JP3302910B2 - Alloyed hot-dip galvanized steel sheet with excellent workability and sharpness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process involving sliding from a low surface pressure to a high surface pressure, such as deep drawing or drawing bending, in which the flow of material is good and no processing crack occurs. For example, the present invention relates to an alloyed hot-dip coated steel sheet that can be effectively used for applications requiring excellent clarity, such as an automotive outer panel.

[0002]

2. Description of the Related Art An alloyed hot-dip galvanized steel sheet is formed by heat-treating immediately after hot-dip galvanizing so that Fe of a base steel sheet is diffused and transferred to the hot-dip galvanized layer to form a galvanized layer.
This alloyed hot-dip galvanized steel sheet has not only excellent coating film adhesion, weldability and corrosion resistance, but also lower cost compared to electro-galvanized steel sheet. Because of the feature that thickening is possible, the demand for automobile parts, building materials and the like tends to increase further.

[0003] By the way, in the galvannealed steel sheet,
As the alloying progresses, that is, as the Fe concentration in the galvannealed layer increases, the surface hardness of the galvanized layer increases and the workability improves. On the other hand, as alloying progresses, an Γ layer with a high Fe concentration (Fe layer) is formed at the interface between the alloyed plating layer and the base steel sheet.
₃ Zn ₁₀ ) grows, and this layer is fragile. If it undergoes tensile deformation or compression deformation during bending or drawing, the plating layer cannot follow the deformation of the base steel sheet and peels off (flaking phenomenon). Not only adversely affect corrosion resistance, but also in some cases, exfoliated powder may cause surface defects during press forming. Has also been confirmed. From such a point, it is necessary to avoid the above-mentioned problems, and there is naturally a limit even if the degree of alloying is increased as a means for improving workability.

[0004] Therefore, in order to obtain an alloyed hot-dip galvanized steel sheet exhibiting excellent workability without causing plating delamination at the time of working, for example, a high-F
e-concentration flash plating, high-lubricity oil on the surface of galvannealed layer, method of forming inorganic or organic lubricating film on plating surface, optimization of plating conditions, etc. The method has been implemented.
However, the flash plating method inevitably raises the cost and the production cost due to the addition of equipment, and the method of applying high lubricating oil can improve workability at low cost, but only this treatment Thus, satisfactory workability improvement effects cannot be obtained, and it is difficult to apply the method to difficult deformation processing under high surface pressure. In addition, in the lubricating film forming method, it is possible to obtain a workability improvement effect equal to or higher than that of the flash plating method described above, but it often involves deterioration in paintability and weldability,
In addition, an increase in manufacturing cost cannot be neglected.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to avoid the above-mentioned problems pointed out by flash plating and lubricating film formation. Another object of the present invention is to provide an alloyed hot-dip galvanized steel sheet having excellent workability and sharpness.

[0006]

According to the present invention, which can solve the above-mentioned problems, the surface roughness is determined by the center line average roughness (Ra).
The plating surface has an opening area of 5 to 100 μm ² , a maximum depth of 0.6 to 6.0 μm, and 100 to 500 concave portions per 1 mm ^{2 of the} plating surface. The number of coarse recesses having an area exceeding 100 μm ² and / or a maximum depth exceeding 6.0 μm is 50 or less per 1 mm ^{2 of the} plating surface, and F in the alloyed plating layer
This is an alloyed hot-dip galvanized steel sheet having an e concentration of 8 to 13% by weight and improved workability and sharpness.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reason for determining the surface roughness and the surface properties of the galvannealed steel sheet according to the present invention will be described. Surface roughness and surface properties of alloyed hot-dip galvanized steel sheet are major factors that affect workability, especially when working with sliding such as deep drawing and drawing bending, and these are properly controlled. This is extremely important for improving workability.

First, regarding the surface roughness, when the center line average roughness (Ra) exceeds 1.4 μm, large irregularities existing on the surface of the plating layer cause an increase in sliding resistance at the time of processing, and the lubricating oil Regardless of the presence or absence, since the mold and the protruding tip of the plating layer surface contact and slide with a small contact area, the contact surface pressure at the protruding tip increases and the sliding resistance increases. It leads to deterioration. However, the center line average roughness (Ra) is 1.4 μm or less, more preferably 1.2 μm.
When the thickness is suppressed to not more than μm, the above-mentioned decrease in slidability hardly causes a problem, and excellent workability can be obtained.

