JPH06344004A - Alloyed hot-dip galvanized steel sheet for automobiles and method for producing the same - Google Patents

Abstract

(57) [Summary] [Object] To provide an alloyed hot-dip galvanized steel sheet having excellent image clarity, press formability, and powdering property, which can be easily produced, and a method for producing the same. [Structure] The surface of the steel sheet surface that corresponds to the exterior surface of the automobile is bright-finished, and a skin pass roll with a roll surface roughness Ra of 0.2 μm or less is used. 2.0 μm or more, 5.
The PGD value is adjusted to 0.8 or more and the surface friction coefficient is adjusted to 0.25 or less, respectively, by temper rolling with elongation rates of 0.4% or more and 2.0% or less using a skin pass roll of 5 μm or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet for automobile body outer panels, which is excellent in sharpness, workability and powdering property, and a method for producing the same.

[0002]

2. Description of the Related Art Thin steel sheets, which are formed by pressing, such as outer panels of automobile bodies, are generally cold rolled steel sheets, which are often used from the viewpoint of coating finish, while from the viewpoint of rust prevention, they are melted. Those that have been galvanized or variously electroplated are used.

However, the alloyed hot-dip galvanized steel sheet, which is useful in terms of rust prevention, has a large unevenness on the surface of the plated layer which has been alloyed, so that the quality of the coated surface, particularly the sharpness, is poor. In addition, since press formability is also difficult and plating film peeling (powdering property) occurs during press forming, it has not yet reached the practical range in applications where paint finish is important.

Here, the "vividness" is an index of the surface smoothness after coating, and the diffused reflection of the coated surface is small and the gloss is high, while the image distortion on the coated surface is small. Is an index that also shows. With its distinctiveness, the image clarity of passenger cars, light vehicles, wagon vehicles, trucks, etc. is directly and visually appealing to the customer of the high overall quality of the automobile. , Which is an important quality control indicator.

Regarding the details of the evaluation means, various methods are being developed, but generally, a simple image clarity measuring device (PGD value) having high correlation with visual evaluation is used. In the present specification, this PGD value is also used as a means for evaluating image clarity.

The sharpness of such a coated surface is of course influenced by the type of coating material and the coating method, but is also affected by the roughness of the surface of the original plate as the coating base, which is especially If the flat portion occupies a small proportion and the irregularities are severe, diffuse reflection occurs and the glossiness is impaired, as well as distortion of the image causes deterioration of the image clarity and deterioration of the image clarity.

[0007] On the other hand, the press formability depends on the alloy composition and properties of the coating base plate, but the surface texture affects the so-called seizure property.
Therefore, in terms of this press formability, it is also required to have a high oil retaining ability, and a steel sheet with severe surface irregularities has a high oil retaining ability,
It is said that the press formability is good. Therefore, the press moldability and the sharpness are two properties that are incompatible with each other.

Further, the powdering resistance tends to increase the coating weight of the plating from the viewpoint of the recent improvement of the rust prevention ability, so that the plating film peeling easily occurs due to the processing, and the securing of the powdering resistance is also a big problem. Has become. The above-mentioned problems are almost the same in the case of any alloyed hot-dip galvanized steel sheet for coating and working with respect to the outer panel of an automobile body.

As a method for improving the clarity of an alloyed hot-dip galvanized steel sheet, Japanese Patent Laid-Open Nos. 2-175004 and 2-274860 have proposed a method of forming a regular pattern of craters by laser processing. A method of transferring the surface pattern onto the surface of the galvannealed alloy by a skin pass roll or performing a temper rolling twice with a skin pass roll having a regulated roll surface roughness is disclosed in JP-A-2-
274853, 2-274854, 2-274855, 2-27
No. 4856, No. 2-274857, No. 2-274859 discloses a method of prescribing the shape of an alloyed hot dip galvanized surface. In Japanese Patent Laid-Open No. 4-285149, a base metal plate before plating is specified. Methods for adjusting the roughness are disclosed, respectively.

However, the laser dull roll has a high manufacturing cost for the roll, which is disadvantageous in cost, and the double skin pass has a large number of steps, which is disadvantageous in cost, and control is required to define the shape of the galvannealed surface. It ’s very difficult,
All the proposed methods have deficiencies, and further improvement is required.

Further, the surface roughness of the skin pass roll is Ra 1.0 to 3.0 μm as in the above-mentioned Japanese Patent Laid-Open No. 2-274860.
Even if it is specified that the surface roughness of the roll is rough, it cannot be expected that the image clarity is improved.

