JP3346157B2 - Alloyed hot-dip galvanized steel sheet with excellent chipping resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvannealed steel sheet having excellent chipping resistance.
In particular, a coated steel sheet having a coating applied to its surface is suitably used for an automobile body or the like.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets have high weldability,
Because of its excellent paintability and corrosion resistance after painting, it is mainly used for automobile bodies. Especially in cold climates, a car is driven while jumping up snowmelt salt particles or pebbles sprayed on the road, but when these particles collide with the body surface, a so-called chipping phenomenon occurs and it is applied to the body. The peeled coating film and plating layer are peeled off, leading to a decrease in corrosion resistance and a deterioration in appearance. In other words, when pebbles collide with the surface of a coated steel plate coated with a coating film, the impact energy generated at that time is transmitted to the coating film and further to the plating layer and the base steel plate under the coating film, and the coating film and the coating layer Peeling occurs.

Therefore, various proposals have been made to avoid such a chipping phenomenon. For example, as a typical countermeasure against chipping, lowering the baking temperature after painting, based on the idea that if the hardness of the coating is reduced and softened, impact energy due to pebbles etc. can be absorbed in the coating,
A method such as using a coating film containing a soft paint is employed. However, when the coating film is softened by these methods, there is a problem that the surface is apt to be scratched, and the method of blending a soft paint causes an increase in cost. Furthermore, as a general property of the coating film, its hardness tends to increase with a decrease in the environmental temperature. Therefore, even when the above-described softening treatment is performed, especially during running at a low temperature, it is as expected. The impact energy absorption effect of
At present, the occurrence of the chipping phenomenon is inevitable.

[0004] Alternatively, Japanese Patent Publication No. 7-62226 discloses a steel sheet in which the amount of the alloy layer is controlled in relation to the average thickness of the Γ phase in the alloy layer in order to prevent chipping and the like and improve low-temperature impact adhesion. Proposed. Stipulated in this gazette.
When the total plating amount is calculated based on the relational expression between the phase thickness and the alloy layer amount (total plating amount), and the Γ layer thickness (0.6 to 1.35 μm), the lower limit value is 39.6 g / m ² . Become. That is, according to the above-mentioned publication, the minimum amount of plating required to absorb the impact energy at the time of chipping is 39.6 g / m ² . However, on the plating side where chipping occurs,
Generally, the amount of plating is reduced to about 30 g / m ² in consideration of plating peeling during processing, cost, and the like, and the steel sheet described in the above publication does not satisfy the required characteristics. Accordingly, there is a strong demand for providing a plated steel sheet having good chipping resistance even when the plating amount is as small as about 30 g / m ² .

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to maintain good appearance and corrosion resistance and to reduce the chipping phenomenon even when the plating amount is small. It is an object of the present invention to provide an alloyed hot-dip galvanized steel sheet that can be efficiently prevented, particularly, has excellent chipping resistance at low temperatures.

[0006]

The alloyed hot-dip galvanized steel sheet of the present invention which can solve the above-mentioned problems is defined as the average thickness of the Γ phase in the coating layer (hereinafter abbreviated as [TΓ]). ): 2.0 μm or less, and the center line average roughness (Ra) of the plating layer surface is 3.0 μm or less, and [T · Γ] and Ra are represented by the following formula (1). The gist lies in that a coating film is formed on the surface of the galvannealed steel sheet satisfying the above. [T · Γ] × Ra ≦ 1.5 (1) Further, an alloyed hot-dip galvanized steel sheet serving as a base material for obtaining the coated steel sheet is also included in the scope of the present invention.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In order to provide an alloyed hot-dip galvanized steel sheet having excellent chipping resistance, the present inventor has described in detail the coating peeling portion caused by the chipping phenomenon and the plating layer peeling portion inside the coating. Study was carried out. As a result, as a form of peeling by chipping, when the entire plating layer including the coating film is peeled from the base steel sheet (hereinafter referred to as A-type peeling), the plating layer does not peel from the base steel sheet but only the coating film is removed. There are two types of peeling from the plating layer (hereinafter, referred to as B-type peeling), and it has been found that these peeling occur at the same time, although the size of the peeled portion differs. Furthermore, A
When the mold release and the B-type release are compared, in the case of the B-type release, the coating film is peeled off, but the plating layer still covers the base steel sheet, so that the rust-preventive force of the plating layer is maintained, whereas A
In the case of mold release, since the plating layer applied directly on the base steel sheet is separated, the base steel sheet is directly exposed to a corrosive environment. As a result, rust is immediately generated on the surface of the base material, which significantly reduces the appearance and promotes perforated corrosion of the steel sheet. Therefore, in order to prevent peeling due to chipping, it is extremely important how the degree of A-type peeling can be suppressed as compared with B-type peeling.

