JP2002060917A - Manufacturing method of galvanized steel sheet - Google Patents

Abstract

(57) [Summary] [Problem] Considering that wiping control immediately after a plating bath cannot completely prevent a ripple pattern, furthermore, use a work roll of high roughness for a steel sheet on which a ripple pattern is observed. The present invention provides a method for producing a hot-dip galvanized steel sheet which is excellent in press formability and sharpness while solving the problem that a ripple pattern is more likely to remain when temper rolling is performed. In a temper rolling of a hot-dip galvanized steel sheet,
By the first temper rolling, the average roughness Ra of the plated steel sheet is 0.25μm
The following, then, by the second temper rolling, the average roughness Ra 0.8
Adjust μm to 2.0 μm and undulation Wca to 0.70 μm or less. In addition, a work roll having an average roughness Ra of 0.20 μm or less was used as the first temper rolling, and then the average roughness was used as the second temper rolling.
Use a work roll with Ra of 1.6 to 4.0 μm and undulation Wca of 1.3 μm or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-dip galvanized steel sheet having good surface properties, and more particularly to a method for producing a hot-dip galvanized steel sheet which is free from defective appearance and excellent in press formability and sharpness. Get involved.

[0002]

2. Description of the Related Art In the field of automobile parts and building materials, demand for galvanized steel sheets is increasing. As for galvanized steel sheets used for press working, relatively large surface roughness is required for the purpose of securing oil retention at the interface between the press die and the steel sheet and preventing mold galling. On the other hand, it is said that it is necessary to reduce long-period undulations on the steel sheet surface before painting for products that require sharpness after painting. For a method for producing a galvanized steel sheet that achieves both such press formability and sharpness, for example, JP-A-5-15901 and JP-A-2-2590
It is disclosed in 74853.

[0003] These prior arts relate to a so-called alloyed hot-dip galvanized steel sheet which is subjected to a galvanizing treatment after galvanizing. On the other hand, recently, the demand for lower-cost galvanized steel sheets has been increasing, and thicker galvanized steel sheets have attracted attention for the purpose of ensuring even more rustproofness for automotive applications. Usually, a manufacturing method without alloying treatment is used, and the plating film is mainly composed of the η phase.

The surface roughness of a galvannealed steel sheet is as follows:
In order to form unique irregularities caused by the alloying reaction, the average roughness Ra is generally adjusted to about 0.5 μm. The average roughness Ra is defined in JIS B 0601, and will be described below using a value based on a cutoff value of 0.8 mm.

On the other hand, a galvanized steel sheet which is not subjected to an alloying treatment does not form a unique surface irregularity during the alloying reaction, and has a higher galvanized steel sheet than the coating of the galvannealed steel sheet. Is mainly composed of the η phase, so that it is soft and has a low melting point, so that it has a property that adhesion to a mold easily occurs during press molding. Therefore, the average roughness Ra required to ensure press formability is 0.8 to 2.0 μm, preferably
It is adjusted to about 1.0 to 1.6 μm, and requires a large average roughness as compared with the galvannealed steel sheet.

On the other hand, increasing the surface average roughness Ra of the steel sheet is a disadvantageous condition for sharpness. That is,
The undulation Wca, which has a strong correlation with the sharpness, is the average roughness Ra
Is larger, the sharpness is increased, and the sharpness is deteriorated. Therefore, it is more difficult to improve the sharpness of the hot-dip galvanized steel sheet than with the galvannealed steel sheet. Here, the undulation Wca is defined by JIS B 0610, and is the average height of the irregularities subjected to the high-frequency cutoff. Specifically, in order to obtain high definition, the cutoff value is 0.8mm as Wca 0.7
It is considered that a galvanized steel sheet having a thickness of μm or less is desirable.

[0007] By the way, in the galvanized steel sheet as described above, a plating base sheet such as a cold-rolled steel strip is continuously annealed and immersed in a zinc bath or a zinc bath containing aluminum so that a film is adhered thereto. By injecting wiping gas onto the surface of the steel strip immediately after lifting, the thickness of the galvanized layer is adjusted, and the surface of the coated steel sheet is adjusted while giving a constant elongation by a temper rolling mill. Manufactured.

[0008] At this time, if the control of the adhesion amount by the wiping gas is inappropriate, a corrugated pattern (referred to as a ripple pattern) is generated on the plating surface, and a steel sheet having an excellent appearance cannot be obtained. Occurs. That is, in the temper rolling, it is possible to adjust the average roughness and undulation values as described above on average, but locally, very long-period irregularities due to ripples remain. , Will impair the appearance. On the other hand, Japanese Unexamined Patent Application Publication No. 11-236658
Japanese Patent Application Laid-Open Publication No. H11-139,086 discloses a method for lowering the injection pressure of a wiping nozzle and narrowing an interval between the nozzle and a steel plate.

