JP3240945B2 - Control method of hot-dip galvanized coating weight - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous hot-dip galvanizing method for controlling the amount of galvanized coating by spraying a wiping gas nozzle onto a steel sheet running continuously through a hot-dip galvanizing bath. More particularly, the present invention relates to a method for controlling the amount of hot-dip galvanized coating with the aim of improving the accuracy of the amount of coating on a thinned material.

[0002]

2. Description of the Related Art Conventionally, the coating weight control of continuous hot-dip galvanizing is performed by a pair of hot-dip galvanizing baths 2 installed above a hot-dip galvanizing bath 2 in a continuous hot-dip galvanizing line coating weight control section as shown in FIG. This is performed by scraping excess molten zinc by injecting gas from the wiping nozzle 3 to the running steel sheet 1.

[0003] As a method for controlling the coating weight of continuous hot-dip galvanizing, the coating weight W is determined based on a coating weight prediction formula using the steel sheet line V, the injection gas pressure P of the wiping nozzle, the nozzle-steel distance B, and the like as factors. A method for controlling the amount of plating has been proposed. A large number of adhesion amount prediction formulas have been published. For example, Japanese Patent Application Laid-Open No. 6-116696 discloses a prediction formula as shown in the following formula (1). Here _{^{W = h 0 P h1 V h2}} B h3 ...... (1), h0, h1, h2, the value of the coefficient of h3 is to be determined by multiple regression calculation from the actual coating weight.

Also, taking into account the effect of the sheet thickness t of the steel sheet on the amount of plating, the following equation (2) combining the above three control factors, line speed, nozzle pressure, and nozzle-steel distance with the sheet thickness t is used. A control method based on, for example, JP-A-5-171
No. 395, disclosed. W = a ₀ Pa ¹ Va ^{2 Ba 3} t ^a4 exp (a5 / V) (2)

Further, Japanese Patent Application Laid-Open No. Hei 6-296923 discloses that a wiping gas jet is divided into a developed region and a fully developed region according to a distance between a nozzle and a steel plate based on a fluid dynamics theory. Taking into consideration that the influence of the distance on the coating weight is different, it is disclosed that different coating weight prediction formulas are used for the developed region and the fully developed region as shown in the following formulas (3) and (4). ing. B / D ≦ C (in the development area) W = a1ρ {(κ-1) / (2ηκP ₀ )｝ ^1/2 B ^1/2 × [μV / (P / P ₀ ) (κ-1) / κ −1｝] ^1/2 (3) B / D> C (in the fully developed region) W = a2ρ {(κ-1) / (2ηκP ₀ )｝ ^1/2 DB ^1/2 × [μV / (P / P ₀ ) (κ-1) / κ-1｝] ^1/2 (4) (ρ: molten zinc density, μ: molten zinc viscosity, P0: atmospheric pressure, κ: gas specific heat ratio, D: distance between wiping nozzle and steel sheet, η: wiping nozzle efficiency) In general, feedforward control and feedback control are performed using a plating weight prediction formula as described in each of the above publications to predict the plating weight. ing. Therefore, in order to control the coating weight with high accuracy, the accuracy of the plating weight prediction formula is important.

[0006]

As described above, various formulas for estimating the wiping conditions (distance between wiping nozzle and steel plate, wiping gas pressure, wiping gas nozzle gap, etc.), line speed, and coating weight have been proposed. However, at present, sufficient adhesion amount prediction has not been performed. For example, even if the wiping conditions and the line speed are the same, the amount of adhesion differs if the steel type or the aluminum concentration in the plating bath is different.

[0007] In order to control the coating weight with high accuracy, especially for thin-coated materials, it is necessary to use a plating weight prediction formula that takes into account the difference in the coating weight due to the difference in the steel type and the concentration of aluminum in the coating weight bath. There is. The present invention has been made in view of the above circumstances, and the amount of hot-dip galvanized coating using a coating weight prediction formula in consideration of the difference in the coating weight due to the difference in the steel type, the coating weight, and the concentration of aluminum in the bath. The purpose of the present invention is to provide a control method.

[0008]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention is to reduce the amount of hot-dip galvanized coating by injecting gas from a wiping nozzle onto a steel sheet that continuously travels through a hot-dip galvanizing bath. An object of the present invention is to provide a hot-dip galvanized coating amount control method, characterized by using a coating amount prediction formula in consideration of an initial alloy layer formed when passing through a plating bath. A method for controlling the amount of hot-dip galvanized coating characterized by using a steel type, an aluminum concentration in a plating bath, a time during which a steel sheet passes through the plating bath, and a plating bath temperature as factors for predicting the amount of coating by the initial alloy layer. Is provided.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have repeatedly studied the cause of the difference in the amount of adhesion when the steel type and the aluminum concentration in the plating bath are different even under the same wiping condition and line speed. It has been found that the cause is that a layer called an initial alloy layer is formed on the surface of the steel sheet by a reaction of iron-zinc while passing through the plating bath. Therefore, in the present invention, the adhesion amount prediction formula that takes into account the initial alloy layer formed when passing through the plating bath is used.

[0010] The formation of the initial alloy layer is affected by, among other things, the type of steel, the aluminum concentration in the plating bath, the time the steel sheet passes through the plating bath, and the plating bath temperature. Therefore, if the quantitative relationship between these factors and the increase in the amount of adhesion is grasped, it is possible to predict the increase in the amount of adhesion due to the formation of the initial alloy layer.