[0012]

DISCLOSURE OF THE INVENTION It is an object of the present invention to further enhance the sharpness and to improve the press formability of a galvannealed steel sheet for coating and working which is used for the above-mentioned purposes. It is the establishment of technology that can achieve both improvement and improvement. A specific object of the present invention is to provide an alloyed hot-dip galvanized steel sheet which has excellent image clarity, press formability, and powdering property and can be easily produced, and a method for producing the same.

[0013]

Means for Solving the Problems As described above, the present inventors have focused on satisfying both of them as a whole by imparting contradictory properties to the front and back surfaces of a galvannealed steel sheet separately. As a result of further studies, it was found that the press formability is mainly controlled by the punch side friction coefficient, and the die side friction coefficient is not so important, and the present invention was completed.

Incidentally, the same idea is shown in Japanese Patent Laid-Open No. 2-274860, but in comparison with that, the present invention is further advantageous in that it is possible to secure the image clarity.

According to the present invention, a temper-rolled alloy in which the image clarity evaluation value PGD of the surface corresponding to the outer surface of an automobile body is 0.8 or more and the surface friction coefficient of the opposite surface is 0.25 or less. It is a hot-dip galvanized steel sheet.

[0016] Further, from another aspect, the present invention provides a surface corresponding to an automobile exterior surface of a steel sheet surface after plating alloying when temper-rolling an alloyed hot-dip galvanized steel sheet for an automobile body outer plate. A skin pass roll with bright finish and roll surface roughness Ra of 0.2 μm or less was used. On the opposite side, electric discharge or shot dull processing was applied to the roll surface roughness Ra of 2.0 μm or more and 5.5 μm or less. A method for producing an alloyed hot-dip galvanized steel sheet having excellent image clarity, press formability and powdering property, which is characterized by performing temper rolling with an elongation of 0.4% or more and 2.0% or less using a roll. is there.

[0017]

According to the present invention, the image clarity evaluation value PGD of the surface corresponding to the outer surface of the automobile body is set to 0.8 or more and the surface friction coefficient of the opposite surface is set to 0.25 or less. In order to satisfy the image clarity and the press formability at the same time, the PGD value was 0.4 to 0.7 and the coefficient of surface friction was 0.25 to 0.27 in the related art.

According to another aspect of the present invention, a bright skin pass roll having a roll surface roughness Ra of 0.2 μm or less is formed on the outer surface of the outer surface of an automobile body in order to improve sharpness and powdering. The surface of the alloyed hot-dip galvanized sheet is stretched using a skin pass roll with a surface roughness Ra of 2.0 μm or more and 5.5 μm or less that has been subjected to electrical discharge or shot dull processing on the opposite side to improve press formability.
The temper rolling is performed at 0.4% or more and 2.0% or less.

The reason for limiting the processing conditions in the present invention as described above will be described. In order to improve the sharpness, it is necessary to smooth the plated surface. Therefore, when temper rolling is performed after the plating alloying treatment, a skin pass roll mat is applied to the surface corresponding to the exterior surface of the alloyed hot-dip galvanized steel sheet for automobile body by bright finishing rolls with a surface roughness Ra of 0.2 μm or less. Rolling elongation 0.4% or more, 2.0
The temper rolling is performed at a rate of not more than%.

Even if a bright finishing roll is used and the rolling elongation is less than 0.4% and the roll surface roughness Ra exceeds 0.2 μm, the plating surface is not sufficiently smoothed and the sharpness is not sufficiently improved. is there. Further, if the rolling elongation exceeds 2.0%, the load on the skin pass device becomes large, which is disadvantageous in terms of cost. In addition, improvement of the sharpness can be seen by performing similar temper rolling with a laser dull skin pass roll,
The powdering property tends to be poor.

Further, in order to improve the press formability, it is necessary to enhance the oil retaining ability, and the plating surface needs to have a certain degree of roughness. However, since the oil retaining ability is inferior in the bright finish skin pass roll, in the present invention, on the surface opposite to the outer surface of the automobile body, which is the press punch side,
In particular, electric discharge or shot dull processing is applied to increase oil retention capacity, and roll surface roughness Ra is 2.0 μm or more, 5.5 μm
The following skin pass rolls also have an elongation of 0.4-2.
0% temper rolling is performed. If the amount is out of these ranges, the oil retaining ability, that is, the effect of improving press formability is insufficient.

Here, the "bright finish roll",
“Electric discharge machining”, further “shot dull machining” and the like are already well known, and they may be used in the present invention, and are not particularly limited per se. Next, the working effects of the present invention will be described more specifically by way of examples.