[0009]

As described above, in order to manufacture a product having excellent press formability, appearance and sharpness as a galvanized steel sheet, it is necessary to prevent ripples generated after hot-dip plating. In addition, it is necessary to uniformly provide appropriate average roughness Ra and undulation Wca by temper rolling.

However, when the interval between the nozzle and the steel plate is reduced as in Japanese Patent Application Laid-Open No. H11-236658, the nozzle and the steel plate may come into contact with each other due to the fluttering of the passing material, and a flaw may be generated. In particular, this is a problem when trying to increase the productivity by increasing the line speed, and it is actually difficult to shorten the distance between the nozzle and the steel plate until the ripple pattern can be sufficiently reduced.

On the other hand, even if the surface roughness is imparted to the steel sheet with the remaining rippled pattern by temper rolling, there is a problem that the pattern remains and the appearance is impaired. In particular, in the case of galvanized steel sheets that require a large average roughness Ra compared to alloyed hot-dip galvanized steel sheets, it is necessary to use temper rolling rolls with higher roughness. When the roughness is transferred, a ripple pattern tends to remain.

The present invention takes into account that the ripple pattern cannot be completely prevented by the wiping control immediately after the plating bath, and furthermore, the steel sheet on which the ripple pattern is observed is formed by using a high-roll work roll. It is an object of the present invention to provide a method for producing a hot-dip galvanized steel sheet which is excellent in press formability and sharpness, while solving the problem that a ripple pattern is more likely to remain when temper rolling is performed.

[0013]

Means for Solving the Problems The present inventors have investigated the form of the ripple pattern seen in the galvanized steel sheet and examined the temper rolling conditions for eliminating such a ripple pattern. On the other hand, the temper rolling conditions that can give the average roughness Ra and the undulation Wca to achieve both the press formability and the sharpness of the product were studied. As a result, the rippled pattern is eliminated by once smoothing the surface of the plated steel sheet, and then, by a high-roughness temper rolling roll,
It has been found that by adjusting the surface to an appropriate value, there is no ripple pattern, and both press formability and sharpness can be achieved.

The present invention is based on this finding, and the means are as follows. (1) The first means is that, in the temper rolling of a hot-dip galvanized steel sheet, the average roughness Ra
Is adjusted to 0.25 μm or less, and then the average roughness Ra is adjusted to 0.8 μm to 2.0 μm and the undulation Wca is adjusted to 0.70 μm or less by a second temper rolling.

(2) The second means is that, in the temper rolling of the hot-dip galvanized steel sheet, the average roughness Ra is used as the first temper rolling.
Use a work roll of 0.20μm or less, and then as the second temper rolling, the average roughness Ra is 1.6 to 4.0μm, and the undulation Wca is 1.
A method for producing a galvanized steel sheet, comprising using a work roll of 3 μm or less. (3) The third means is the method for producing a galvanized steel sheet according to the above (1) or (2), wherein the galvanized steel sheet is a galvanized steel sheet whose plating film mainly comprises an η phase. is there.

In the present invention, the temper rolling step is separated into two stages, and the first temper rolling is adjusted to an average roughness of a certain value or less to eliminate the ripple pattern, and is appropriately applied in the second temper rolling. By imparting a high surface roughness, both press moldability and sharpness can be achieved.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION FIG. 3 is a schematic diagram of a production line for a hot-dip galvanized steel sheet used for describing an embodiment of the present invention. The steel strip 10 manufactured through the cold rolling is unwound from the pay-off reel 11, passed through an annealing step 12, and then immersed in a hot-dip galvanizing bath 13 to be plated. For the steel strip 10 coming out of the plating bath 13, the wiping nozzle 1
In step 4, the plating amount is adjusted. Further, two stands of temper rolling mills 15 and 16 are arranged downstream of the plating equipment. Each of the temper rolling mills 15 and 16 is a quadruple rolling mill. In each stand, a constant elongation rate is imparted, the material is adjusted, and the necessary surface is adjusted. The galvanized steel sheet that has undergone temper rolling is wound up by a tension reel 17 to form a product.

In the present invention, it is not necessary to shorten the distance between the wiping nozzle and the steel sheet as disclosed in Japanese Patent Application Laid-Open No. 11-236658, and even if the steel sheet flaps, no contact with the nozzle occurs. The distance may be set to about the same. At this time, a ripple pattern remains on the surface of the steel sheet after passing through the nozzle, and schematically shows a surface irregularity as shown in FIG. 2 (a). The height of the unevenness at this time is 2 to 6
μm, and the pitch is about 3 to 7 mm. Further, the average roughness Ra was 0.3 to 0.4 μm.