It is necessary to consider the amount of increase in the adhesion due to the formation of the initial alloy layer, the amount of the initial alloy layer itself, and the amount of lifted molten zinc that increases due to the formation of this layer. As shown in FIG. 1, the increase in the amount of molten zinc lifted by the formation of the initial alloy layer is caused by the uneven iron-zinc crystals (ζ phase) formed on the strip surface, as shown in FIG. 1.
This is because the frictional drag on the surface of the steel sheet increases as compared with a case where the initial alloy layer is not formed smoothly.

[0012]

Embodiments of the present invention will be described below. First, an example will be described in which even if the wiping conditions and the line speed are the same, the plating adhesion amount is different. For steel A and steel B shown in Table 1, a plating bath having a line speed of 80 mpm, a bath temperature of 460 ° C., and an aluminum concentration of 0.12 wt% was used. The wiping conditions were a nozzle gap of 0.7 mm and a header pressure of 0.8 kg /. Galvanization was carried out at a distance of 4 mm between the nozzle and the steel plate at a density of ² cm. At this time, the measured adhesion amount per side was 31 g / m ² for Steel B and 26 g / m ² for Steel A.

[0013]

[Table 1]

As described above, depending on the type of steel, even when the wiping conditions and the line speed were the same, the amount of plating was different. When the initial alloy layers of both were observed, B
The steel phase height was higher. Therefore, it can be said that the difference in the amount of adhesion between the two is due to the difference in the form of the initial alloy layer formed while passing through the plating bath.

In the case of the wiping conditions and the line speed shown in the above example, the adhesion amount predicted based on the jet theory was about 23 g / m ² . Therefore, the difference between the theoretical predicted value and the measured value is the amount of adhesion increase due to the formation of the initial alloy layer, and it is possible to perform highly accurate adhesion amount control without using the adhesion amount prediction formula incorporating the adhesion increase amount. Obviously you can't.

When the plating was carried out with the bath temperature changed to 500 ° C., no difference was observed in the adhesion amount and the initial alloy layer height as in the case of the bath temperature of 460 ° C. It was confirmed that the temperature was also affected by the bath temperature.

Further, when the aluminum concentration is 0.14 wt%
In this case, the adhesion amount was reduced by about 5 g / m ² compared to the case where the aluminum concentration was 0.12 wt%. This can be said to be because the alloying reaction of iron-zinc was suppressed by increasing the aluminum concentration.

FIG. 2 shows an example of the relationship between the increase in the amount of adhesion (difference from the amount of adhesion when the initial alloy layer is not formed) and the height of the initial alloy layer (層 phase). As shown in this figure, it is understood that the two have a substantially linear relationship. That is, as the Δ phase increases, the amount of increase in the amount of adhesion also increases.

For example, as shown in FIG. 3, the Δ phase height has a strong correlation with the amount of iron diffusion into the galvanized film. Furthermore, the amount of iron diffusion depends on the type of steel, the aluminum concentration in the plating bath, the plating bath passage time, and the plating bath temperature. For example, if the aluminum concentration in the bath is 0.12 wt%
In this case, the bath residence time and the amount of iron diffusion have a relationship as shown in FIG. 4 according to the bath temperature.

That is, the iron diffusion amount Fe is given by Fe = at
^{It is calculated} with ^0.5 . Here, a is a coefficient determined by the bath temperature, aluminum concentration, and steel type, and t is the bath passage time. Further, if the iron diffusion amount is obtained, the ζ-phase height h is obtained by h = f (Fe) from the relationship as shown in FIG. 3, and the increase ΔC in the amount of adhesion from this h is shown in FIG. As shown, ΔC = b
h + c. Here, b and c are coefficients depending on the kinematic viscosity coefficient and the line speed of the molten zinc. As described above, by adding the increased adhesion amount due to the formation of the initial alloy layer and the adhesion amount when the initial alloy layer is not formed, the adhesion amount can be predicted with high accuracy.

In the present invention, the formula for predicting the amount of deposition when the initial alloy layer is not formed is not particularly limited. For example, a high aluminum concentration bath (for example, 0 (20 wt% aluminum concentration bath), based on a prediction formula obtained by summarizing the results of the adhesion amount obtained from the operation using the jet flow theory described in JP-A-6-296923. The constructed prediction formula or the like can be used.

[0022]

As described above, according to the present invention,
Since the accuracy of the plating weight prediction equation can be improved, the control of the plating weight, particularly the control of the plating weight of the thinned material, can be performed with high accuracy.

[Brief description of the drawings]

FIG. 1 is a view showing an image of an initial alloy layer (ζ phase).

FIG. 2 shows the increase in the amount of galvanized coating and the initial alloy layer (ζ
The figure which shows an example of the relationship with phase) height.

FIG. 3 is a diagram showing an example of the relationship between the phase height and the amount of iron diffusion into a zinc plating film.

FIG. 4 is a diagram showing an example of the effect of each factor on the amount of iron diffusion into a galvanized film.

FIG. 5 is a schematic configuration diagram showing an outline of a continuous hot-dip plating line.

[Explanation of symbols]

1; steel plate; 2; hot-dip galvanizing bath; 3; wiping nozzle; 4; sink roll.

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Junichi Ozaki 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Takaharu Nagayama 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Keishi Yamashita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-7-41925 (JP, A) JP-A 9-157822 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] 1. A method for controlling the amount of hot-dip galvanized coating by injecting a gas from a wiping gas into a steel sheet continuously running through a hot-dip galvanizing bath, the initial alloy formed when passing through the hot-dip galvanizing bath. A method for controlling the amount of hot-dip galvanized coating, characterized by using a coating amount predicting formula in consideration of the layer. 2. The amount of hot-dip galvanized coating characterized by using a steel type, an aluminum concentration in a plating bath, a time during which the steel sheet passes through the plating bath, and a plating bath temperature as factors for predicting the amount of coating by the initial alloy layer. Control method.