjected to pH =
FIG. 4 shows the results of crack generation density, flaking resistance (slidability), and low-temperature chipping resistance when immersed in hydrochloric acid adjusted to 0.5 to 4 for 20 seconds. The powdering resistance is not affected by the post-processing, and the amount of powdering is as follows:
There was no problem at 8 mg / unit.

Next, the effect of the pickling treatment after the alkali treatment is described with reference to examples.

The test materials had the same chemical component values as in Example 1,
Using a cold-rolled steel sheet having a large thickness, a post-treatment was performed in a laboratory in an actual production line using a steel sheet subjected to a galvannealing treatment.

The plating film composition had a basis weight of 55 g / m ² , an Al content of 0.13% in the bath, an alloying holding temperature of 520 ° C., a cooling rate after the alloying treatment of 10 ° C./sec, and a mechanical strain. Was based on a method in which temper rolling in an actual production line was not performed. The alloy phase structure at that time,
9 wt%, [delta] ₁ Airyo 86 wt%, gamma Airyo is 5 wt%, cracking density was 2 / mm.

Based on the alloyed hot-dip galvanized steel sheet, it was immersed in an alkaline aqueous solution having a pH of 9.0, a temperature of 70 ° C. and a concentration of 1% for 2 seconds, and then 5% containing Fe ions = 2 g / l.
Figure 5 shows the results of cracking density, anti-flaking properties (sliding friction coefficient), and low-temperature chipping resistance (plated area ratio) when immersed in a sulfuric acid solution (pH = 1.8) for 0 to 10 seconds. Was. The powdering resistance was not affected by the post-treatment, and the powdering amount was 7 mg / piece, and there was no problem.

FIG. 4 shows that the pickling treatment improves the low-temperature chipping resistance and the flaking resistance. At this time, if the pH of the acid treatment liquid is 0.5, surface abnormalities occur, which is not preferable, and it can be seen that the pH of the acid treatment liquid for securing a very excellent region is 1.0 to 2.0.

From FIG. 5, it can be seen that by performing the pickling treatment, cracks can be generated in the plating film without applying mechanical strain, and the alkali treatment alone has no effect of crack generation. However, the flaking resistance (slidability) is rather deteriorated, which is not preferable. By performing the acid treatment, cracks are generated, and the flaking resistance (slidability) and the low-temperature chipping resistance are improved.

These results are compared with those of FIG.
By performing the alkali treatment, it is easier than pickling only
It can be seen that good performance can be obtained in a shorter time, and that the combination of the alkali treatment and the pickling treatment is more effective.

[0065]

According to the present invention, the flaking resistance of an alloyed hot-dip galvanized steel sheet, in particular, the slipperiness at the time of working can be improved, so that the press cracking of the plated film at the time of working is improved and the steel sheet is stabilized. Workability can be ensured and powdering resistance is improved, so that surface press defects due to plating peeling during processing can be reduced. Since the low-temperature chipping resistance, which is a plating cracking phenomenon at the time of low-temperature impact after coating, which was an obstacle to ensuring such workability, can be improved, alloying melting, which has been conventionally studied for automotive exterior materials, has been studied. Relatively easy and inexpensive alloyed hot-dip galvanized steel sheet without resorting to expensive and complicated process alloyed hot-dip galvanized steel sheet for applying upper flash plating on galvanized steel sheet Can be provided. Such features are most suitable for alloyed hot-dip galvanized steel sheets that are required to ensure low cost and high performance stably for automobile body outer panels, and will be able to accurately respond to expanding needs in the future. The industrial value of the present invention is extremely large.

[Brief description of the drawings]

FIG. 1 is a schematic view of the composition of an alloyed hot-dip galvanized film which is a premise of the present invention.

FIG. 2 is a schematic diagram when viewed from a surface and a cross section when a crack is generated.

FIG. 3 is a schematic diagram when a planar sliding test is performed for evaluating flaking resistance.

FIG. 4 is a graph illustrating a relationship between a pickling treatment liquid and product performance when a pickling treatment is performed.

FIG. 5 is a graph illustrating the relationship between the pickling treatment and the product performance after performing the alkali treatment.
This shows the superiority of the effects of the present invention.

Claims (4)

[Claims] An alloyed hot-dip galvanized film obtained by heat-treating a hot-dip coated film on a surface of a steel sheet, wherein the plated film is substantially composed of a single δ ₁ phase, and 1 mm An alloyed hot-dip galvanized steel sheet with excellent low-temperature chipping resistance and workability, characterized by having 10 or more cracks per sheet. 2. A coated steel sheet provided with a coating film on the galvannealed coating film according to claim 1. 3. A method for producing an alloyed hot-dip galvanized steel sheet, wherein the steel sheet is immersed in a hot-dip galvanizing bath, and the obtained hot-dip galvanized film is subjected to an alloying treatment by a heat treatment.
In the alloying treatment, the maximum plate temperature of the heat treatment is set to 500 ° C. or higher, and in the subsequent cooling process, the steel plate is cooled to 420 ° C. at a cooling rate of 10 ° C./sec or more.
A method for producing an alloyed hot-dip galvanized steel sheet having excellent low-temperature chipping resistance and workability, wherein strain is imparted at an elongation of 2.0% or less and cracks are generated. 4. A method for producing an alloyed hot-dip galvanized steel sheet in which a steel sheet is immersed in a hot-dip galvanizing bath and the resulting hot-dip galvanized film is subjected to an alloying treatment by heat treatment.
In the alloying treatment, the maximum plate temperature of the heat treatment is set to 500 ° C. or more, and in the subsequent cooling process, the temperature is cooled to 420 ° C. at a cooling rate of 10 ° C./sec or more, and then, the pickling treatment is performed. A method for producing an alloyed hot-dip galvanized steel sheet having excellent low-temperature chipping resistance and workability, wherein an alkali treatment is performed before the treatment to cause cracks.