[0008] In view of the above, it is necessary to consider cost reduction and securing scratch resistance.
From the point of view, normal paint was used and normal paint treatment was applied.
Alloys affecting the occurrence of A-type and B-type delaminations in steel sheets
The factors on the galvannealed steel sheet side were investigated. Concrete
In general, alloyed hot-dip zinc alloy with various plating layer properties
Steel plate (coating weight: 60g / m^Two ), For automobiles
General three-coat coating (electrodeposition coating, intermediate coating, top coating)
(Three layers of coating) were subjected to a chipping test.
The chipping test was performed on a test piece cooled to -20 ° C.
Spray rock (size 5mm, 500g) at 90 ° angle
Pressure: 4kgf / cm^Two After projecting with a microscope, the surface is
(50x magnification) and observe the interface between the plating layer and the base steel sheet.
Release diameter (A type release diameter) and interface between coating film and plating layer
From each other (B-type peel diameter) were measured. Like this
10 of the total measured values obtained
Of the A-type peeling system and the average
And a B-type release system. The properties of the plating layer
Fe and Al concentration in plating layer, plating layer
Phase [す る phase, δ₁ And Γ phases (Γ and Γ ₁ of
Thickness), surface roughness [central gland average roughness (R
a), maximum roughness (Rmax), etc.]. these
Investigation of the relationship between the factors of
As described above, the two factors of the average thickness of the が phase and Ra are
Was found to have a significant effect on Specifically, A
As the mold release diameter increases, the B-type release diameter decreases, and conversely
It is said that as the B-type release diameter increases, the A-type release diameter decreases.
And there is an inverse correlation between the two
I understood. Furthermore, the average thickness of the Γ phase ([T
And Ra also increases, the A-type release diameter increases (
Therefore, in this case, the B-type peeling diameter becomes smaller.)
When [TΓ] is thin and Ra is small, the B-type peeling diameter increases.
(Therefore, in this case, the A-peel diameter is small)
).

The inverse correlation between the A-type release diameter and the B-type release diameter is based on the adhesion at the interface between the coating film and the plating layer (the coating film
This can be explained by comparing the adhesion at the interface between the plating layer and the base steel sheet (referred to as the adhesion at the interface between the plating layer and the base). That is,
The impact energy generated by chipping is consumed for peeling at the interface side where the adhesion strength is small among both,
Peeling on the interface side where the adhesion is large is suppressed.
Therefore, when the adhesion between the coating film and the plating layer interface is lower than the plating layer / substrate interface, the impact energy due to chipping is consumed for peeling at this interface. Peeling at the interface is suppressed. Conversely, when the adhesion at the coating film / plating layer interface is higher,
Peeling at the plating layer / substrate interface is promoted.
In other words, in order to efficiently suppress the A-type peeling, which has a large influence of the peeling due to the chipping phenomenon, as compared with the B-type peeling,
It is useful to lower the adhesion at the interface between the coating film and the plating layer as compared to the interface between the plating layer and the base material.

From such a viewpoint, the properties of each phase constituting the plating layer will be examined. Among Γ phase is generated in the base steel sheet interface, and a property that is high and brittle hardness than other ζ phase and [delta] ₁ phase. Accordingly, when the thickness of such a Δ phase is increased, the adhesion at the interface between the plating layer and the base material is reduced, so that the A-type peeling becomes remarkable. Next, regarding the surface roughness of the plating layer, when Ra increases, the adhesion between the coating film and the plating layer increases due to the anchoring effect, so that the A-type peeling is likely to occur.

From the above results, in order to obtain a plated steel sheet which is excellent in chipping resistance, has good appearance and can maintain corrosion resistance by suppressing the degree of A-type peeling as compared with B-type peeling, It is useful to increase the adhesion at the substrate interface (that is, to lower the adhesion at the coating film / plating layer interface). For this purpose, it is necessary to reduce [TΓ] and reduce Ra. It is.