The present inventors have studied conditions for eliminating the ripple pattern by temper rolling. FIG. 1 shows that the average roughness Ra of the galvanized steel sheet after the temper rolling was changed by using only the temper rolling mill 15 and changing variously the surface roughness of the work roll and the elongation rate to be applied. In this case, the relationship between the average roughness Ra and the degree of remaining ripples is shown. The remaining degree of the ripple pattern was evaluated by visual appearance evaluation, and was classified into 1 to 5. Note that the score 1 was such that little improvement was seen from the rippled pattern of the unregulated material.
Reference numeral 5 denotes a state in which the ripple pattern is completely eliminated, and an area where a score of 3 or more can be satisfied. This indicates a state in which the ripple pattern is not observed after the second temper rolling.

From this figure, it can be seen that the average roughness Ra after temper rolling is
It can be seen that the acceptable range has been reached when the thickness becomes 0.25 μm or less. That is, when tempering rolling in a single pass is performed on a galvanized steel sheet in which ripples are generated, the ripple pattern becomes more conspicuous as the average roughness is increased.

[0021] The first means of the present invention is based on such knowledge, and by adjusting the average roughness Ra of the galvanized steel sheet to 0.25 µm or less by the first temper rolling, the ripple pattern is substantially reduced. Then, the average roughness Ra of the galvanized steel sheet is adjusted to 0.8 to 2.0 μm by the second temper rolling using the temper rolling mill 16.

FIG. 2 schematically shows such an effect.
It is. FIG. 2 (a) is a galvanized steel sheet having a ripple pattern, and shows a cross section before temper rolling. In the case of imparting high roughness with an average roughness Ra of 0.8 μm or more by one-pass temper rolling, as shown in FIG. Inconsistency in appearance is caused by the noticeable difference between the part that does not and the part that does not. On the other hand, by making the ripple pattern once smoothed by the first temper rolling, as shown in FIG. Roughness can be obtained. At the same time, the surface of the galvanized steel sheet after imparting high roughness has a small undulation, and it is possible to obtain a steel sheet having excellent sharpness after painting.

Incidentally, in order to reduce the average roughness of the galvanized steel sheet by the first temper rolling, it is necessary to adjust the surface roughness of the temper roll to a certain value or less. In particular, a steel sheet having an average roughness Ra of 0.3 to 0.4 μm before temper rolling is required to be adjusted to a smoother roll surface in order to further reduce the average roughness Ra by temper rolling. . Further, in the temper rolling, the upper limit of the elongation rate is set due to restrictions on the material, and when performing multiple pass temper rolling as in the present invention, if the elongation rate of the first temper rolling is too high. In addition, the elongation required for providing high roughness in the second temper rolling cannot be secured. Therefore, since the elongation rate in the first temper rolling is naturally limited, in the present invention, the elongation rate of the first temper rolling is limited to an upper limit of 0.5%.
As a work roll capable of reducing the average roughness Ra of a steel sheet to 0.25 μm or less, the average roughness Ra is limited to 0.20 μm or less.

In the second temper rolling, the average roughness Ra: 0.8-2.0 μm and the undulation Wca: 0.70 μm for achieving both press formability and sharpness after coating as a galvanized steel sheet.
The temper rolling conditions for the following are specified. At this time, since the average roughness Ra after the first temper rolling is 0.25 μm or less, it is necessary to use a work roll having high roughness in the second temper rolling. In particular, the upper limit of the elongation rate
In the case of about 1.5%, the transfer rate of the average roughness is 40 to 50
% And the average roughness Ra of the work roll is 1.6
4.04.0 μm.

On the other hand, in order to reduce the undulation Wca of the galvanized steel sheet to 0.70 μm or less by the second temper rolling, the undulation Wca of the work roll is reduced to 1.3% because the undulation transfer rate is about 50 to 55%. It must be less than μm.

The second means of the present invention is based on such knowledge, and the average roughness of the work roll for adjusting the average roughness Ra of the galvanized steel sheet to 0.25 μm or less by the first temper rolling. By setting the Ra to 0.20 μm or less, the ripple pattern is eliminated to a level that does not substantially cause a problem, and then the average roughness Ra is adjusted to 0.8 to 2.0 μm and the undulation Wca to 0.70 μm or less by the second temper rolling. The average roughness Ra of the work roll is set to 1.6 to 4.0 μm, and the undulation Wca is set to 1.3 μm or less.

As an embodiment of the present invention, a two-stand temper rolling mill is not necessarily required, but a one-stand temper rolling mill provided in a production line for hot-dip galvanized steel sheets is used. The same effect can be obtained by performing the first temper rolling and then performing the second temper rolling using a batch type temper rolling mill that is a separate line.