For example, FIGS. 1 and 2 show the relationship between the surface roughness (center line average roughness: Ra) and the coefficient of friction in an alloyed hot-dip galvanized steel sheet in which the Fe concentration in the plating layer is fixed to about 10% by weight. FIG. The coefficient of friction in these figures was such that the test piece was pressed from both sides with a plane tool of 20 mm square at a surface pressure of 30 N / mm ² (FIG. 1) and about 100 N / mm ² .
mm ² (FIG. 2) and a value obtained from the pulling load when drawing at a speed of about 150 mm / min.

As is clear from these figures, when the Fe concentration in the galvannealed layer is kept constant,
At low surface pressure, the workability becomes worse as the surface roughness (Ra) increases. On the other hand, even at a high surface pressure, the workability tends to deteriorate as the surface roughness increases, and it can be confirmed that the workability deteriorates even when the surface roughness is too small. Therefore,
As a result of research to ensure excellent workability even under high surface pressure conditions, the surface properties of the plating layer, that is, the size and maximum depth of the recesses present on the plating surface, greatly affect the workability. I found that. Hereinafter, the surface properties of the alloyed plating layer will be described.

Regarding the surface properties, when the size of the concave portion is less than 5 μm ^{2 in the} opening area or the maximum depth is less than 0.6 μm, the concave portion is easily flattened by sliding during processing. In this case, the effect of the lubricating oil as an oil reservoir is not effectively exhibited, and processing defects such as galling tend to occur particularly in difficult molding at high surface pressure. On the contrary, the opening area of the concave portion exceeds 100 μm ² or the maximum depth is 6.0 μm.
If it exceeds m, the sharpness deteriorates or the surface roughness (Ra) becomes large, and the workability is adversely affected for the above-mentioned reason. Therefore, while effectively exhibiting the effect as an oil pool expected in the concave portions present on the plating surface, the appropriate concave portion for not adversely affecting the sharpness has an opening area of 5 to 100 μm ² and the maximum. It was confirmed that the depth was in the range of 0.6 to 6.0 μm. The opening area of the concave portion is more preferably 50 μm ² or more and 80 μm ² or less, and more preferably the maximum depth is 2.0 μm or more and 5.0 μm or less for achieving both workability and sharpness.

Next, if the number of the appropriate concave portions is less than 100 per 1 mm ^{2 of} the plating surface, the lubricating oil will not be sufficiently effective as an oil pool during processing, and the oil will run out due to difficulty in forming under high surface pressure. It is easy to cause processing defects such as galling due to cracks. On the other hand, when the number exceeds 500, not only the sharpness deteriorates, but also the surface roughness (Ra) becomes large, making it difficult to satisfy the requirement of “Ra ≦ 1.4 μm”, and the slidability is reduced. This results in poor workability. Therefore, it is essential that the number of the concave portions having the above-mentioned size and the maximum depth be 100 to 500 per 1 mm ² of the plating surface.

FIG. 3 shows a concave portion on the surface of an alloyed hot-dip galvanized steel sheet in which the Fe concentration in the plating layer is adjusted to about 10% by weight (proper concave portion has an opening area of about 50%).
７０70 μm ² , maximum depth 1.0-4.0 μm, opening area exceeds 100 μm ² and / or maximum depth 6.0 μm
Is a graph showing the relationship between the number of coarse concave portions exceeding 50 and the coefficient of friction. The coefficient of friction in this figure was about 1 N / mm ² by pressing the test piece from both sides with a plane tool of 20 mm square.
It is a value obtained from the pulling load when the drawing is performed at a speed of 50 mm / min.

As is clear from this figure, 1 mm ²
When the number of the appropriate recesses per hit is in the range of 100 to 500, the friction coefficient is low and excellent workability is exhibited. However, when the number is less than 100 or more than 500, the friction coefficient becomes large, and the workability is deteriorated. Can be confirmed. From the viewpoint of achieving both workability and sharpness, the more preferable number of appropriate concave portions is 1
The number is from 00 to 300.

However, even if the opening area and the number of concave portions having the maximum depth are appropriate, the opening area on the plating surface is 10
When the number of coarse recesses exceeding 0 μm ² and / or the maximum depth exceeding 6.0 μm increases, the overall image sharpness is significantly adversely affected, and the object of the present invention cannot be achieved. In order to ensure satisfactory sharpness, it is necessary to suppress the number of the large concave portions having a large opening area and the maximum depth to 50 or less, more preferably 30 or less per 1 mm ² . Thus, when the number of the coarse recesses as described above increases, the sharpness significantly deteriorates the sharpness, and the object of the present invention cannot be achieved.