[0023]

[Example] Coating weight 25 g / m ² or more, degree of alloying 9 to 11%
The alloyed hot-dip galvanized steel sheet was subjected to temper rolling with a roll mat and skin pass elongation changed using a laboratory rolling roll. The test steel sheet had a plate thickness of 0.8 mm, and the surface roughness of the base material used was Ra ≦ 0.2 finished with a bright roll and Ra: 0.7 to 1.0 μm finished with a dull roll. The material used was as shown in the table below.

[0024]

[Table 1]

The alloyed hot-dip galvanized steel sheet is produced by plating at a bath temperature: 460 ° C., Al concentration in the bath: 0.11 wt%, plate temperature: 500-550 ° C., treatment time: 10-60 seconds. Treatment and alloying treatment were performed.

Skin pass conditions were changed on the galvannealed steel sheet obtained as described above (roll mat,
Elongation rate) The hot-dip galvanized steel sheet after temper rolling was examined for image clarity, press formability (sliding property), and powdering property. The test method in this example is as follows.

(1) Visibility The alloyed hot-dip galvanized steel sheet after temper rolling was subjected to phosphate treatment, cationic electrodeposition coating, intermediate coating, and topcoating in that order, and then with a picture clarity measuring device. The PGD value was measured.
A PGD value of 0.8 or more was regarded as good image clarity.

(2) Press Formability (Sliding Property) The surface friction coefficient was determined by subjecting the alloyed hot-dip galvanized steel sheet after temper rolling to a deformation Bowden test. A surface friction coefficient of 0.25 or less was defined as good press formability.

(3) Powdering property A disc-shaped test piece having a diameter of 90 mm that has been temper-rolled is subjected to a cylindrical drawing, and a tape peeling test of the plating film is performed on the side surface of the obtained cup, and powdering resistance is obtained. The weight difference before and after the test was evaluated. If the weight difference before and after the test is 10 mg / TP or less,
The powdering resistance was considered to be good. The powdering amount is indicated by the weight difference per TP (test piece).

The results are shown graphically in Table 2 and FIGS. As can be seen from the results shown in Table 2, the alloyed plated steel sheet produced under the conditions of the present invention is divided into functions of a sharpness-oriented aspect and a formability-oriented aspect, and as a whole, both functions are sufficiently performed. You can see that you are satisfied.

FIG. 1 is a graph showing the image clarity when the laboratory rolling roll mat is changed. From the results shown in the figure, the image clarity is improved by using bright finish rolling rolls and an elongation of 0.4% or more. It can be seen that is dramatically improved.

FIG. 2 shows the press formability when the laboratory rolling roll mat is changed. From these results, it is understood that the press formability is improved by using the electric discharge dull finishing roll. .

FIG. 3 shows the test results of the powdering property when the laboratory rolling roll mat was changed. From these results, the bright finish rolling roll was used.
It can be seen that the powdering property is secured.

[0034]

[Table 2]

[0035]

The alloyed hot-dip galvanized steel sheet as the rust-preventive steel sheet for the outer surface of the automobile body according to the present invention has conventionally been considered to have a trade-off between properties, that is, image clarity after coating, press formability, and powdering. Both of them satisfy the above-mentioned requirements, and the excellent action and effect obtained by dividing the functions on the front and back sides in this way have never been seen before. Also, in terms of the emphasis on the clearness, the improvement in the clearness was observed regardless of the surface roughness of the base material.

[Brief description of drawings]

FIG. 1 is a graph showing image clarity when a rolling roll mat is changed.

FIG. 2 is a graph showing press formability when a rolling roll mat is changed.

FIG. 3 is a graph showing powdering properties when a rolling roll mat is changed.

Claims (2)

[Claims] 1. A temper rolling is performed to adjust a PGD value of 0.8 or more, which is a sharpness evaluation value of a surface of a steel sheet surface corresponding to an automobile exterior surface, and adjust a surface friction coefficient of the opposite surface to 0.25 or less.
Alloyed hot-dip galvanized steel sheet for automobile body outer panels with excellent image clarity, press formability and powdering. 2. A roll surface roughness obtained by subjecting a surface of a steel sheet surface after plating alloying corresponding to an automobile exterior surface to a bright finish when temper-rolling an alloyed hot-dip galvanized steel sheet for an automobile body outer panel. A skin pass roll with Ra 0.2 μm or less was used, and the opposite surface was subjected to electric discharge or shot dull processing to obtain roll surface roughness Ra 2.0 μm or more, 5.5 μm
Elongation rate 0.4% by using skin pass roll with m or less
As described above, it is characterized by performing temper rolling of 2.0% or less,
A method for producing a galvannealed steel sheet having excellent image clarity, press formability, and powdering property.