[0028]

EXAMPLES The results of temper rolling on a hot-dip galvanized steel sheet with a 0.8 mm-thick cold-rolled steel sheet as a base will be described.
In this example, a work roll having a diameter of 600 mm and a body length of 2000 mm was used as the work roll for the first temper rolling. At this time, the average roughness was changed by changing the number of the grindstone of the grindstone or by performing shot blasting. Furthermore, as a work roll related to the second temper rolling,
The average roughness Ra and the undulation Wca were controlled by subjecting rolls of the same size to discharge dulling.

At this time, as the galvanized steel sheet of the temper rolled original sheet, the shape of the rippled pattern is a bumpy height: 3.2 μm, the pitch: 5.5 mm, and the first and second work rolls are used. Temper rolling and second temper rolling were performed. For comparison, temper rolling of only one pass using the work roll used for the second temper rolling was also performed.

The average roughness Ra of the galvanized steel sheet after the first temper rolling, the presence or absence of a ripple pattern, the average roughness Ra and the undulation Wca of the galvanized steel sheet after the second temper rolling were investigated. Table 1 shows the survey results.

[0031]

[Table 1]

In the present invention examples (Nos. 1 to 7), in the first temper rolling, the average roughness Ra of the work roll is set to 0.20 μm or less, so that the average of the galvanized steel sheet after the first temper rolling is obtained. The roughness Ra can be reduced to 0.25 μm or less, and the ripple pattern of the galvanized steel sheet after the second temper rolling is eliminated. In addition,
In the second temper rolling, by using a work roll having an average roughness Ra of 1.6 to 4.0 μm and waviness Wca of 1.3 μm or less, the average roughness Ra of the galvanized steel sheet after the second temper rolling is reduced.
0.8 μm to 2.0 μm, undulation Wca is 0.70 μm or less, and the average roughness Ra and undulation Wca necessary for achieving both press formability and sharpness are obtained.

On the other hand, in the first temper rolling, the average roughness Ra is 0.20
In Comparative Examples (Nos. 9 and 10) using work rolls exceeding μm, the average roughness Ra of the galvanized steel sheet after the first temper rolling was 0.
It is more than 25 μm, and the ripple pattern of the galvanized steel sheet after the second temper rolling is not eliminated. In the second temper rolling, swell W
Comparative example using a work roll with ca over 1.3μm (No. 7)
Then, the undulation Wca of the galvanized steel sheet after the second temper rolling is 0.
It exceeds 70 μm, and the undulation Wca required for achieving both press formability and sharpness has not been obtained. Moreover, in the comparative examples (Nos. 11 to 13) in which only one pass of the temper rolling was performed using the work roll used for the second temper rolling, the ripple pattern was not eliminated.

[0034]

According to the present invention, even if a galvanized steel sheet has a ripple pattern, the surface is smoothed by the first temper rolling, the ripple pattern is eliminated, and the second temper rolling is performed. As a result, an appropriate surface property is imparted, thereby making it possible to produce a hot-dip galvanized steel sheet excellent in press formability and sharpness while preventing appearance defects.

[Brief description of the drawings]

FIG. 1 is a view showing a relationship between an average roughness Ra of a steel sheet after a first temper rolling and a residual degree of a ripple pattern on a steel sheet surface in the present invention.

FIG. 2 is a schematic diagram illustrating a process of smoothing a ripple pattern in the present invention.

FIG. 3 is a schematic diagram of a production line for a hot-dip galvanized steel sheet used for describing the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Steel strip 11 Payoff reel 12 Annealing equipment (annealing process) 13 Hot-dip galvanizing bath 14 Gas wiping nozzle 15 First temper rolling mill 16 Second temper rolling mill 17 Tension reel

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Yasuhiro Soya 1-2-1 Marunouchi, Chiyoda-ku, Tokyo F-term (reference) 4E002 AD06 BA01 BB09 CB01 CB03 4K027 AA02 AA22 AB02 AB14 AB15 AB35 AB42 AC87 AD15 AD28 AE25

Claims (3)

[Claims] In the temper rolling of a hot-dip galvanized steel sheet, an average roughness Ra of the coated steel sheet is set to 0.2 by a first temper rolling.
5μm or less, then by the second temper rolling, the average roughness Ra
Is adjusted to 0.8 μm to 2.0 μm and waviness Wca to 0.70 μm or less. 2. In temper rolling of a hot-dip galvanized steel sheet, a work roll having an average roughness Ra of 0.20 μm or less is used as first temper rolling, and then an average roughness Ra of 1.6 is used as second temper rolling. A method for producing a galvanized steel sheet, comprising using a work roll having a swell of about 4.0 μm and a swell Wca of 1.3 μm or less. 3. The method for producing a galvanized steel sheet according to claim 1, wherein the galvanized steel sheet is a galvanized steel sheet whose plating film mainly comprises an η phase.