FIG. 4 shows that the surface roughness (Ra) is 1.
0 to 1.3 μm, and the opening area is 5 to 100 μm
^{2. With} respect to various alloyed hot-dip galvanized steel sheets in which 100 to 400 concave portions having a maximum depth of 0.6 to 6.0 μm exist per 1 mm ^{2 of the} plating surface, the opening area is 100 μm ^2.
And / or a graph showing the relationship between the number of coarse recesses per 1 mm ² and the sharpness which exceeds 6.0 μm and / or the maximum depth, and as is clear from this graph,
It can be seen that when the number of the coarse recesses exceeds 50, sharpness deteriorates extremely.

Factors that greatly affect the workability of an alloyed hot-dip galvanized steel sheet include surface hardness in addition to the surface roughness and surface properties described above. It can be said that the hardness is determined by the Fe concentration in the galvannealed layer. Therefore, also in the present invention, it is important to keep the Fe concentration in the alloyed hot-dip galvanized layer appropriately in order to obtain more excellent workability, and it can be said that the range of 8 to 13% by weight is good. The basis is as shown in FIG. 5. When the Fe concentration is less than 8% by weight, the surface hardness of the plating layer is too low, so that the plating layer easily adheres to the mold even at a low surface pressure, and the surface pressure is reduced. Regardless of the size, the slidability is reduced and satisfactory workability cannot be obtained. When the Fe concentration exceeds 13% by weight, the slidability is improved by improving the surface hardness. However, when the Fe concentration in the alloyed hot-dip galvanized layer is increased, the alloy is hard near the interface between the plated layer and the base steel sheet. Fragile Γ layer (Fe ₃ Zn
₁₀ ) grows, and when tensile deformation or compressive deformation is applied by processing, the plating layer cannot follow the deformation of the base steel sheet, and plating peeling starts from layer Γ.

In practicing the present invention, as an even more preferable requirement, it is desirable to control the Al concentration in the galvannealed layer within the range of 0.1 to 0.6% by weight. That is, Al in the plating layer is composed of Fe and F in the base steel sheet.
It has an effect of forming an e-Al alloy layer and appropriately controlling the Zn-Fe alloying reaction. When the Al content is less than about 0.1% by weight, the Zn-Fe alloying reaction is performed. Rapidly progresses to form a brittle Γ layer at the interface between the base steel sheet and the plating layer, and the plating adhesion tends to deteriorate. On the other hand, when the Al content exceeds 0.6% by weight, Zn- Fe
The part where the alloying reaction progresses quickly and the part where the progress is slow become apparent, and the Zn-Fe alloy layer generated in the early part grows thick by taking in the Zn in the slow part, while the alloy layer in the slow part becomes thin. This is because irregularities are easily formed on the plating surface, which may increase the surface roughness or adversely affect the sharpness.

As described above, according to the present invention, the surface roughness, the surface properties, and the Fe content in the alloyed hot-dip galvanized layer
By properly controlling the concentration, an alloyed hot-dip galvanized steel sheet having excellent workability and sharpness can be provided. Here, the surface roughness and surface properties of the alloyed hot-dip galvanized steel sheet can be easily controlled by skin pass rolling after the alloyed hot-dip galvanizing or the surface roughness of the material steel sheet to be the base plate for plating. Further, the Fe concentration in the alloyed hot-dip galvanized layer can be easily controlled by the temperature and time of the alloying treatment after the plating.

[0020]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited to the following Examples, and the present invention is not limited thereto. It is also possible to implement the present invention with modifications, and all of them are included in the technical scope of the present invention.

Example An Al-killed cold-rolled steel sheet was used as a base steel sheet, and alloying hot-dip galvanizing was formed by changing the alloying treatment time after hot-dip galvanizing, and about 100 g / m ² per side of the base steel sheet. An alloyed hot-dip galvanized steel sheet having a plating layer formed thereon was manufactured.

The obtained alloyed hot-dip galvanized steel sheet was subjected to skin pass rolling under various rolling and roll conditions, and the surface roughness and surface properties of each coated steel sheet were confirmed by the following method, and the workability was confirmed. And the sharpness and plating peeling resistance were examined, and the results shown in Table 1 were obtained.

The surface roughness was measured by a center line average roughness Ra using a stylus type roughness meter. Regarding the surface properties, the opening area and the maximum depth of the concave portion were determined by the surface roughness profile using a stylus type roughness meter, and the S
From the EM image, the number of appropriate concave portions and coarse concave portions having the above opening area and maximum depth per unit area was measured.