In view of the above findings, FIG. 2 summarizes the relationship between [TΓ] and Ra on the occurrence of A-type peeling and B-type peeling. In the figure, ○ indicates A-peel diameter <B-peel diameter (B-peel controlled area), and X indicates A-peel diameter ≧ B-peel diameter (A-peel controlled area). As is clear from the figure,
The boundary line between the B-type controlled area and the A-type controlled area is [T ·
Γ] × Ra almost coincides with the parabola indicated by 1.5, and in the region indicated by [TΓΓ × Ra ≦ 1.5 (hatched portion in the figure), B-type peeling is dominant. It can be seen that the A-type peeling area is suppressed. However, as described above, [T
・ Γ]> 2 μm, the adhesion between the plating layer and the substrate interface is significantly reduced. In this case, A-type peeling occurs even if Ra is controlled to be small. Similarly, Ra> 3 μm
In this case, the adhesion at the coating film / plating layer interface becomes extremely high,
Even if [TΓ] is controlled to be thin, A-type peeling occurs. Therefore, in order to obtain a desired plated steel sheet, it is necessary to satisfy all of these requirements. [TΓ] is preferably 1
μm or less, and more preferably 0.5 μm or less. On the other hand, Ra is preferably 1.5 μm or less, more preferably 1.0 μm or less. It is needless to say that the values of [T · Γ] and Ra are preferably smaller according to the gist of the present invention, but should be determined in consideration of the production cost and the production efficiency at the actual operation level. A coated steel sheet in which a coating is applied to the surface of the steel sheet of the present invention that satisfies such requirements has particularly excellent chipping resistance at low temperatures.

In the present invention, [T · Γ] and Ra are controlled from the viewpoint that the degree of A-type peeling can be prevented as compared with B-type peeling by increasing the adhesion at the plating layer / substrate interface side. It is characterized by the point, and other requirements are not specified. Therefore, as for the component composition of the steel sheet and the coating conditions (such as the type of coating material, coating method, thickness of the coating film, baking temperature after coating, etc.), suitable conditions are appropriately set within the range not impairing the effects of the present invention. You can choose.

Hereinafter, the present invention will be described in detail with reference to examples.
However, the following examples do not limit the present invention,
All modifications and alterations without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.

[0015]

【Example】

Example 1 Various alloys shown in Table 1 were obtained by passing an ultra-low carbon cold-rolled steel sheet containing Ti and Nb through a continuous hot-dip galvanizing line and changing [T · Γ] and Ra in the plating layer. A hot-dip galvanized steel sheet was manufactured. [TΓ] and Ra were controlled by changing alloying temperature, line speed, skin pass conditions (roll surface roughness, rolling reduction) after alloying treatment, and the like.

The alloyed hot-dip galvanized steel sheet obtained in this manner is subjected to the same method as described above [that is, electrodeposition coating,
After coating with three layers of an intermediate coating and a top coating (all of which are usually coated), a chipping test was performed, and the A-type release diameter and the B-type release diameter were measured. In addition, No. 4 and No.
In No. 5, the coating conditions were partially changed, and the total coating thickness of No. 4 was 130 μm (all others were 100 μm).
Is a hard coat applied to the top coat compared to the normal coat. Table 1 also shows the obtained results.

[0017]

[Table 1]

As is clear from the results shown in Table 1, the steel sheets of Nos. 1 to 3 satisfying the requirements of the present invention were able to significantly suppress the A-type peel diameter as compared with the B-type peel diameter. Also, it was found that good chipping resistance was obtained even in No. 4 and No. 5 having different coating conditions.

[0019]

As described above, the alloyed hot-dip galvanized steel sheet of the present invention can effectively prevent the chipping phenomenon while maintaining good appearance and corrosion resistance.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of [TΓ] and Ra on the peel diameter.

FIG. 2 is a graph showing the relationship between [TΓ] and Ra in FIG. 1;

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-34213 (JP, A) JP-A-3-191045 (JP, A) JP-A-6-93402 (JP, A) (58) Field (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] 1. An average thickness of a Γ phase in a plating layer: 2.0 μm
m or less, and the center line average roughness of the plating layer surface: 3.
With 0μm or less, der which coating is formed on the surface of the alloy galvannealed steel sheet you satisfy the following formula (1)
A coated steel sheet with excellent chipping resistance . [T · Γ] × Ra ≦ 1.5 (1) (where [T · Γ] represents the average thickness of the Γ phase, and Ra represents the center line average roughness of the plating layer surface, respectively) 2. A method for obtaining the coated steel sheet according to claim 1.
Alloyed hot-dip galvanized steel sheet that serves as a base material .