As for the workability, as shown in FIG.
The test piece 1 is pressed from both sides with a surface pressure of about 30
N / mm ² and about 100 N / mm ²
From the pulling load F and the pressing load P when the drawing was performed at a speed of 50 mm / min, the friction coefficient (μ = F
/ 2P) was determined and used as an evaluation standard for workability.

The sharpness was measured using a stylus type roughness meter.
_{The CA} was measured and used as an evaluation index for sharpness. Regarding the plating peeling resistance, a bending test of the test piece 1 was performed using a V-shaped punch 3 and a die 4 having a bending angle of 60 ° and a bending radius of 1 mm as shown in FIG. Was peeled off with a tape, and the peeling state was visually evaluated.

[0026]

[Table 1]

As is clear from Table 1, test material 1
(Comparative material) has a low surface pressure because the surface roughness (Ra) is large,
Both high surface pressure and poor workability and poor clarity. Test material 6
Since the (comparative material) has a large maximum depth of the concave portion and a large surface roughness (Ra), the workability is poor and the sharpness is poor in both low surface pressure processing and high surface pressure processing.

In the test material 9 (comparative material), the number of recesses having the opening area and the maximum depth per 1 mm ² is small.
The workability and sharpness at low contact pressure are excellent, but the workability at high contact pressure is poor. Sample 10 (comparative material) has the opening area of 5 to 100 μm ² and the maximum depth of 0.6 to 6.0 μm.
Since the number of appropriate concave portions per 1 mm ² is too large, not only the sharpness is poor, but also the workability is slightly poor at both low surface pressure and high surface pressure. The test material 13 (comparative material) is good in both the surface roughness Ra, the shape and the number of concave portions on the surface, but is excellent in sharpness because of the low Fe concentration in the alloyed hot-dip coating layer. The workability at a high surface pressure is slightly inferior as well as the workability at a high surface pressure. In addition, the test material 14 (comparative material) also has a high Fe concentration in the alloyed hot-dip galvanized layer, and thus has excellent workability at both low surface pressure and high surface pressure, and also has excellent clarity. However, the plating peeling resistance is poor. The test material 15 (comparative material) has excellent workability at both high and low surface pressures because the number of coarse recesses is too large, but the sharpness is extremely poor.

On the other hand, test materials 2, 3, 4, 5, 7,
8, 11, and 12 (all examples) have surface roughness (R
a), the nature and number of the concave portions on the surface, and the Fe concentration in the alloyed hot-dip galvanized layer are all appropriate, so that both low surface pressure and high surface pressure are excellent in workability and excellent in sharpness. Further, it is understood that the plating resistance is excellent.

[0030]

The present invention is constructed as described above, and exhibits excellent workability not only in low surface pressure but also in difficult processing in which high surface pressure is applied, causing problems such as processing cracks. Nothing. In addition, for example, the sharpness required when used as an outer panel material for automobiles is also good, and it is possible to provide an alloyed hot-dip galvanized steel sheet that satisfies all the characteristics required by consumers at a relatively low price. became.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the surface roughness (Ra) of an alloyed hot-dip galvanized layer and the friction coefficient during low surface pressure working.

FIG. 2 is a graph showing the relationship between the surface roughness (Ra) of an alloyed hot-dip galvanized layer and the coefficient of friction during high surface pressure working.

FIG. 3 is a graph showing the relationship between the number of recesses on the surface of a galvannealed layer and the coefficient of friction.

FIG. 4 is a graph showing the relationship between the number of coarse recesses present on the surface of a galvannealed steel sheet and the sharpness.

FIG. 5 is a graph showing the relationship between the Fe concentration in the galvannealed layer and the coefficient of friction.

FIG. 6 is a schematic view showing a sliding test method for measuring a coefficient of friction.

FIG. 7 is a schematic diagram showing a V-bending test method for evaluating plating peel resistance.

[Explanation of symbols]

 Reference Signs List 1 test piece 2 plane tool 3 punch 4 die

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-18402 (JP, A) JP-A-7-18403 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. A surface roughness having a center line average roughness (Ra) of 1.4 μm or less, a plating surface having an opening area of 5 to 100 μm ² and a maximum depth of 0.6 to 6.0 μm. There are 100 to 500 depressions per 1 mm ² of the plating surface, and coarse depressions having an opening area exceeding 100 μm ² and / or a maximum depth exceeding 6.0 μm are present on the plating surface.
50 or less per mm ² , and Fe in the alloyed plating layer
An alloyed hot-dip galvanized steel sheet excellent in workability and sharpness, characterized in that the concentration is 8 to 13% by weight.